roundabouts and access management final report€¦ · roundabouts and access management, and to...

RoundaboutsandAccessManagement

FDOTProjectBDK77977‐22

FinalReport

March2014

Preparedfor:FloridaDepartmentofTransportation

605SuwanneeStreet,MS19Tallahassee,FL32399

ProjectManager:GinaBonyani

SystemsPlanningOffice

Preparedby:

PrincipalInvestigatorDr.RuthL.Steiner

DepartmentofUrban&RegionalPlanningUniversityofFlorida

431ArchitectureBuildingGainesville,FL32611

Dr.ScottWashburn,Dr.LilyElefteriadou

EngineeringSchoolofSustainableInfrastructureandtheEnvironmentUniversityofFlorida

365WeilHallGainesville,FL32611

Dr.AlbertGan

DepartmentofCivilandEnvironmentalEngineeringFloridaInternationalUniversity

10555WestFlaglerStreet,EC3603Miami,FL33174

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DisclaimerTheopinions,findings,andconclusionsexpressedinthispublicationarethoseoftheauthorsandnotnecessarilythoseoftheStateofFloridaDepartmentofTransportation.

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MetricConversionTable

SYMBOL WHENYOUKNOW MULTIPLYBY TOFIND SYMBOLLENGTH

in. inches 25.4 millimeters mmft. feet 0.305 meters myd. yards 0.914 meters mmi miles 1.61 kilometers km

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TechnicalReportDocumentationPage1.ReportNo. 2.GovernmentAccessionNo. 3.Recipient'sCatalogNo.

4.TitleandSubtitleRoundaboutsandAccessManagement

5.ReportDateMarch2014

6.PerformingOrganizationCode7.Author(s)RuthL.Steiner,ScottWashburn,LilyElefteriadou,AlbertGan,PriyankaAlluri,DimitraMichalaka,RuoyingXu,ShantyRachmat,BenjaminLytle,AmyCavaretta

8.PerformingOrganizationReportNo.

9.PerformingOrganizationNameandAddressDepartmentofUrbanandRegionalPlanning

UniversityofFloridaP.O.Box115706

Gainesville,FL32611‐5706

10.WorkUnitNo.(TRAIS)11.ContractorGrantNo.BDK77‐977‐22

12.SponsoringAgencyNameandAddressFloridaDepartmentofTransportation

605SuwanneeStreet,MS30Tallahassee,FL32399

13.TypeofReportandPeriodCoveredFinalReportSeptember2012–March2014

14.SponsoringAgencyCode

15.SupplementaryNotesGinaBonyani,FDOTProjectManager16.AbstractTransportationengineersandplannersarebecomingmoreinterestedinusingroundaboutstoaddressaccessmanagementandsafetyconcernsinthetransportationsystem.Whileroundaboutsarebeingusedincreasinglyinavarietyofcontexts,existingresearchdoesnotprovidedetailedguidanceonhowtoevaluatetheuseofroundaboutsasaformofaccessmanagement.ThisFloridaDepartmentofTransportation(FDOT)researchprojecthasthreeprimarycomponents:areviewandassessmentofnationalandstateguidancerelatedtoroundaboutsandaccessmanagement,asafetyanalysisofall283roundaboutsinFlorida,andanoperationalanalysisofselectedroundabouts.Literaturerelatedtosafety,accessmanagement,andmultimodaltransportation(especiallyforbicyclistsandpedestrians,androadwaycapacityassociatedwiththeuseofroundabouts)isreviewed,andgapsinknowledgeregardingtheuseofroundaboutsareidentified,particularlyastheyapplytosafety,access,andcapacity.Oneofthefindingsoftheliteraturereviewisthatlittleresearchhasbeencompletedonaccessmanagementnearroundabouts.AreviewofnationalandstateguidanceidentifiesmajorstudiesincludingNCHRP672andguidanceinKansas,WisconsinandVirginiathatrecommendintersectionanddrivewayspacingsimilartothatrecommendedforun‐signalizedintersections.Thesafetyandoperationalanalysisidentifiesfourareasofconcern:cornerclearance,includingstoppingsitedistance(SSD)andintersectionsightdistance(ISD);theneedforguidanceonthefunctionalareanearroundaboutsincludingdrivewayandintersectionspacing,andtheuseofmedians;accesstomajoractivitycenters;andsafetyofvulnerableroadusers,especiallybicyclistsandpedestrians.Theoperationalanalysisconfirmspreviousresearchthatshowsthatroundaboutsaresimilartoun‐signalizedintersections,butthedifferencesmayinfluencetheoperationsandsafetywithinthefunctionalareaoftheroundabout.AnassessmentoftheprimaryFDOTutilizedsoftwaretoolsfocusesonthecurrentsuitabilityofthesesoftwaretoolstoassistpractitionersinassessingthesuitabilityofincorporatingroundaboutsintoexistingandproposedroadwayconfigurations.Recommendationsaremadeforadditionalnationalresearchonguidanceondrivewayandintersectionspacing,medians,andSSDandISDinthedifferentcontextsinwhichroundaboutsareinstalled.ChangestotheFDOT’sAccessManagementTools,MedianHandbookandDrivewayInformationGuidearealsorecommendedalongwiththedevelopmentofFlorida‐specificparametersforcapacityandsafetyanalysis.Modificationstoroundaboutdesignguidelinesandhandbooksforaccessmanagementwillleadtosafer,moreeffective,andultimately,betterperformingroundaboutsforallusersofFlorida’stransportationsystemandthroughouttheUnitedStates.17.KeyWords:Roundabout,accessmanagement,safety,capacity,operationalanalysis

18.DistributionStatementNorestrictions.

19.SecurityClassif.(ofthisreport)Unclassified.

20.SecurityClassif.(ofthispage)Unclassified.

21.No.ofPages177

22.Price

FormDOTF1700.7(8‐72) Reproductionofcompletedpageauthorized

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AcknowledgementsTheprojectteamwouldliketothankGinaBonyaniandGarySokolow,oftheFloridaDepartmentofTransportation(FDOT)SystemsPlanningOffice,fortheirassistanceinunderstandingthescopeoftheprojectandfortheirfeedback.Theresearchteamwishestoacknowledgetheeffortsofnationalexpertsonroundabouts,includingAndreaBill,PhilDemosthenes,PatrickFlemming,HillaryIsebrands,MarkJohnson,HowardMcCullough,LeeRodegerdts,EugeneRussell,JeffShaw,KenSides,MichaelWallwork,andBrianWalsh,fortheirwillingnesstoparticipateinthisresearchinavarietyofways,includingdiscussingdesign,safety,policy,andothertopicsrelatedtoaccessmanagementnearroundabouts,providingtimeinmeetingsoftheITERoundaboutsandtheTransportationResearchBoard(TRB)RoundaboutsCommittee,andtoreviewingthedraftsofthisfinalreport.

ExecutiveSummarySummaryofFinalReport,BDK77977‐22

March2014BackgroundOverthelasttwentyyears,engineersandplannershavebecomeincreasinglyinterestedintheuseofroundaboutsbecausetheyofferseveraladvantagesoverothertrafficcontrols;theymaycostlesstoinstall,havegreatersafetypotentialbyreducingthenumberofconflictpoints,canaccommodateaseriesofU‐turnsandleft‐turnlanesandreducedelayinacorridor,and,mayhaveloweroperationsandmaintenancecosts.Floridahasrecentlybeguntoencouragetheuseofroundaboutsonthestatehighwaysystemandissystematicallyupdatingitsguidancedocuments(e.g.,PlansPreparationManual,IntersectionDesignManual,andManualonUniformTrafficStudies)butneedsguidanceonwhattoincludeintheMedianHandbook,andDrivewayInformationGuideandotheraccessmanagementdocuments.ObjectivesThepurposeofthisstudyistounderstandpreviousresearchandstateandnationalguidanceonroundaboutsandaccessmanagement,andtoconductempiricalresearchonthesafetyandoperationofroundaboutsinFlorida.Adviceonimplementingroundaboutsandaccessmanagementintostateguidancedocumentswillbeprovided.Theresearchobjectiveswereachievedbycompletingthefollowingtasks:

1. Literatureandbackgroundreviewofnationalandstateguidance;2. Safetyanalysisofall283roundaboutsinFlorida;3. OperationalanalysisofthirteenselectedroundaboutsitesinFlorida;and4. Softwaretoolsreviewforroundaboutsimulationandevaluation.

FindingsandConclusionsThereviewofnationalguidanceonroundaboutandaccessmanagementshowsthatonlyfivefederalaccessmanagementreportsrefertoroundabouts:AASHTOGreenBook,NCHRPReport672–Roundabouts:AnInformationalGuide,SecondEdition,NCHRPReport572–RoundaboutsintheUnitedStates,NCHRPReport674–CrossingSolutionsatRoundaboutsandChannelizedTurnLanesforPedestrianswithVisionDisabilities,andNCHRPSynthesis264–CrossingSolutionsatRoundaboutsandChannelizedTurnLanesforPedestrianswithVisionDisabilities.NCHRPReport672,whichisthemostrelevanttothisreport,referstotheaccessmanagementinthecontextofroundaboutsandreinforcestheideathatmanyoftheaccessmanagementprinciplesthatapplytoconventionalintersectionscanbeappliedtoroundabouts.

Stateguidanceonroundaboutsandaccessmanagementprovidesvaryinglevelsofspecificity,withmoststatesadoptingnationalguidancefromNCHRPReport672–Roundabouts,AnInformationalGuide;afewstatesprovidestate‐specificparametersandguidance.Whileseveralstatesadoptlocalparametersforroundabouts,onlythreestates–Wisconsin,Virginia,andKansas–addresstheuseofaccessmanagementwithinthebroadercontextofthedesignofroundabouts.ThesafetyandoperationalanalysesofexistingroundaboutsinFloridaidentifythreeareasofconcernaboutaccessmanagementnearroundabouts:(1)conflictswithinthefunctionalareaofroundabouts;(2)safetyofvulnerableroadusers,includingpedestriansandbicyclists;and(3)roundaboutsthatprovidedirectaccesstoactivitycenters.Ofatotalof2,941crashesthatoccurredfrom2007–2011within500ft.ofthe283roundaboutsinthestate,1,882crashesweredirectlyrelatedtoaroundabout;thisisanaverageof6.65crashesperroundaboutwithanaverageof8.10and5.4crasheseacharoundcommercialandresidentiallanduses,respectively.Consistentwiththepreviousfindings,thesafetyandoperationalanalysisofroundaboutsshowedarelativelowrateofcrashes,butsomeareasofconcern.Theoperationalanalysisidentifiedsituationsinwhichaleft‐turningvehicleorpedestrianscouldcausedelaysinvehiclesmovingthroughtheroundabout.Thesafetyanalysisshowedthatcrashesinvolvingvehiclesturningleftatmedianopeningswererelativelyrare.Whilethesafetyanalysisshowedthatthedownstreamdrivewaycornerclearancehasagreatersafetyimpactthantheupstreamdrivewaycornerclearance,theoperationalanalysisdidnotidentifysuchconflicts.Highpedestrianandbicyclevolumescanaffectthecapacityandthe

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effectiveoperationofroundabouts.Crashesinvolvingpedestriansandbicycleswereabout4%ofallcrashes,butnogeneralconclusionscanbedrawnduetothesmallsamplesizeandthelackofgoodexposuredata.Thesafetyandoperationalanalysishadsomewhatconflictingresultsforroundaboutsatactivitycenters.Roundaboutswiththreeorfourlegs,withdirectaccesstoactivitycenters,areequallyassafeasroundaboutswithoutdirectaccesstoroundabouts.However,inactivitycenterswithhighvolumesofpedestriansandbicyclists,erroneousdriverbehavior,suchasstoppinginthemiddleoftheroundabouttopick‐upordrop‐offpedestrians,causesdelaysforotherdrivers.Theoperationalanalysisdidnotidentifyotherconcernsfoundintheliterature,includingspillbackintotheroundaboutfromadownstreambottleneck,whichwouldresultincompletelylockingtheroundabout.Amajorconclusionofthisresearchisthat,whilemuchresearchhasbeenconductedaboutroundaboutsandaboutaccessmanagement,littleresearchhasbeenconductedonroundaboutsincombinationwithaccessmanagementandroundaboutsasaformofaccessmanagement.Roundaboutsareaformofaccessmanagementbecausetheycanaccommodateleftturnsandallowtheremovalofdirectionalleft‐turnlanes;yettheyfunctionasintersections.Howqueuesformandtrafficoperatesinthefunctionalareaaroundroundaboutsislesswellunderstoodthanforothertypesofintersections.Thedifferencesinroundaboutsafetyandoperationalcharacteristicsfromothertypesofaccessmanagementandotherintersectionsmeansthatthesitedistances,stoppingdistances,functionalareacharacteristics,andintersectionanddrivewayspacingmaybedifferentfromothertypesofintersections.RecommendationsAsFloridastartsincorporatingroundaboutsintoitspractices,consistentguidanceontheuseofroundaboutsthataddressthediversesituationsunderwhichroundaboutsareimplementedshouldbeprovided.Ofthe283roundaboutsinFlorida,onlyfourarelocatedonthestatehighwaysystem;therestarelocatedinavarietyofregionalcontexts–urban,suburbanandrural–withdiversedesignsandaccessconsiderations,andatdifferentdistancesfromthenearestcommunitycenters,highways,interstates,andstatehighways.Essentialtothisguidanceisconsiderationofthedifferencesbetweenroundaboutsandothertypesofintersections,andtotypesofaccessmanagement,suchasdriveways,andmedians.Itisessentialtounderstandtheeffectsofroundaboutsontrafficconditions,safetyandtrafficnetworkoperations.Thefindingsofboththesafetyandoperationalanalysisreinforcetheneedtoaccommodatebicyclistsandpedestriansaroundroundabouts.Whilethisresearchdidnotidentifysignificantproblemswithtrucksandotherlargevehicles,theneedtoaccommodatethemislikelytobecomeanissueasroundaboutsaremorewidelyusedalongstateroadwaysandotherhigh‐capacityroadwayswhereroundaboutdesignneedstoaccountforadequatelateralclearanceandlargerradius.FloridahasalreadyadoptedNCHRP672,Roundabouts,AnInformationalGuidebutthestateshouldconductandsupportadditionalresearchontheuseofroundabouts.TheFDOTshouldsupportnationalresearchthatspecificallyfocusesonthefunctionalareaofroundaboutsonmajorarterials.Thestateshouldconsidertheuseoflocally‐developedparametersforvariousaspectsofdesignandoperationalanalysisofroundabouts.Recently,theCityofSarasota,inconsultationwiththeFDOT,hasproposedaseriesofroundaboutsonUS41.TheFDOThasauniqueopportunitytocompleteabefore‐and‐afterstudyontheoperationalandsafetycharacteristicsofcorridorsofroundaboutsinsteadofconventionalintersectionsinthiscorridor.BenefitsRoundaboutsofferseveraladvantagesoverothertrafficcontrols:theymaycostlesstoinstall,havegreatersafetypotentialbyreducingthenumberofconflictpoints,canaccommodateaseriesofU‐turnsandleft‐turnlanesandreducedelayinthecorridor,andcanhaveandmayhaveloweroperationsandmaintenancecosts.Theguidanceresultingfromthisresearchcancertifythatroundaboutsareimplementedinamannerthatensuresimprovedsafetyandcapacitywhilemaintainingaccesstonearbybusinesses.ThisresearchprojectwasconductedbyRuthL.Steiner,oftheUniversityofFlorida.Formoreinformation,contactGinaBonyani,ProjectManager,at850‐414‐4707,[email protected].

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TableofContents

Disclaimer...................................................................................................................................................................................................ii

MetricConversionTable.....................................................................................................................................................................iii

TechnicalReportDocumentationPage.........................................................................................................................................iv

Acknowledgements.................................................................................................................................................................................v

ExecutiveSummary...............................................................................................................................................................................vi

Background..........................................................................................................................................................................................vi

ListofFigures..........................................................................................................................................................................................xii

ListofTables..........................................................................................................................................................................................xiv

ListofAbbreviations............................................................................................................................................................................xv

ChapterOne:Introduction...................................................................................................................................................................1

1.1Background...................................................................................................................................................................................1

1.2ResearchQuestions....................................................................................................................................................................2

1.3ObjectiveofResearch................................................................................................................................................................3

1.4ScopeofWorkandSupportingTasks.................................................................................................................................3

1.5OrganizationoftheReport.....................................................................................................................................................6

ChapterTwo:LiteratureReview.......................................................................................................................................................7

2.1Overview........................................................................................................................................................................................7

2.2Roundabouts.................................................................................................................................................................................7

2.2.1ModernRoundabouts......................................................................................................................................................7

2.2.2GeometricDesign...............................................................................................................................................................8

2.2.3ContextsofRoundabouts..............................................................................................................................................11

2.2.4ComparingRoundaboutstoOtherTypesofIntersectionTrafficControls..............................................13

2.3AccessManagement.................................................................................................................................................................13

2.3.1AccessManagementElements...................................................................................................................................14

2.3.2SpacingStandardsandRoadwayClassifications................................................................................................14

2.3.3AccessManagementMechanismsandIntersectionControls........................................................................16

2.3.4ImpactofRoundaboutsonAccessManagement................................................................................................18

2.4OperationalEffectsofRoundabouts.................................................................................................................................19

2.4.1EffectofTrafficFlowandDriverBehavior............................................................................................................19

2.4.2EffectofGeometry...........................................................................................................................................................20

2.4.3OperationalAnalysisofRoundabout.......................................................................................................................20

2.4.4RoundaboutCapacityunderDifferentConditions.............................................................................................21

2.4.5SummaryofRoundaboutOperationLiteratureReview..................................................................................22

2.5RoundaboutsandSafety........................................................................................................................................................23

2.5.1OverallSafetyEffectsoftheRoundabouts............................................................................................................24

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2.5.2AspectsofSafetyPerformanceofRoundabouts.................................................................................................25

2.5.3SafetyforDifferentRoundaboutUsersandModes...........................................................................................29

2.5.4MethodsinRoundaboutSafetyAnalysis................................................................................................................35

2.5.5RoundaboutsandSafety:Conclusion......................................................................................................................39

2.6EvaluationofGapsinRoundaboutLiterature..............................................................................................................40

2.6.1LiteratureGapsinAccessManagement.................................................................................................................40

2.6.2LiteratureGapsinRoundaboutOperationsandCapacity..............................................................................40

2.6.3LiteratureGapsinRoundaboutSafety....................................................................................................................41

ChapterThree:Methodology............................................................................................................................................................43

3.1AccessManagementandRoundaboutGuides’Selection.........................................................................................43

3.2SiteIdentification......................................................................................................................................................................45

3.3SafetyAnalysis...........................................................................................................................................................................46

3.3.1CategorizeRoundaboutLocations............................................................................................................................46

3.3.2ExtractCrashData...........................................................................................................................................................47

3.3.3CorrectCrashLocationsandReviewPoliceReports........................................................................................47

3.4OperationalAnalysis...............................................................................................................................................................50

3.4.1DataCollectionSiteSelection.....................................................................................................................................50

3.4.2DataCollection..................................................................................................................................................................53

3.4.3DataAnalysis.....................................................................................................................................................................54

ChapterFour:ReviewofNationalandStatePractices..........................................................................................................56

4.1NationalandStateGuidebooksforRoundaboutsandAccessManagement....................................................56

4.1.1NationalGuidanceforAccessManagement..........................................................................................................56

4.1.2States’GuidanceforAccessManagement..............................................................................................................61

4.1.3NationalandStateGuidebooksforRoundabouts..............................................................................................62

4.1.4StateGuidanceforRoundabouts...............................................................................................................................65

4.2StateofFloridaGuidance.......................................................................................................................................................68

4.2.1AccessManagementGuidanceinFlorida..............................................................................................................68

4.2.2RoundaboutsGuidanceforFlorida...........................................................................................................................75

4.3NationalGuidanceonAccessManagementintheContextofRoundabouts....................................................76

4.4States’GuidanceonAccessManagementintheContextofRoundabouts........................................................77

4.5RoundaboutLocationGuidelines.......................................................................................................................................80

4.6GeometryDesignGuidelines................................................................................................................................................81

ChapterFive:SafetyAnalysis...........................................................................................................................................................88

5.1OverallCrashStatistics...........................................................................................................................................................88

5.1.1AreaType............................................................................................................................................................................88

5.1.2CrashType..........................................................................................................................................................................88

5.1.3CrashSeverity...................................................................................................................................................................90

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5.1.4NumberofVehiclesInvolved......................................................................................................................................91

5.2ImpactofDrivewayCornerClearancesonRoundaboutSafety.............................................................................91

5.3SafetyImpactofMedianOpeningsintheVicinityofRoundabouts.....................................................................94

5.4SafetyatRoundaboutsThatProvideDirectAccesstoActivityCenters..........................................................100

5.5SafetyofVulnerableRoadUsers.....................................................................................................................................104

5.5.1Pedestrians......................................................................................................................................................................104

5.5.2Bicyclists...........................................................................................................................................................................105

5.6SummaryofFindings...........................................................................................................................................................106

ChapterSix:OperationalAnalysis...............................................................................................................................................109

6.1OverviewofDataCollectionSites...................................................................................................................................109

6.2AnalysisofAccessManagementIssuesAffectingOperations.............................................................................109

6.2.1ConflictsatAccessPointwithinRoundabout’sFunctionalArea...............................................................109

6.2.2ConflictswithPedestrians.........................................................................................................................................110

6.2.3ViolationofTrafficRules...........................................................................................................................................111

6.2.4SummaryofOperationalAnalysis.........................................................................................................................112

6.3AssessmentofSoftware......................................................................................................................................................112

6.3.1HCS......................................................................................................................................................................................113

6.3.2Synchro.............................................................................................................................................................................114

6.3.3SIDRA.................................................................................................................................................................................115

6.3.4RODELandARCADY....................................................................................................................................................115

6.3.5VISSIM...............................................................................................................................................................................115

6.3.6CORRIDORSIMULATION(CORSIM).....................................................................................................................116

6.3.7Summary..........................................................................................................................................................................118

ChapterSeven:Discussion..............................................................................................................................................................119

7.1Overview...................................................................................................................................................................................119

7.2RoundaboutsandAccessManagementinFlorida...................................................................................................120

7.2.1SummaryofSafetyAnalysis.....................................................................................................................................120

7.2.2SummaryofOperationalAnalysis.........................................................................................................................122

7.3RoundaboutsandAccessManagementGuidance....................................................................................................124

7.3.1SummaryofNationalandStateGuidanceonRoundabouts.......................................................................124

7.3.2SummaryofNationalandStateGuidanceonAccessManagement.........................................................125

7.3.3SummaryofNationalandStateGuidanceonRoundaboutandAccessManagement......................126

7.3.4SummaryofFlorida’sGuidanceonRoundaboutsandAccessManagement........................................127

7.4SynthesisofFindingsoftheResearch...........................................................................................................................128

7.5Recommendations.................................................................................................................................................................131

7.5.1RecommendationsforFlorida’sGuidanceonRoundaboutsandAccessManagement...................131

7.5.2RecommendationsforAdditionalResearch......................................................................................................134

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ChapterEight:Conclusions............................................................................................................................................................136

8.1ConclusionsoftheReviewofNationalandStateGuidance.................................................................................136

8.2ConclusionsAboutSafetyAnalysisofRoundaboutsinFlorida..........................................................................136

8.3ConclusionsAboutOperationalAnalysisofRoundaboutsinFlorida..............................................................137

8.4FinalRemarks.........................................................................................................................................................................137

8.5AdditionalResearchNeeds................................................................................................................................................138

ReferencesCited.................................................................................................................................................................................140

AppendixA:RoundaboutsFeaturesandDimensions.........................................................................................................149

KeyFeaturesofaModernRoundabout..........................................................................................................................149

Dimensions.................................................................................................................................................................................149

AppendixB:StatePolicies..............................................................................................................................................................151

AppendixC:AccessManagementTechniquesinStateGuidelines................................................................................161

AppendixD:SiteSelection..............................................................................................................................................................165

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ListofFiguresFigure1.GeometricDesignFeaturesofaTypicalModernRoundabout(FDOT,2007)...........................................8 Figure2.GeometricDesignFeaturesofaTypicalModernRoundabout:Single‐lane(b)andMultiple‐LaneRoundabouts(c)......................................................................................................................................................................................9 Figure3.IntersectionSightDistance(FHWA,2006,p.18)................................................................................................11 Figure4.StoppingSightDistance(FHWA,2006,p.19).......................................................................................................11 Figure5.AccessandRoadClassification....................................................................................................................................15 Figure6.RelationshipbetweenAccessManagement,RoadwayDesign,TrafficOperationsandLandUse(Roseetal.,2005)..................................................................................................................................................................................16 Figure7.CrashTypesonaTypicalRoundabout(ArndtandTroutbeck,1998,p.28‐3)........................................24 Figure8.VehicleConflictsandVehicle‐PedestrianConflictsatSignalizedIntersectionsandSingle‐LaneRoundabouts(Rodegerdtsetal.,2010,Exhibit5‐2,p.5‐7).................................................................................................26 Figure9.DifferentMarkingSystems(Bieetal.,2005).........................................................................................................28 Figure10.(1)Mixedtraffic;(2)adjacentbikelanes;(3)separatedbikelaneswithpriorityforbicyclists;and(4)separatedbikelaneswithoutpriorityforbicyclists(DanielsandWets,2005,p.6‐8)............................31 Figure11.CrashFrequenciesinRoundabouts(Isebrands,2009b)...............................................................................36 Figure12.DataRequiredforChi‐SquareAnalysis(FlanneryandDatta,1996,p.6)..............................................36 Figure13.ExamplesofRoundaboutsLocatedinEachLandUseType.........................................................................47 Figure14.CrashesDisplayedbyCrashTypeataRoundabout........................................................................................48 Figure15.CrashesDisplayedbyCrashSeverityataRoundabout..................................................................................48 Figure16.AnExampleofaCrashThatWasNotDirectlyRelatedtotheRoundabout...........................................49 Figure17.DataCollectionusingWeb‐basedTool..................................................................................................................50 Figure18.RoundaboutsitesinFloridaSelectedforOperationalAnalysis.................................................................52 Figure19.CameraLocationofVideoRecordingforIndependentDriveandSouthLauraStreetinJacksonville..............................................................................................................................................................................................54 Figure20.RoadwayFunctionClassificationinFlorida(FDOT,2010,p.24)..............................................................69 Figure21.DrivewayDesignandSpacing(FDOT,2008,p.9)............................................................................................69 Figure22.EffectiveRadiusandCurbRadius(FDOT,2008)..............................................................................................70 Figure23.RampSpacing(FDOT,2008,p.78).........................................................................................................................71 Figure24.RoundaboutatanInterchange(FHWA,2006,p.8).........................................................................................72 Figure25.CornerClearance(FDOT,2008,p.73)..................................................................................................................72 Figure26.CornerClearanceforDownstream(FDOT,2008,p.76).................................................................................73 Figure27.SightDistanceandDriverEyeSetbackDrivewayInformationGuide(FDOT,2008,p.62)...........74 Figure28.JointandCrossAccess(FDOT,2008,p.86)........................................................................................................74 Figure29.TypicalDimensionsforLeft‐turnAccessnearRoundabouts(Rodegerdtsetal.,2010,p.6‐98)...77 Figure30.MeasuredDistancefromSplitterIslandtoFirstAccessPoint(KsDOT,2013,p.4‐26)....................79 Figure31.MinimumSpacingStandardsforCommercialEntrances,Intersections,andCrossovers(VDOT,2007,p.F‐23)..........................................................................................................................................................................................80 Figure32.TheEffectofDesignElements(WisDOT,2011,p.38).....................................................................................81 Figure33.ExampleSolutionDesignwithCirculating‐ExitingPathConflict(Caltrans,2007,p.62).....................83 Figure34.SolutionOptionsforCirculating‐ExitingPathConflict:(i)ModifyLaneConfiguration,and(ii)RealignApproaches(Caltrans,2007,p.63‐64).........................................................................................................................83 Figure35.AngleofVisibility:(i)theAngleisTooSevere(ii)RealignedRampTerminalApproachtoHaveBetterAngleofVisibility(Caltrans,2007,p.65)......................................................................................................................84 Figure36.StatisticsbyAreaType.................................................................................................................................................89 Figure37.TotalandNighttimeCrashStatisticsbyCrashType......................................................................................90 Figure38.StatisticsbyCrashSeverity........................................................................................................................................90 Figure39.UpstreamandDownstreamDrivewayCornerClearances...........................................................................92

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Figure40.RoundaboutonSRA1A,NassauCounty,FloridawithReducedSightDistanceatDownstreamCornerClearance...................................................................................................................................................................................94 Figure41.Case1‐VehiclesTurningontoaDrivewayDownstreamoftheRoundabout.....................................95 Figure42.Case2‐VehiclesTurningLeftfromaDrivewayUpstreamofaRoundabout.......................................96 Figure43.AnExampleofaCrashataMedianOpeningInvolvingaVehicleTurningLeftFromtheMainStreetOntoaDriveway.......................................................................................................................................................................97 Figure44.AnExampleofaCrashataMedianOpeningInvolvingaVehicleTurningLeftFromaDrivewayOntotheMainStreet............................................................................................................................................................................97 Figure45.ANon‐incapacitatingInjuryInvolvingaVehicleTurningLeftfromDrivewayandaBicyclist....97 Figure46.ExamplesofCrashesInvolvingHeavyVehiclesatRoundabouts..............................................................98 Figure47.ClosingMedianOpeningsPreventVehiclesFromTurningLeftFromtheDriveway.......................99 Figure48.ClosingMedianOpeningsPreventVehiclesFromTurningLeftFromtheMainStreet...................99 Figure49.ACorridorwithTwoRoundaboutsonSegoviaStreet,MiamiDadeCounty,Florida....................100 Figure50.AnActivityCenterwithAccessThroughaMajorDriveway....................................................................101 Figure51.AnActivityCenterwithDirectAccessfromaRoundabout......................................................................101 Figure52.ExamplesofSix‐leggedRoundaboutsthatExperiencedHighCrashes.................................................103 Figure53.FatalCrashInvolvingaPedestrian(CrashID:772427040)....................................................................104 Figure54.CorridoronSW2ndAvenue,Gainesville,AlachuaCounty,Florida........................................................106 Figure55.ConflictofLeft‐turnVehicleatRoundabout(SW2ndAvenueandSW6thinAlachuaCounty)..110 Figure56.RoundaboutObservationonSpillBackofEnteringTrafficintoanAdjacentAWSCIntersection(NE10thCt.andSW152ndAve.,Miami)...................................................................................................................................110 Figure57.RoundaboutObservationwithPedestrianConflict(IndependentDr.andS.LauraSt.,DuvalCounty)...................................................................................................................................................................................................111 Figure58.RoundaboutObservationwithDriverViolationofTrafficRules(IndependentDr.andS.LauraSt.,DuvalCounty)...............................................................................................................................................................................111 Figure59.RoundaboutObservationwithSpillBackfromDrivewayintoCirculatingLanes(CausewayBlvd.andMandalayAve.,PinellasCounty).........................................................................................................................................112 Figure60.InterfaceofHCS2010................................................................................................................................................113 Figure61.UserInterfaceofSynchro(Trueblood,2013).................................................................................................114 Figure62.ExampleofRoundaboutSimulationinVISSIM(FHWA,2011)................................................................116 Figure63.ExampleofModelingRoundaboutinCORSIM(Elias,2009).....................................................................117 Figure64.ConditionalTurnMovementinCORSIM(Elias,2009)................................................................................117 Figure65.ConflictandSpillbackassociatedwithLeft‐turnAccesstoDriveway...................................................122 Figure66.Solution1‐DedicatedLeft‐turnLaneforAccesstoDriveway..................................................................123 Figure67.Solution2–Right‐laneAccess...............................................................................................................................123 FigureD.1.CameraLocationofRoundaboutatCausewayBlvdandMandalayAve.............................................165 FigureD.2.CameraLocationofRoundaboutatSW2ndAveandSW6thSt...............................................................166 FigureD.3.CameraLocationofRoundaboutatMLKBlvd.andN.CentralAve......................................................167 FigureD.4.CameraLocationofRoundaboutatEagle’sReserveBlvdandDyerBlvd..........................................168 FigureD.5.CameraLocationofRoundaboutatIndependentDr.andS.LauraSt..................................................169 FigureD.6.CameraLocationofRoundaboutatCR‐707andAveA.............................................................................170 FigureD.7.CameraLocationofRoundaboutatCR‐210andMicklerRd...................................................................171 FigureD.8.CameraLocationofRoundaboutatNE10thCt.andSW152ndAve.....................................................172 FigureD.9.CameraLocationofRoundaboutatGreenwayDr.andSegoviaSt.&CoralWay............................173 FigureD.10.CameraLocationofRoundaboutatBiltmoreWayandSagoviaSt....................................................174 FigureD.11.CameraLocationofRoundaboutatHolmbergRd.andParksideDr.................................................175 FigureD.12.CameraLocationofRoundaboutatPonceDeLeonBlvd.andRuizAve..........................................176 FigureD.13.CameraLocationofRoundaboutatMargateBlvd.andNW58thSt...................................................177

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ListofTablesTable1.SelectionofAnalysisTool(Rodegerdtsetal.,2010)............................................................................................23 Table2.DetailedCountermeasuresforDesignElements(Lordetal.,2007,p.429)..............................................32 Table3.AdvantagesandDisadvantagesofRoundaboutforPedestrians(Furtado,2004)...................................33 Table4.MainDocumentsonAccessManagement–RelatedStateDOTGuidebooks.............................................44 Table5.TheSourcesofRoundaboutStates’DesignGuidebooks....................................................................................45 Table6.SummaryofRoundaboutsinFloridabyDesignandContext...........................................................................46 Table7.CriteriaforSelectingRoundaboutsforOperationalAnalysis..........................................................................51 Table8.SummaryofRoundaboutSelectionProcess............................................................................................................52 Table9.SummaryofFeaturesandSurveyTimeofSelectedRoundaboutsofThirteenRoundaboutsandDataCollectionTimesforOperationalAnalysis.......................................................................................................................53 Table10.MainDocumentsoftheAccessManagement‐RelatedStateDOTsGuidebooks...................................62 Table11.RoundaboutStates’DesignGuidebooksReviewedinthisDocument.......................................................65 Table12.AccessManagementStandardsfromRule14‐97(FDOT,2006,p.15)......................................................73 Table13.RoundaboutDesignSpeed...........................................................................................................................................82 Table14.RecommendedHeadwayValues(WisDOT,2011,p31)...................................................................................82 Table15.WisconsinDOTMinimumVisibilityDistance.....................................................................................................84 Table16.TypicalInscribedCircleDiameterRanges(Caltrans,2007,p.67)..............................................................84 Table17.CommonRangesofInscribedCircleDiameters(Caltrans,2007,p.68)....................................................85 Table18.TheGuidelinesComparisonforDesignVehiclesonMulti‐laneRoundabouts(Caltrans,2007).....86 Table19.StatisticsbyAreaType..................................................................................................................................................88 Table20.StatisticsbyCrashType................................................................................................................................................89 Table21.StatisticsbyCrashSeverityandAreaType...........................................................................................................91 Table22.StatisticsofSingle‐vehicleandMulti‐vehicleCrashesbyAreaType.........................................................91 Table23.StatisticsofSingle‐vehicleandMulti‐vehicleCrashesbyCrashSeverity.................................................92 Table24.Driveway‐relatedCrashesThatOccurredwithinUpstreamandDownstreamDrivewayCornerClearances................................................................................................................................................................................................93 Table25.StatisticsofRoundaboutswithThreeandFourLegs...................................................................................102 Table26.StatisticsofRoundaboutswithFiveandSixLegs..........................................................................................102 Table27.PedestrianCrashStatisticsbyMedianType.....................................................................................................105 Table28.BicycleCrashStatisticsbyLocationandCrashSeverity.............................................................................106 Table29.InputandOutputforRoundaboutComponentsinHCS2010....................................................................114 Table30.RecommendedSelectionofAnalysisToolforDifferentApplicationsRegardingRoundaboutsandAccessManagement..........................................................................................................................................................................118 TableA.1.KeyFeaturesofaModernRoundabout..............................................................................................................149 TableA.2.DimensionsofRoundabouts...................................................................................................................................149 TableB.3.StateWebsitesandGuidanceonRoundaboutsandAccessManagement...........................................151 TableB.4.RoundaboutGuidelinesinDrivewayorHighwayManuals.......................................................................153 TableB.5.SpecificManualsonRoundaboutGuidance......................................................................................................154 TableB.6.StateGuidanceonAccessManagementManuals...........................................................................................156 TableB.7.OtherDocumentsRelatedtoAccessManagement........................................................................................160 TableC.8.SpacingRequirements...............................................................................................................................................161 TableC.9.AccessManagementElementsontheStates(GluckandLorenz,2010,page48)............................162 TableC.10.AccessManagementTechniquesappliedbytheStateDOTs(GluckandLorenz,2010,pages49‐50).............................................................................................................................................................................................................163

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ListofAbbreviationsAADT AverageAnnualDailyTrafficAASHTO AmericanAssociationofStateHighwayandTransportationOfficialsADA AmericanswithDisabilitiesActANOVA AnalysisofVarianceARCADY AssessmentofRoundaboutCapacityandDelayAWSC All‐WayStopControlledAzDOT ArizonaDepartmentofTransportationCMF CrashModificationFactorsCORSIM CorridorSimulationCS ConflictingSpeedDCEE DepartmentofCivilandEnvironmentalEngineeringDOT DepartmentofTransportation(general;appliestoanystateorstatescollectively)ESSIE EngineeringSchoolofSustainableInfrastructureandtheEnvironmentFDOT FloridaDepartmentofTransportationFHWA FederalHighwayAdministrationFIU FloridaInternationalUniversityft. FeetFTA FederalTransitAdministrationFWSC Four‐wayStopControlledGIS GeographicInformationSystemsHCM HighwayCapacityManualHCS HighwayCapacitySoftwareHSM HighwaySafetyManualICD InscribedCircleDiameterINDOT IndianaDepartmentofTransportationIowaDOT IowaDepartmentofTransportationISD IntersectionsightdistanceITE InstituteofTransportationEngineerskm/h KilometersperhourKSU KansasStateUniversityKYTCKentuckyTransportationCabinetLOS LevelofServiceLOSPLAN LevelofServicePlanningMEV MillionEnteringVehiclesMDOT MichiganDepartmentofTransportationmi. MilesMNDOT MinnesotaDepartmentofTransportationmph MilesperhourMPO MetropolitanPlanningOrganizationNCHRP NationalCooperativeHighwayResearchProgramNHDOT NewHampshireDepartmentofTransportation ODOT OregonDepartmentofTransportationPDO PropertyDamageOnly

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PennDOT PennsylvaniaDepartmentofTransportationPHB PedestrianHybridBeaconRCI RoadwayCharacteristicsInventoryRTM Regression‐to–the‐meanSPF SafetyperformancefunctionsSSD StoppingsightdistanceTRB TransportationResearchBoardTWSC Two‐waystopcontrolledUF UniversityofFloridaURP DepartmentofUrbanandRegionalPlanningVISSIM VerkehrinStädten–SimulationsModelWisDOT WisconsinDepartmentofTransportationWSDOT WashingtonStateDepartmentofTransportation

Chapter1Introduction

Roundabouts and Access Management Page 1

ChapterOne:Introduction1.1BackgroundTransportationengineersandplannersareincreasinglyinterestedinusingroundaboutstoaddressaccessandsafetyconcernsinthetransportationsystem.Severalstateshavestronglyencouragedtheuseofroundaboutsbecausetheymaycostlesstoinstallthansignalizedintersections,mayhaveagreatersafetypotentialbyreducingthenumberofconflictpoints,anddependinguponthecontext,loweroperationsandmaintenancecosts(TRB,2010a).Roundaboutshave“seenunprecedentedgrowthacrosstheUnitedStates,fromjustahandfuladecadeagotomorethan2,000andcounting”(Schroederetal.,2011,p.1).ArecentFederalHighwayAdministration(FHWA)andFederalTransitAdministration(FTA)(Rueetal.,2010)publicationdescribesthebenefitsofroundaboutsfromalivabilityperspective:

…theymanagequeuingandcongestionatintersectionsbyallowingsimultaneousoperationofsomecrossingmovements;theybreakpotentialvehicle‐pedestrianconflictsintotwodiscretepointsbyuseoftheirsplitterislands;andtheyslowtrafficmovingthroughtheintersection,whileincreasingcapacity.Theyoffergreatersafety,eliminatingthepotentialforhead‐oncollisionsandfocusingdrivers’attentionontheroadwayahead,andtowardothercarsandpedestrians.Althoughtheyrequireconstructionadjustmentstoexistinggeometryoftheintersectingroadways,theyoffersafetyandoperationalbenefitsthatmakethemworkmoreeffectivelythantrafficsignalsbymostmeasures(Rueetal.,2010,p.6).

Althoughroundaboutsareinuseinmanycontexts,existingresearchdoesnotprovidedetailedguidanceonhowtoevaluatetheuseofroundaboutsasaformofaccessmanagementoraspartofalargerroadwaynetwork.Roundabouts,AnInformationalGuidesuggeststheadvantageofroundaboutsasamethodto“facilitateU‐turnsthatcansubstituteformoredifficultmid‐blockleftturns,especiallywherethereisnoleftturnlane”(Rodegerdtsetal.,2010,p.29).ExamplesfromothercommunitiessuggestthatacorridorusingmultipleroundaboutscanaccommodateaseriesofU‐turnsandleft‐turnlanesandreducedelayinthecorridor.However,Roundabouts,AnInformationalGuidealsosuggeststhatroundabouts“mayreducethenumberofavailablegapsformid‐blocksignalizedintersectionsanddriveways”(Rodegerdtsetal.,2010,p.29)andthusreducethecapacityoftheseaccesspoints.Attheveryleast,thetrafficalongacorridorchangeswiththeintroductionofroundabouts;thetrafficmaybemoreuniformlydistributedwithalargernumberofsmallergapsratherthanfewerlargeones.Thechallengesofusingroundaboutsalongacorridoraredescribedingreaterdetailinthefollowing:

Itiscommonpracticetocoordinatetrafficsignalsonarterialroadstominimizestopsandtraveltimedelayforthroughtrafficonthemajorroad.Aroundaboutwithonlyyieldcontrolcannotbeactivelymanagedtoprovideprioritytomajorstreetmovementsinthesameway.Asaresult,thecoordinatedplatoonsoftrafficthatimprovetheefficiencyoftrafficsignalscanbedisruptedbyroundabouts,thusreducingtheefficiencyofdownstreamintersections.Roundaboutscannotbemanagedusingacentralizedtrafficmanagementsystemtofacilitatespecialevents,diverttrafficflows,andsoonunlesssignalsattheroundaboutorinthevicinityareusedforsuchapurpose(TRB,2010a,pp.2‐6).

However,thebenefitsofaroundaboutmayvaryfordifferentusers.Priorresearchshowsgenerallyconsistentresultsaboutcrashratesbuttheperceptionsofthesafetyofroundaboutsvariesamongdiverseusers.Researchisalsoneededontheoperationalaspectsofroundabouts,especiallyasitrelatestoallroadwayusers;priorityforonetypeofusermaycausedelaysforothertypesofusers.Accessmanagementmayalsorequireestablishingpriorityforspecificmovementsatornearroundaboutsthataffecttheiroperations.



AccessmanagementbenefitshavebeendocumentedinvariousNationalCooperativeHighwayResearchProgram(NCHRP)reports,bothforsignalizedandunsignalizedintersections;roundaboutsaregenerallyincludedasunsignalizedintersections.Themostrecentdocumentonaccessmanagement—NCHRPReport548(TRB,2003)—statesthataccessmanagementhasanumberofpositivebenefits:improvedsafety,reductionindelay,increasedenvironmentalfriendlinessintermsoffuelconsumptionandemissions,improvedaccesstoproperties,integrationoflanduseandtransportation,andtheprovisionofappropriatefunctionforhighwayswithreducedcut‐throughtraffic.Tomaximizeroundaboutbenefitsandtoachievethemainpurposesofroundaboututilization,theintegrationofroundaboutandaccessmanagementisrequired.Insummary,roundaboutshavethepotentialtoincreasesafetyandreducedelaybycontrollingaccessandmorereadilyaccommodatingU‐turnandleft‐turnmovements.However,lessisknownabouthowtoevaluateroundaboutscomparedtootherformsofaccessmanagementandintersectioncontrolwithrespecttotraveldelay,safety,andothercommunityperformancemeasures.Additionally,manyofthemicro‐scaledetailsaboutaccessmanagementnearroundaboutsandalongcorridors,likethelocationofdrivewaysandtheplacementanduseofmedians,arenotwelldefinedintheliteratureandarepotentiallymoreflexiblewithroundaboutsthanconventionalintersectiondesigns.NCHRPProject03‐65:ApplyingRoundaboutsintheUnitedStates,hasresultedintwomajornationalresearchreportsontheuseofroundabouts:NCHRPReport572:RoundaboutsintheUnitedStates(Rodegerdtsetal.,2007)andNCHRPWeb‐OnlyDocument94:AppendicestoNCHRPReport572;RoundaboutsintheUnitedStates(Rodegerdtsetal.,2006).ThesereportsincludeaninventoryofroundaboutsintheUnitedStatesatthetimeofthepublicationofthedocument,andadatabaseofgeometric,operational,andsafetyinformation.TheresultsofthisresearchhavebeenincorporatedintotheHighwaySafetyManual(HSM)(TRB,2010b)andtheHighwayCapacityManual(HCM)(TRB,2000).Roundabouts,AnInformationalGuidewasfirstpublishedin2000andupdatedthroughNCHRPProject03‐65AtoproduceNCHRPReport672:Roundabouts:AnInformationalGuide,SecondEdition(Rodegerdtsetal.,2010).Thisguidecontainssectionsonroundaboutconsiderations,planning,operationalanalysis,safety,geometricdesign,applicationoftrafficcontroldevices,illumination,landscaping,andconstructionandmaintenance.TheFHWAOfficeofSafetyhasaRoundaboutOutreachandEducationToolbox(FHWA,2013)thatincludesavarietyofcasestudiesfromdifferentstates,focusingonhowtoeducatethepublictoproperlyandsafelyuseroundabouts.1.2ResearchQuestionsThemainquestionaddressedinthisresearchis,“WhataspectsofaccessmanagementshouldbeincorporatedintothestateguidancedocumentsinthestateofFloridaonroundaboutswithrespecttotheirusageneardrivewaysandalongcorridors?”Thismainquestionisaddressedthroughanexplorationofthefollowingsub‐questions:

(a) Whatcanwelearnfromexistingliteratureabouttheoperation,capacity,safetyandaccessassociatedwithroundabouts?

(b) Howhaveroundaboutsbeenincorporatedintonationalandstateguidancedocumentsonaccessmanagement?

(c) Whatguidanceonoperation,capacity,safety,accessmanagement,anddesignhasbeenincorporatedintonationalandstateguidancedocumentsonroundabouts?

(d) Howhaveaccessmanagement,safety,operations,andcapacityconsiderationsassociatedwithroundaboutsbeenincorporatedintocurrentpractices?

(e) HasaccessmanagementinfluencedthesafetyofexistingroundaboutsinFlorida?(f) HastheFloridastategovernmentincludedroundaboutsintheiraccessmanagementanddriveway

managementdocuments?Howdoesaccessmanagementfigureintoroundaboutdesigndocuments?



(g) WhatdoStateofFloridadocumentsrecommendinregardtoaccessmanagementinthevicinityofroundabouts?

1.3ObjectiveofResearchThemainobjectiveof this research is toprovideguidance for transportationprofessionals inFloridaonhow access management around roundabouts should be managed. This objective is achieved throughseveraltasksstartingfromareviewofpreviousliteratureandotherstateguidelinesonroundaboutstoseehow these guidelines are applied throughout the United States. The goal is to understand how accessmanagement,capacity,andsafetyareaddressed; toevaluate thegaps inknowledgeregardingtheuseofroundabouts; to analyze crashes near roundabouts; to conduct an operational analysis of a sample ofroundabouts; and to assess the primary software tools for analyses of roundabouts. The researchrecommends changes to guidance documents in Florida, including the access management resources,MedianHandbook,andDrivewayInformationGuide.ResearchersattheUniversityofFlorida(UF)andFloridaInternationalUniversity(FIU)accomplishedthesegoals through a series of tasks including: review of literature and other research on roundabouts,evaluationofthegapsinknowledgeregardingtheuseofroundabouts,safetyanalysisofcrasheswithin500feetofall283roundaboutsinthestateofFlorida,operationalanalysisofasampleofthirteenroundabouts,review of software used to evaluate roundabouts, and development of recommendations for additionalresearch and specific guidance on the deployment of roundabouts. The Department of Civil andEnvironment Engineering at FIU completed the safety analysis,made recommendations regarding theiranalysis and reviewed the entire document. Faculty from the UF’s Transportation Institute in theEngineering School of Sustainable Infrastructure and Environment (ESSIE) directed the operationalanalysisandthereviewofsoftwareforanalysisofroundabouts.ResearchersintheDepartmentofUrbanandRegionalPlanning(DURP)atUFcompletedtheremainingtasks,includingthereviewofliterature,theevaluationofthegapsinknowledgeabouttheuseofroundabouts,thereviewofnationalandstatepolicydocumentsandthepreparationofthefinalreport.1.4ScopeofWorkandSupportingTasks Task1:LiteratureandBackgroundReviewLiteraturerelatedtothesafety,accessmanagement,multimodaltransportation(especiallyforbicyclistsandpedestrians),androadwaycapacityassociatedwiththeuseofroundaboutswasreviewed.Theresearchteamalsoexaminedroundaboutpoliciesandguidelinesfromotherstates.Documentationonthedesignandplacementofroundaboutsissummarizedinaseparatespreadsheet.Inataskthatwascompletedaftertheliteraturereview,nationalandstatepoliciesandguidelinesonroundaboutsafety,access,andcapacitywerereviewedanddocumented;theresultsofthispolicyscanareincorporatedintoaseparatechapterthatreportstheresultsofthisresearch.Task2:EvaluationofGapsinKnowledgeRegardingUseofRoundaboutsInthistask,theresearchteamcriticallyevaluatedavailableliteratureandstatepoliciesandidentifiedthegapsinknowledgeregardingtheuseofroundabouts,especiallyastheyapplytosafety,access,operations,androadwaycapacity.Theliteratureisusedtodefineatypologyofcontextsinwhichroundaboutsareimplemented.Thistypologyexpandsthedefinitionofcontextfromurban,suburban,andrural,toincludeotherfactorsthataffectsafety,access,androadwaycapacitysuchasaccesspoints(threevs.four);numberoflanes(onevs.two);isolatedroundaboutsvs.roundaboutsinacorridor;roundaboutsinaresidentialneighborhoodvs.roundaboutsincommercialdistrictsornearinterchanges;andotherfactorsasdefinedintheliterature.Thistaskassessedanddocumentedthestateoftheartinaccessmanagementinthevicinity



ofroundabouts(forexample,policiesandassessmentregardingthepositioningofdrivewaysclosetoaroundabout,oronalinkconnectingtworoundabouts).Thisevaluationalsodevelopedatypologyofcontextsinwhichroundaboutsareimplemented,andthiswasusedintheselectionofroundaboutsfordetailedinvestigationintheoperationalanalysis.Task3:SafetyAnalysisTheresearchteamusedthetypologydevelopedintheprevioussteptounderstandsafetyissuesassociatedwithroundabouts.Thesafetyanalysisdetermineswhethercrashcausationisrelatedtothepresenceofspecificdrivewayandmediancharacteristicsandprovidesrecommendationsforaccessdesignfeatureswithrespecttosafety.

Subtask3‐1:IdentifyPotentialStudyLocationsInthistask,FDOT’sRoadwayCharacteristicsInventory(RCI)wasusedtoidentifythelocationofallroundaboutsinthestate.TheRCIincludesroadwaydataforallstateroadsandafewoff‐systemroads.The2011RCIhas219locationsclassifiedas“roundabouts.”Anadditional64roundaboutswerefoundusingGoogleMapforatotalof283roundaboutsthroughoutthestate.UsingsatelliteimagesalreadycapturedfromGoogleMapsforeachoftheselocationsandGoogle’sStreetView,allpotentialstudylocationswereidentifiedforuseinthesafetyandoperationalanalysis.Forthesafetyanalysis,allroundaboutlocationswereusedtounderstandthegeneraltrendsincrashesnearroundaboutsandalargersamplewasusedforspecificanalysis.Asdescribedbelow,theoperationalanalysisconsidersseveralfactorsusedtoselectroundaboutsfordetailedstudy:thepresenceofsignificantmainlineanddrivewaytraffic,andtheproximityoftheroundaboutstodrivewaysand/ormediandesignfeatures,aswellascommercialormixedresidentialandcommerciallanduseareas.

Subtask3‐2:CreateConditionDiagrams,CollectFieldData,andEstimateDrivewayTrafficUsingacombinationofGoogleEarth,BingMaps,andGoogle’sStreetView,scaledconditiondiagramsofeachpotentiallocationidentifiedintheprevioussubtaskwereconstructedinMicroStation.Eachsitewasvisuallyinspectedtocollectinformationonthelandusesassociatedwithadjacentdriveways,aswellastoverifyexistinggeometricconditions.Theinformationcollectedincludeslandusetypes(e.g.,restaurants,gasstations,apartments,etc.),numberofunits,yearestablished,andwhereapplicable,numberofemployees,floorspace,numberofgaspumps,andotherrelatedcontextinformation.ThelanduseinformationwasthenusedtoestimatedrivewaytrafficusingtheInstituteofTrafficEngineers(ITE)TripGenerationManual.

Subtask3‐3:ReviewPoliceReportsandCompileCrashInformationHardcopiesofpolicereportsdocumentinguptofiveyearsofcrashesthatoccurredwithinthefunctionalarea(500feet)ofeachselectedroundaboutlocationweredownloadedfromageographicinformationsystem(GIS)currentlybeingdevelopedbyDr.IlirBejlerioftheUFDURP.Crashdatafrompolicereportswereextracted,includingcrashlocation,crashtype,crashseverity,vehicletype,driver’sage,lightingconditions,andothercontributingfactors.Additionally,theillustrativesketchanddescriptionofeachcrashwasrecorded.Sincetheconstructiondateofsomeofthelocationswasnotavailableandthegeometricconditionshavechangedovertime,policesketchesanddescriptionswereusedtofurtherverify,totheextentpossible,thatgeometricconditionsdidnotchangeoverthestudyperiod.Inthosecaseswherepolicereportsindicategeometricchanges,crashesthatoccurredbeforethechangeswereexcludedaswerecrashesnotdirectlyrelatedtotheroundabout.



Subtask3‐4:ConstructCollisionDiagramsandPerformSafetyAnalysis

Inthissubtask,crashinformationcompiledpreviouslywasusedtoconstructacollisiondiagramforeachstudylocation.Fromthesediagramsandtheassociatedcrashcharacteristics,crashpatternsastheyrelatetodrivewayandmediandesignfeatureswereidentified.Thesepatternswerefurtheranalyzedbasedonvehicletype,timeofday,lightingcondition,driverage,estimateddrivewaytrafficvolumes,andotherfactors,toidentifythecausesofover‐representedcrashes.Thestatisticswerealsostratifiedbycrashinjuryleveltodeterminetheseverityofthecrashes.Asampleofthequestionstheanalysisattemptedtoanswerincludes:

Dospecificdrivewayandmedianconditions(e.g.,proximityofdrivewayandmedianopeningtoroundabouts;directvs.indirectdrivewayconnection)contributetocertaintypesofcrashesinvolvingaccesstraffic?

Doesthepresenceofdrivewaysandmedianopeningsresultinmoreseverecrashes? Issafetyaffectedbycertaingeometriccharacteristicsofroundaboutswhencombinedwithspecific

drivewayandmedianopenings? Arethereasignificantnumberofcrashesinvolvingpedestriansnearroundabouts? Howhavepedestriancrossingsbeenaffectedbydrivewaylocations?

Basedontheresultsoftheanalysisdonehere,specificrecommendationsondrivewayandmediandesignfeaturesnearoratroundaboutlocationsaremade.Thistaskdocumentstheresultsofthesafetyanalysisandprovidesinformationabouthowsafetyconsiderationsaffectthecontextinwhichroundaboutsareplaced.Task4:AnalysisofSelectedFieldRoundaboutSitesInthistask,theresearchteamidentifiedseveralroundaboutsitesinFloridafordirectstudyandanalysis.Trafficoperationspotentiallyaffectedbydrivewaysandmediansapproachingandexitingtheroundabout,werestudied.TheresultsofthisanalysiswerecomparedwiththefindingsofTask1.Duringpeakoperatingtimes,betweentwoandfourhoursofvideodatawerecollectedateachroundaboutlocation.Task5:DevelopmentofRecommendationsforIncorporatingAccessManagementintoFloridaPracticeInthistask,theresearchteamtooktheresultsoftheliteraturereviewandanalysisofgapsinknowledgeandmaderecommendationsonhowtoincorporateaccessmanagementintoroundaboutdesigninFlorida.Thisincludesrecommendationsforadditionalresearch,andchangestoFDOT’sAccessManagementTools,MedianHandbookandDrivewayInformationGuide.Task6:AssessmentofPrimaryFDOT‐UtilizedSoftwareToolsforRoundaboutEvaluationAsappropriate,FDOTregularlyimplementsvariousanalysismethodologiesintocustomsoftwareproducts,andrecommendstheuseofcertainsoftwareproductsthatimplementFDOT‐approvedanalysismethodologies.Forexample,FDOTsupportsthedevelopmentofcustomsoftwarefortrafficoperationsandlevelofserviceanalysis(i.e.,LOSPLAN).LOSPLANisgenerallyintendedforplanningandpreliminaryengineeringanalyses,andemploysdeterministic,macroscopicanalysistechniquesconsistentwiththeHCM.Fortrafficanalysisscenariosinvolvingahighlevelofcomplexity,themicroscopic,stochasticsimulationprogramCORSIM(corridorsimulation)isgenerallyrecommended.AsFDOThasdecidedtoadopttheHSMmethodologyforsafetyanalysis,thecurrentcapabilityofHSMinanalyzingandpredictingthesafetyperformanceofroundaboutswasassessed,andpotentialapplicationgapswereidentifiedandrecommendedforHSMimplementation.



Inthistask,anassessmentoftheprimaryFDOT‐utilizedsoftwaretoolswasmade.Thisassessmentfocusedonthecurrentsuitabilityofthesesoftwaretoolstoassistwiththeevaluationoftheissuespreviouslyidentified.Wheretheymaybedeficient,recommendationsweremadeonhowtoimprovethesetoolstomakethemmoreeffectivefortheevaluationofroundaboutsandaccessmanagement.Task7:PreparationofDraftandFinalReports

ThedraftfinalreportwaspreparedandsubmittedforreviewbytheFDOTSystemsPlanningOfficeandtheResearchCenterstaff.Thedraftfinalreportwasreviewedforgrammar,clarity,organization,andreadabilitypriortosubmissiontoFDOTfortechnicalapproval.Towardtheendofthistask,ameetingwasorganizedwiththestaffoftheSystemsPlanningOfficetodiscussthefindingsandrecommendations,andthedraftfinalreport.Thereportwasalsodistributedtootherresearchersandpractitionerswithexpertiseinthedesignanddeploymentofroundabouts.Theresearchteampreparedarevisedfinalreportbasedonthecommentsreceivedbythepanel,andsubmittedittoFDOTandthetechnicalreviewandprojectimplementationpanel.1.5OrganizationoftheReportThisreportisorganizedintoeightchaptersbeginningwiththeIntroduction.ChapterTwocontainstheliteraturereviewthatintroducestheconceptsofroundaboutandaccessmanagement;examinesthepriorstudiesandreportsonthesimilartopics;andidentifiesgapsinknowledge.ChapterThreedescribesthemethodologiesutilizedinthisresearch.ChapterFourdescribesthereviewofnationalandstateguidanceregardingroundabouts,accessmanagementandthecombinationofroundaboutsandaccessmanagement.ChapterFivereportsthefindingfromthesafetyanalysis.ChapterSixdiscussesthefindingsfromtheoperationalanalysisandexploresthesoftwarethatisavailableforuseinanalysisofroundaboutsandaccessmanagement.ChapterSevendiscussesaccessmanagementintheroundabouts,incorporatingacomparisonoftheinformationfoundintheliteraturereviewandinthestateguidance,includingwhathasbeenimplementedintheStateofFlorida,tomakerecommendationsforfurtherresearchandguidancetoimproveFloridaguidancedocumentsonroundaboutsandaccessmanagement.InChapterEight,theresearchissummarized.

Chapter2LiteratureReview


ChapterTwo:LiteratureReview2.1OverviewThisreviewofpriorresearchhastwoparts.First,theavailableliteratureregardingtheuseofroundabouts,especiallyastheyapplytosafety,roadwaycapacity,andaccessissummarized.Next,asummaryofthestate‐of‐the‐artinroundaboutpracticeisdeveloped,includinganevaluationofgapsinknowledgeregardingresearchaboutroundaboutsandaccessmanagement.Theliteraturereviewisorganizedaroundscholarlyandpractice‐basedresearchonroundabouts,roundaboutcapacity,roundaboutsafety,andaccessmanagement.Ofparticularinterestinthissectionarearticlesthataddressaccessmanagementandmultimodaltransportation,especiallyforbicyclistsandpedestrians.Theliteraturedefinesatypologyofcontextsinwhichroundaboutsareevaluated,including:thetype—urban,suburban,andrural;thenumberofaccesspoints—threeandfour;thenumberoflanes—oneandmulti‐lane;thenumberofroundabouts—oneandcorridor;andlocationoftheroundabouts—residential,commercial,mixed‐use,andinterchanges.2.2RoundaboutsPriorliteraturedifferentiatesmodernroundaboutsfromtrafficcirclesorcircularintersections.Thetrafficcircle,introducedin1905,canbeseenasaprecursortoroundaboutsintheUnitedStates(Jacquemart,1998).IntheRoundaboutsGuide,2ndedition,Rodegerdtsetal.(2010)definedthreetypesofcircularintersections:rotaries,neighborhoodtrafficcircles,androundabouts.TheUnitedKingdominitiatedthemodernroundaboutin1966withthe“give‐way”ruleforenteringtraffic,byallowingcirculatingtraffictocontinuedrivinginroundaboutsratherthanyieldingtoenteringvehicles.ThefirstmodernroundaboutsintheUnitedStateswereconstructedin1990,andwerebasedontheprofessionaldesignexperienceofothercountries,particularlyAustraliaandtheUnitedKingdom.Thedifferencebetweenroundaboutsandothercircularintersectionsisthe“give‐way”rulethatprioritizestrafficcirculatingintheroundaboutorthesmallerneighborhoodtrafficcircles(Rodegerdtsetal.,2010).2.2.1ModernRoundabouts.Thisprojectfocusesonthemodernroundabouts;throughoutthedocumenttheterm“modernroundabouts”isusedinterchangeablywith“roundabouts”asdefinedhere.Roundaboutscanbedescribedas:

circularintersectionswithspecificdesignandtrafficcontrolfeatures.Thesefeaturesincludeyieldcontrolofallenteringtraffic,channelizedapproaches,andappropriategeometriccurvaturetoensurethattravelspeedsonthecirculatoryroadwayaretypicallylessthan50km/h(30mph).Thus,roundaboutsareasubsetofawiderangeofcircularintersectionforms(Rodegerdtsetal.,2010,p.5).

Withthisdefinition,threekeyfeaturesofroundaboutsaredistinguishedfromthoseofotherformsoftrafficcircles,suchasrotaries,mini‐trafficcircles,andothernon‐modernroundabouts.Thesefeaturesaretheyield‐at‐entryrule,channelizedapproaches,andgeometriccurvaturedesignstoslowdownthespeed.AtyandHosni(2001)addedtwoothercharacteristicsofmodernroundaboutsthatareimportanttothisresearch:prohibitingbothparkingonthecirculatingroadway,andpedestrianactivitiesonthecentralisland.Figure1andFigure2showthefeaturesofatypicalroundaboutandthedifferencesandsimilaritiesbetweensingleandmulti‐laneroundabouts.



2.2.2GeometricDesignGeometricelementsoftheroundaboutinclude:inscribedcirclediameter,entrywidth,circulatoryroadwaywidth,centralisland,entrycurves,exitcurves,pedestriancrossinglocationandtreatments,splitterisland,stoppingsightdistance(SSD),intersectionsightdistance(ISD),verticalconsiderations,andbicycleprovisions.2.2.2.1KeyFeaturesandDimensions.AccordingtothesecondeditionofRoundabouts,AnInformationalGuide(Rodegerdtsetal.,2010),thekeyfeaturesofroundaboutsincludethecentralisland,splitterisland,circulatoryroadway,apron,yieldline,accessiblepedestriancrossings,bicycletreatments,andlandscapingbuffer.Furthermore,theroundaboutdimensionsaddresstheinscribedcirclediameter,circulatoryroadwaywidth,approachwidth,departurewidth,entrywidth,exitwidth,entryradius,andexitradius.AdditionalexplanationsabouteachfeatureareincludedinAppendixA.

Figure1.GeometricDesignFeaturesofaTypicalModernRoundabout(FDOT,2007)



(a)single‐laneroundabout(FDOT,2007,p.2‐21)

(b)Multi‐laneroundabout(FDOT,2007,p.2‐21)

Figure2.GeometricDesignFeaturesofaTypicalModernRoundabout:Single‐lane(a)andMultiple‐LaneRoundabouts(b)



Designspecificationsandguidelinesforeachindividualgeometrycomponentareprovidedinnationalandstateguides(e.g.,GluckandLorenz,2010;FDOT,2007;IowaDOT,2010;Maryland,2012;andWisDOT,2013).Thefirstelementsthatshouldbedefinedandoptimizedinthegeometricdesignofaroundaboutarethesize,position,alignment,andarrangementofapproachlegs.Then,otherdetailsofgeometrycanbedetermined.Eachtypeofroundabout(single,double,multi‐lane,rural,ormini)hasspecificdesignguidelines,soitisdifficulttostandardizethem.However,basedonNCHRP672,Roundabouts,AnInformationalGuide(Rodegerdtsetal.,2010,pp.6‐8),everyroundaboutdesignshouldmeetthefollowingsetofobjectives:

1. “Slowentryspeedsandconsistentspeedsthroughtheroundaboutbyusingdeflection;”2. “Theappropriatenumberoflanesandlaneassignmenttoachieveadequatecapacity,lanevolume

balance,andcontinuityoflanesthroughtheroundabout;”3. “Smoothchannelizationthatisintuitivetodriversandresultsinvehiclesnaturallyusingthe

intendedlanes;”4. “Adequateaccommodationforthedesignvehicles;”5. “Adesignthatmeetstheneedsofpedestriansandbicyclists;”and6. “Appropriatesightdistanceandvisibility”(Rodegerdtsetal.,2010,pp.6‐8).

2.2.2.2EntryandExitDesign.Sinceaccessmanagementfocusesonlandusesanddrivewaysadjacenttoaroundabout,thetwomostobviouslocationstoexamineaccessinrelationtogeometricdesignaretheentryandtheexit.Entrywidthshouldbedesignedtoaccommodatethedesignvehiclewhileensuringadequatedeflection(Layton,2012,44).Typically,theminimumwidthforasingle‐laneentranceonastatefacilityroundaboutis14ft.Whenacurbispresentonbothsides,andthesplitterislandislongerthan33ft.,theminimumwidthshouldbe17ft.(thecriteriaforpassingastalledvehicle).Deflectionisdefinedas:“thechangeintrajectoryofavehicleimposedbygeometricfeaturesoftheroadway”(Rodegerdtsetal.,2010,Glossary,p.3).Itisusuallydesignedfortheentrancetoaroundaboutandshouldsupportthedesignprinciplesofdeflectiontoslowdriversdown,althoughitcanbesignificantlyaffectedbythelocationandspacingofdrivewaysbeforetheroundabout.Deflectionisanimportantaspectofroundaboutdesign,bothforsafetyandcapacity.Aspectsofdeflectioninroundaboutsforcethedrivertoreachtheintendedcirculatingspeedrange(usuallybetween20‐30mph),andincreasethedriver’sawarenessoftrafficbeforeenteringtheroundabout,whileinit,andafterexitingtheroundabout.Deflectionisoftenachievedwiththeuseofreversecurvesontheentrancetoaroundabout.AccordingtotheOregonDOT,areversecurve“shouldhavethesameoraslightlylargerradiusthantheradiusofthecurvedpaththatavehiclewouldbeexpectedtotravelthrough.Thespeedofthecurveoftheapproachshouldbenomorethan10mphfasterthanthemaximumnegotiationspeedthroughtheroundabout”(Taekratok,1998,p.45).Toslowtrafficandindicatetheupcomingpresenceofaroundabout,splitterislandsorlanemarkingsareusedinconjunctionwithreversecurves.Ifdrivewaysorotheraccesspointsareplacedtooclosetoaroundabout,properlevelsofdeflectioncanbeinhibited,potentiallyaffectingtheoperationoftheroundaboutandmakingitlesssafeforusers.Toavoidthis,roundaboutsplitterislandsshouldextendbackfromtheroundaboutentryatalengthadequatetohinderdrivewayaccessmovementsthatcouldcausesafetyorqueuingconcerns.2.2.2.3SightDistance.AccordingtoTaekratok(1998,p.52),“visibilityisanimportantconcerninthedesignofroundabouts.”Severalaspectsofsightdistanceshouldbeevaluatedtodetermineadequatespacingdistanceandaccesstoaroundabout:SSD,decisionsightdistance,ISD,minimumaccessspacing,andrecommendedspacing.SSDsarecalculatedbasedonapproachspeedsandotherfactors,andcanbefoundintheHCM2010.Evaluationsaboutsightdistanceandconflictpointsaresignificantfactorsinrelationtothesafetyofaroundaboutandadjacentlanduses.SeeFigure3andFigure4,below.



Figure3.IntersectionSightDistance(FHWA,2006,p.18)

Figure4.StoppingSightDistance(FHWA,2006,p.19)

2.2.3ContextsofRoundabouts2.2.3.1Single‐LaneRoundabouts.Convertingcontrolledintersectionsintoaroundabout,especiallysingle‐laneroundabouts,hasreceivedalotofresearchattentionbecauseofthesafetyeffects.Asanexample,Flannery,Elefteriadou,KozaandMcFadden(1998)studiedthesafetyandoperational



performanceoffivesingle‐laneroundaboutsthatwereconvertedfromstop‐controlledintersections.Overall,thestudylocationsexperiencedareductionincrashfrequencies,crashrates,andcontroldelay.TheauthorscomparedcontroldelaymeasuredinthefieldwiththedelaypredictedbySIDRA,asoftwarepackagethatanalyzesat‐gradecontrolledintersectionsandroundabouts.Roundaboutdesignersshouldcarefullyconsiderthenumberoflanesplannedforinclusioninaroundaboutbeforeinitiatingitsdesign,construction,andimplementation.Studiesshowthatfewercrashesoccurinsingle‐laneroundaboutsthandouble‐laneroundabouts(Wang,OngandRakha,2013;Mahdalová,SeidlerandCihlářová,2010).However,two‐laneroundaboutswerefoundeffectiveregardlessofthedegreeofdemand.Also,anincreaseinthetotalnumberofcrashesoccursatthree‐laneroundaboutsthatwereconvertedfromsignalizedintersections(Mcintosh,RedingerandBagdade,2011).2.2.3.2UrbanandRuralRoundabouts.Designingforroundaboutsinurbanareascanbechallengingwhentheimpactsofdrivewayaccessandnearbyintersectionsaretakenintoaccount.Thesizeandgeometricdesignofaroundaboutishighlydependentuponthenatureofthearea(urbanvs.suburban),speedlimits,roadwaynature,ornumberoflanes,anditmaybecomplicatedbytheneedtoensureaccessforotherlandusesinneighboringurbanareas(IsaacsandBarrett,2003).Itiseasiertomakeanevaluationforaccessmanagementforruralareasforroundaboutsascomparedwithurbanareasbecausetherearefewerspatialconstraints.Thegreaterdistancesbetweentrafficintersectionsresultinlessinteractionwiththeroundaboutfromneighboringdriveways.However,becauseroadwaysinruralareastypicallyhavehigherspeedlimitsthanthoseinurbanareas,trafficsafetyissuesmustbeseriouslyconsideredregardingaccessandsafety.2.2.3.3UrbanRoundabouts.Increasedsafetyatroundaboutscomparedtocontrolledintersectionsisafunctionofreducedspeedandfewerpotentialconflictpoints(IsaacsandBarrett,2003).However,highercrashfrequencymaybecausedbyinadequatedesignstandardsandproblematicdriverbehavior(Sacchi,BassaniandPersaud,2011).Sacchietal.(2011)showedthatinadequategeometricdesign,particularlyanexcessiveradiusofdeflectionandalowangleofdeviationoftheenteringapproach,contributedto60%ofthecrashesintheItaliancitiesofNovaraandTrento.Anotherissueregardingthedesignandconstructionofurbanroundaboutsistheaccommodationofdifferenttypesofroadusers,especiallypeoplewithdisabilitiesandvisuallyimpairedpedestrians(IsaacsBarrett,2003).Whenitcomestoroundaboutsandpeoplewithdisabilities,theliteraturefocusesmoreonvisuallyimpairedpedestriansbecausethoseindividualshavedifficultyinidentifyingwhenandwheretocrossaroundaboutlegduetothelackofdetectable warnings.2.2.3.4RuralRoundabouts.Aconversiontoroundaboutusealongruraltwo‐laneroadwaysreducedcrashfrequencies,crashrates,injurycrashes,andanglecrashes(Isebrands,2009b;IsebrandsandHallmark,2012).Thetwostudiesdefinedruralareasas“completelyruralorlessthan2,500urbanpopulation,notadjacenttoametroarea.”Inthefirststudy,Isebrands(2009a)studied17roundabouts,themajorityofwhichwereconvertedfromtwo‐way‐stopcontrolled(TWSC)intersectionswithflashingyelloworredwarninglights.Thestudyfounda52%reductionintotalcrashes,a67%reductionincrashrate,an84%reductionininjurycrashfrequency,andan89%reductionininjurycrashrate.Especiallysignificantisthefactthatfatalcrasheswerereducedfrom11inthebefore‐periodtononeintheafter‐period.Inaddition,thefrequencyofanglecrasheswasalsoreducedby86%(Isebrands,2009b).Inanotherstudy,IsebrandsandHallmark(2012)evaluatedthesafetyeffectivenessofconverting19intersectionsthatwerelocatedonhigh‐speedruralroadwaysintoroundabouts.Specifically,therewasa62to67%reductionintotalcrashesandan85to87%reductionininjurycrashes.Moreover,anglecrashesweresignificantlyreducedby91%.2.2.3.5RoundaboutsWithinaCorridor.Roundaboutsinteractwithotherstreetsaspartoflargercorridors,oftenwithotherroundaboutsorothertrafficcontroldevicessuchassignalizedintersections.Streetsystemsshouldbedevelopedtocirculateanddistributetraffictomanageaccessto“landusesinthe



areawithaminimalimpactonthemainlineandcrossroad”(Layton,2012,p.3).Forspecialevents,whichmayexceedsuitabledesign‐hourconditionsfortheroundaboutandothertrafficdevicesinthecorridor,thedesignofaccessfacilitiestospecialeventlandusesshouldtakeintoaccountincreaseddelays,queues,safetyimpacts,andlargerthannormalspacingstandards(Layton,2012).ProjectNCHRP03‐100EvaluatingthePerformanceofCorridorswithRoundaboutswasrecentlycompletedonthistopic;thefinalreportthefinalreporthasbeenacceptedandwillbepublishedintheNCHRPseries(seeTRB,2014)2.2.4ComparingRoundaboutstoOtherTypesofIntersectionTrafficControlsThereviewofnationalandstateguidanceonroundaboutsandaccessmanagementsuggestthatoperationsof roundabouts are similar to unsignalized intersections. HCM 2010mentions that “[t]he operation ofroundaboutsissimilartothatoftwo‐waystop‐controlledintersections.Inroundabouts,however,enteringdriversscanonlyonestreamoftraffic—thecirculatingstream—foranacceptablegap.”(TRB,2010a,p.4‐14).Also,“roundaboutsdischargevehiclesmorerandomly,creatingsmall(butnotnecessarilyusable)gapsin traffic at downstream locations” (p. 8‐5). These gaps are different than signalized intersections, acharacteristicsharedwithall‐waystopcontrolled(AWSC)intersections.2.2.4.1Roundaboutsvs.Stop‐ControlledIntersections.Right‐anglecollisionsarethemostcommoncrashtypesatAWSCintersections.Roundaboutsareconsideredtobeunfavorableatlocationswheretrafficflowonapproachlegsisunbalanced,atlocationswherespaceislimited,andatlocationsnearpersistentbottlenecks(Vlahosetal.,2008).Whenroundaboutsareproperlylocated,theyprovidebetterperformance(i.e.,reduceddelayandincreasedcapacity)comparedtoAWSCintersectionswithsimilartrafficvolumeandright‐of‐waylimitations(Vlahosetal.,2008,pp.88).Inaddition,totalcrashfrequencies,totalcrashratesandinjurycrashratesmaybereducedafterstop‐controlledintersectionsareconvertedtoroundabouts(Flannery,2001).Thesestudieswereconductedasbefore‐and‐aftersafetyevaluationsusingvideo‐recordeddataforfourhoursduringthepeakperiodsateightsingle‐laneroundaboutswithaminimumoftwoyearsofdataaftertheroundaboutswerebuilt(Flannery,2001).2.2.4.2Roundaboutsvs.SignalizedIntersections.Manypriorstudiesagreethatconvertingsignalizedintersectionstoroundaboutsresultsinabettersafetyperformance(Saccomanno,Cunto,GuidoandVitale,2008;Mcintoshetal.,2011;JensenandApes,2013;Gross,Lyon,PersaudandSrinivasan,2013;Uddin,HeadrickandSullivan,2012;Wangetal.,2013;andDixonandZheng,2013).However,specificconditionssuchasgeometry,trafficvolumes,andapproachspeedarerelatedtosafetyperformance.First,theconflictinthesignalizedintersectionisaffectedbygeometryandvolume(Saccomannoetal.,2008).Inturn,fewerrear‐endcrashesoccuronroundaboutsthanonsignalizedintersections(Saccomannoetal.,2008).JensenandApes(2013)madeasimilarargumentwhentheyconcludedthatcentralislandsthataremorethantwom(6.6ft.)high,hadabettersafetyperformancecomparedtolowercentralislands.However,DixonandZheng(2013)foundthatthewidthofthecirculatinglaneandtheradiusoftheinscribedcirclewereinsignificantinthemodels.Mostlikely,thisconclusionisduetothesimilarityofgeometricfeaturesinthestudycomparisonofOregonroundabouts.Saccomannoetal.(2008)andGrossetal.(2013)makesimilararguments,andagreethatthesafetybenefitsofroundaboutconversiondeclineswithanincreaseintrafficvolumeintermsoftotalcrashes(Grossetal.,2013).Safetyimprovementswerealsodocumentedwhenintersectionswithhighapproachspeedswereconvertedtoroundabouts(JensenandApes,2013).Observationsshowasignificantsafetybenefitforinjurycrasheswithroundaboutconversions;evenincaseswhereoverallcrashfrequencyincreases(i.e.somemultilaneroundabouts),thereareconsistent,notabledecreasesinseverecrashes(Grossetal.,2013).2.3AccessManagementAccessmanagementisdefinedas“thesystematiccontrolofthelocation,spacing,design,andoperationofdriveways,medianopenings,interchanges,andstreetconnectionstoaroadway”(TRB,2003,pp.3).Much



ofaccessmanagementisachievedthroughpolicyandgovernance,unlikedesignstrategiesmandatedbynationalguidelinesforotheraspectsoftransportationdesignandplanning.Accessmanagementishighlycontext‐sensitive;however,theAccessManagementManualdoesofferguidanceandgeneralconsiderationsforuse.Thoughaccessmanagementcanoftenbethoughtofassimpleregulationofdrivewaysandaccessontoroadways,thetermencompassesasignificantlymorediverserangeofprinciples,particularlyinthecontextofroundaboutdesignandplanning.Accessmanagementrepresentsatoolboxofstrategiesthatmunicipalities,planners,andengineerscanemploytoprovidemobilitytousersoftheroadwaysystemwhilealsoensuringaccesstopropertiesinuse,surroundingandadjacenttotheroadway.Foraccessmanagement,“safety,capacity,continuity,andconnectivityoftheroadwaynetworkarekey”(WilliamsandLevinson,2008,p.26).Clearconnectionsexistbetweenaccessdesign,capacity,andsafety,sinceaccessmanagementhasseveralimplicationsonsomeaspectsofroadwaysystems(WilliamsandLevinson,2008).Accessmanagement,asappliedtotransportationplanningingeneral,enablesaccesstolanduseswhileprovidingsignificantbenefitsto“motorists,bicyclists,pedestrians,transitriders,businesspeople,governmentagencies,andcommunities”(Roseetal.,2005,p.4).AccordingtoFrawleyandEisele(2005,p.3),accessmanagementhasthreegoals:toimprovesafetyandmobility,toprovidereasonableaccesstodevelopments,andtopromotelocalgovernmentpartnerships.Itcanalsobedefinedas“asetoftoolsusedtobalancetheneedsofmobilityonaroadwaywiththeneedsofaccesstoadjacentlanduses”(FrawleyandEisele,2005,p.2).AccordingtotheTRBAccessManagementCommittee,thetenkeyprinciplesofaccessmanagementare:

Provideaspecializedroadwaysystem Limitdirectaccesstomajorroadways Promoteintersectionhierarchy Locatesignalstofavorthrough‐movements Preservethefunctionalareaofintersectionsandinterchanges Limitthenumberofconflictpoints Separateconflictareas Removeturningvehiclesfromthroughtrafficlanes Usenon‐traversablemedianstomanageleft‐turnmovements Provideasupportingstreetandcirculationsystem

Accessmanagement,inthecontextofroundabouts,seekstodefinehowroundaboutsrelatetoadjacentlanduses,particularlythesupportingstreetandcirculationsystem,drivewaysandotheraccesspointstotheroadway,andenteringandexitingtheroundabout,aswellasmovementwithinit.SinceboththeuseofroundaboutsandthestudyofaccessmanagementarerelativelynewintheUnitedStatesatboththenationalandstatelevels,littleliteratureexistsregardingtheapplicationofaccessmanagementtoroundaboutdesignandplanning.2.3.1AccessManagementElementsEventhoughgeometricdesignelementsdonotregulateaccessmanagementdirectly,theygreatlyinfluencetheoperationofandaccesstotheroundaboutforusersandneighboringlandusesandplayasignificantroleinthespacingofdrivewaysandnearbyintersections.AsseeninFigure8,thedistancebetweendrivewaysaffectsthenumberofconflictpointsforpotentialvehiclecollisions.2.3.2SpacingStandardsandRoadwayClassificationsAccordingtotheAccessManagementGuidebook,NCHRPReport548(Roseetal.,2005,p.39),higherfunctionroadscommonlyhavefeweraccessopportunities.Similarly,localstreetsmaximizeaccesstoresidenceswhilesupportinglessthroughtraffic.However,abasicprincipletodeterminetheaccesslevelis



theproposedfunctionoftheroadways.TheAccessManagementGuidebookalsoshowsthat,astheproportionofthroughtrafficincreases,accessdecreases.Forexample,freewayshaveverylimitedcontrolledaccesswhilelocalstreetsprovidefullaccess.

Figure5.AccessandRoadClassification

TheAccessManagementGuidebook(Roseetal.,2005)proposesroadwayclassificationdefinitionsbasedoncharacteristics(Roseetal.,2005,p.49)suchasfunctionalclassification,traveldistanceofmotorists(e.g.,shortvs.longtrips),natureofthetravel(e.g.,throughvs.local),travelspeeds,landuse,locationoftheroadwayfacility(e.g.,urbanvs.rural),andphysicalcharacteristicsoftheroadway(e.g.,dividedvs.undivided).Inadditiontothesecharacteristics,theplanninganddesignelementsincludedintheaccessmanagementforeachroadwayclassificationarethefollowing:

Permittedandprohibitedaccesslocations; Drivewaydesignandspacing; Cornerclearance; Medianopeningdesignandspacing; Signallocation,spacing,andcoordination; Turn‐lanelocationanddesign; Auxiliary‐lanelocationanddesign;and Service/frontageroadlocationanddesign.

Inaddition,accordingtoDemosthenes(2007),roadwaydesignandtrafficoperationsintersectwithaccessmanagementandlandusedesign(seeFigure6).



Figure6.RelationshipbetweenAccessManagement,RoadwayDesign,TrafficOperationsandLandUse(Roseetal.,2005)

2.3.3AccessManagementMechanismsandIntersectionControlsAccordingtotheAccessManagementGuidebook,NCHRPReport548(Roseetal.,2005),themostreliablemethodsofaccessmanagementforgeneralhighwaymanagementintersectioncontrolsinclude:acquisitionofaccessrights;accessmanagementregulations;policies,directives,andguidelines;landdevelopmentregulations;geometricdesign;anddevelopmentreview/impactassessments(Roseetal.,2005,pp.8‐10).2.3.3.1AcquisitionofAccessRights.Localmunicipalitiescanacquirerightstopropertiesthatadjoinorareadjacenttoroundaboutstomaintainaccess.Ifthelocationofaroundaboutwouldblockaccesstoaneighboringproperty,sometimesthemunicipalitymaypurchasethepropertyandprovidefinancingtohelptheownerrelocatetoanalternatelocationwithadequateaccess(Roseetal.,2005).Inothercircumstances,however,drivewaysmayremainincloseproximitytoaroundabout,oreveninthemiddleofaroundabout,asseeninsomeroundaboutsinWisconsin(M.Johnson,Personalcommunication,February7,2013).2.3.3.2AccessManagementRegulations.Mostmunicipalitiesincludetransportationdesignpolicyregulationsaspartofaccessmanagementstandards.Theseareoftenbaseduponnationalandstatestandards,althoughtheycanvoluntarilygointofurtherdetailtoaddressissuesofcontextoroflocaltransportationpatterns.Thesearecommonfortraditionalstop‐controlledandsignalizedintersections,andarebecomingincreasinglypopulartoaddressroundaboutdesignandplanningissueswithinalocality.Agencieswhichfrequentlyuseroundaboutsgenerallyhaveinternalconsensusaboutthetypesofcontextsinwhichroundaboutsareappropriateandwheretomanageaccess(P.Demosthenes,Personalcommunications,March14,2013).2.3.3.3Policies,Directives,andGuidelines.Comprehensiveplanningandzoningdesignationsshouldrecognizetheroleofcontextsensitivetransportationfacilities,whichmayincludeincorporatingminimumspacingstandards,andaddressanyuniquecharacteristicsofthespecificroundaboutinpolicies.Therelevantlocalgovernmentoragencyshoulddesignatetheappropriatelandusecontrolsandcomprehensiveplanningguidelines,becausenationalpolicyalwaysincludesexemptions(P.Demosthenes,Personalcommunications,March14,2013).



Comprehensiveplansshouldincluderegulationsanddesignguidelinesforaccessmanagementoftrafficcontroldevices.Whenconsideringfutureexpansionofcertaincorridors,alternativetrafficcontroldesignssuchasinterchangesorroundaboutsrequiremoreplanninganddesignconsiderationsthanacorridorthatofonlysignalizedintersections(Layton,2012).Thephysicalexpansionofintersectionsshouldbeexaminedincomprehensiveplans,specificallythenumberoftravellanes,auxiliarylanes,high‐occupancyvehiclelanes,transitways,modificationstoexistinginterchanges,andplannednewinterchanges.Eachoftheseprojectedchangesrequiresadditionalright‐of‐wayconsiderationsforthemunicipality.Inthesecases,Layton(2012,p.4)arguesthatthemunicipalityshouldinsurepropertyforexpansion,notingthatprotectivebuyingmaybemorecost‐effectivethanpurchasingthepropertyinthefuture.2.3.3.4GeometricDesign.Geometricdesignforroundaboutsshouldacknowledgetheneedforroundabouttraffictobedistributedtoavoidatrafficqueueintheroundabout,andensureaccesstoneighboringproperties.Inlocaltrafficdesignregulationsandpolicies,designguidelinesshouldbeincludedthatensurebothmobilityandaccesstoneighboringproperties(Schroeder,2011). Evaluationofthelanduseandgeographiccontextsoftheroundaboutisakey.Theoptimumspacingbetweenurbanroundaboutswithinadowntownurbancorecoulddifferfromthatofruralroundaboutsoncountyroads.Minimumspacingandgeometricdesignoftheroundaboutmustallowforweavingdistanceandaqueuelengthsetatacomfortableoperatingcondition(Layton,2012,p.5).2.3.3.5SightDistance.Themostpertinentguidelinesforsightdistancerelatingtoaccessmanagementarethoseoftheexternalapproachexitandthecirculatingroadway.Theexternalapproachsightdistanceisthedistanceadriverhastotravelfromthemomentofapproachingtheyieldlineoftheroundaboutentrancetoanyentrancepath.AccordingtoTaekratok(1998),“adriverwhoisapproachingtheyieldlineshouldhaveaclearlineofsighttoapproachingtrafficenteringtheroundaboutfromanapproachimmediatelytotheleft,foratleastadistancerepresentingthetraveltimeequaltothecriticalgap.Aminimumdistanceis70m(230ft.)”(1998,p.38).Driversenteringtheroadwayfromadrivewayoraccesspointshouldbeabletoseevehiclesupstreamontheroadwaytoensureasafeturn.Forinstance,thespacingandlocationofthedrivewayclosesttotheroundaboutshouldenableadriverexitingthatdrivewaytobeabletoturnontotheroadwaywithaclearviewofvehiclesapproachingandexitingtheroundabout.Thisappliestodrivewayaccesspointsforboththeenteringandexitingsidesoftheroundabout.Whilethepreviousexampletakesintoaccountlocationandsightdistancewithnoqueue,theeffectsofqueuesmustalsobeconsideredwithregardtosightdistance.Anexaminationofstoppingdistanceandqueuelengthshouldbeconsideredwhendeterminingminimumspacingbetweenadrivewayaccesspointandanintersection(Layton,2012).2.3.3.6DevelopmentReview/ImpactAssessments.Oneofthemostimportantwaysaccessmanagementcanbecontrolledwithinamunicipalityisinthedevelopmentreviewprocess.Evenifaroundaboutdesignclaimstofollowaccessmanagementprinciples,itistheresponsibilityofthemunicipalorregionaltrafficengineertoreviewthedesignandpoliciestoensurethedesigndoesachievethestatedgoalsandensuresaccesstoneighboringlanduses.2.3.3.7ImplementingMechanisms.Agenciesneedtoworktogetheracrosstheboardtoimplementaccessmanagementmechanisms.Theseentitiesincludestateagencies,statelegislatures,metropolitanplanningorganizations(MPOs),regionalplanningagencies,localplanningagencies,andlocalelectedofficials.Roseetal.(2005)identifyaccessmanagementimplementingmechanisms,classifiedbyauthority,



agencypolicy,accessmanagement,advocacy,managementaccountability,projectprogramming,andprojectdevelopment,andtheimplementingagency.Collectively,theirworkreinforcestheimportanceofthewiderangeofstateandlocalpoliciesandguidanceonaccesscontrol,landuseandsiteplanreview,drivewayandotherpermittingstandardsandprocesses,fundingforcorridorpreservation,designstandards,andarea‐wideandcorridoraccessmanagementplans.2.3.4ImpactofRoundaboutsonAccessManagementForthemostpart,thesmallbodyofexistingliteratureonaccessmanagementandroundaboutssuggeststheymayhaveperformancecharacteristicssuperiortosignalizedintersections.Roundaboutsenhancetheachievementofaccessmanagementgoalsinmultipleways:maintainingthecontinuityoftheroadway’sflow,improvingsafety,reducingcongestion,balancingmobilityandaccess,andbyextendingthelifeofinfrastructure(personalcommunications,MarkJohnson,February7,2013).Thedifferingoperationalcharacteristicsofroundaboutscanprovideversatilityandflexibilityintheapplicationofaccessmanagementtechniques:lessqueuing,slowerspeedsatentryandexit,consistentspeeds,reducedspeeddifferentials,geometricflexibility,anddriveway/intersectionspacingflexibility.Insomecases,roundaboutsmayalsoprovideincreasedcapacityatintersections,reducingtheneedtoexpandentireroadways.Physicalgeometriccharacteristicsofroundaboutscanalsoalteraccessmanagementpatterns,changingthesideofstreetanddrivewayaccessspacingneedsandrequirements.Often,drivewayaccessandspacingcanbeeasiertoplanbecauseoflessqueuing,slowerspeeds,andeasierdecisionmaking.Inresponsetothescarcityofliteratureonthetopic,KansasStateUniversity(KSU)studiedtheimpactofroundaboutinstallationonbusinessaccess.Russell,LandmanandGodavarthy(2012)concludethattheoperationalcharacteristicsofroundaboutsallowbusinessestobelocatedmuchclosertointersectionsthandotraditional,signalizedintersections(Russelletal.,2012,p.16).Intraditional,signalizedintersections,queuedtrafficatredlightsforthroughtrafficandturnmaneuverscanblockaccesstobusinesses.Withproperaccessmanagementofroundaboutandflowingtraffic,“roundaboutscanbedesignedwithacommercialorbusinessentrancedirectlyofftheroundabout”(Russelletal.,2012,p.16).JohnsonandIsebrands(2008),reachthesameconclusionsasRusselletal.(2012),thattheoperationalcharacteristicsofroundaboutsprovide“lowdelayandimprovedsafety,providesexcellentmobility,ingress,andegressthroughequalopportunityforlefts,throughmovements,andU‐turns”(JohnsonandIsebrands,[2008]ascitedinRusselletal.,2012,p.16).2.3.4.1BusinessAccess.Inseveralcases,roundaboutshaveincreasedaccesstobusinesses.Inthepreviouslymentionedstudy,Russelletal.,(2012)foundthat76.9%ofbusinessesinTopeka,Kansasclassifiedtheimpactoftheroundaboutsasfair,good,orverygood(Russelletal.,2012,p.vi).InadditiontointerviewswithTopekabusinessowners,simulationstudiesoftheroundaboutinstallationdepictedsignificantreductionsindelayandqueuingforalltrafficmovements.Intheirstudy,Russelletal.,(2012)referredtoseveralbusinessownerswhosaidtheyowedtheirsuccesstotheconstructionoftheroundabout.Priortotheroundabout,heavytrafficandqueueshadbeendiscouragingpeoplefrommakingleftturnsinandoutofbusinesses.However,aftertheroundaboutwasinstalled,trafficdelaywasreducedanddriverswereabletomakeleftturnsmoreeasilyandaccesstheadjacentbusinessesmorefrequently(Russelletal.,2012,p.7).InGolden,Colorado,theintroductionofaseriesofroundaboutsprovedmoreefficientinmanagingtrafficflowandcreatedacorridorthatslowedtrafficandallowedpedestrianstoaccessmanybusinessesalongthecorridor(Ariniello,2004).MarkLenters,presidentofOurstonRoundaboutEngineering,foundroundaboutshadapositiveinfluenceonbusinessaccessinanumberoflocations,including(Lenters,n.d.):LinvilleRoadinBrownCounty,Wisconsin;SouthGoldenRoadinGolden,Colorado;LeeRoadinBrighton,Michigan;numerousintersectioninCarmel,Indiana;VailInterchangesinVail,Colorado;RockyMountainAvenueinLoveland,Colorado;andAvonRoad;Avon,Colorado.



However,roundaboutconstruction,likeallintersectionconstruction,isnotoriousforinhibitingaccesstoadjacentpropertiesandbusinessesduringthatperiod.Decreasedaccessduringthistimecancontributetonegativeconnotationsofroundabouts,eventhoughaccesswillreturntonormalorevenimproveonceconstructioniscompleted.2.3.4.2AccessPoints.Severalstudiesfindthatroundaboutsaresuccessfulwhenthe“reorganization”ofaccesspointsispartoftheroundaboutdesignandengineeringprocess.TheaforementionedcasestudyfromGolden,Coloradoinvolvedacorridorthatwasdescribedasbeingan“unpleasanttravelcorridor”withwideroads,poorsafetyconditions,acenterturnlane,and“numerousunorganizedaccesspoints”(Russelletal.,2012,p.9).Inevaluatingdifferentoptions,thecityfavoredtheroundaboutselectionbecauseit“wouldprovidebetteraccessoptionsandbetterpedestrianaccess”thantraditionaltrafficsignals(Russelletal.,2012,p.10).Aftertheconstructionoffourroundaboutsinplaceofsignalizedintersectionsandaftermakingsignificantstreetscapeimprovements,thecorridorwascitedasa“vibrantcommunitycorridor,”with“improvedbusinessaccess,”includingbetterpedestrianaccesstobusinesses,improvedsafety,anda6%increaseinretailsalestaxrevenue(Russelletal.,2012,p.10).Adescriptionofthecorridoranditscharacteristicsispresentedbelow:

SouthGoldenRoadisatypicalsuburbanstripcommercialcorridor.Theinstallationoffourroundaboutswithinthishalf‐milelongarterialhasresultedinslowerspeeds,butlowertraveltimesandlessdelayatbusinessaccesspoints.…[S]alestaxrevenueshaveincreased60%sinceinstallationoftheroundabouts,and75,000squarefeetofretail/officespacehasbeenbuilt.InGolden,Colorado,businesseshavesaid,“Yes,roundaboutsaregoodforbusiness.”(Ariniello,2004inRusselletal.,2012,p.12).

2.4OperationalEffectsofRoundaboutsIngeneral,operationalaspectsofroundaboutscanbeassessedintermsofcapacityandthelevelofservice(LOS),whichcombinesseveralmeasuresofeffectivenesssuchasdelayandqueuelength.Thefollowingdesignaspectshaveanimpactontheoperationsofroundabouts:geometricdesignofroundabouts;trafficflowanddriverbehavior;placementofdrivewaysnearroundabouts;andseriesofroundabouts.2.4.1EffectofTrafficFlowandDriverBehaviorThecapacityofaroundaboutentrydecreasesastheconflictingflowincreases(Rodegerdtsetal.,2010).Incapacitymodelspecifications,thecapacityofaroundaboutdecreasesfromthemaximumentryflowrateperhourwiththeincreaseofthevehicleconflictrate.Additionally,avarietyofconditionsexistinreal‐worldsituationsthatmightaffecttheaccuracyofagivenmodelingtechnique.Rodegerdtsetal.(2010)summarizetheseconditionsasfollows:

Effectofexitingvehicles.Exitingflowattheimmediatelyupstreamlegcanaffectadriver’sdecisiononwhetherornottoentertheroundabout.

Changesineffectivepriority.Whentheenteringflowandcirculatingflowvolumesarebothhigh,acirculatingvehiclemightadjustsitsheadwaytoallowentering,andagap‐acceptancemodelmaynotgivereliableresults.

Capacityconstraint.Thismayoccurwhenanapproachoperatesovercapacity.Duringthiscondition,theactualcirculatingflowislessthanthedemandresultingfromtheover‐saturatedapproach.Thereductioninactualcirculatingflowmaythereforedecreasethecapacityoftheotheraffectedentries.

Origin‐destinationpatterns.Thiscouldcauseanunbalancedflowataroundaboutwithcertainapproachesoperatingovercapacity.



2.4.2EffectofGeometryGeometriccharacteristicsgreatlyaffecttheoperationofroundabouts.Roundaboutsarenormallysaferiftheyaredesignedtoforcevehiclestoreducetheirspeedwhenenteringthecirculatoryroadway.Ontheotherhand,lowspeedsdecreaseroundaboutcapacity.Therefore,geometricdesignshouldbebalancedbetweensafetyandoperationalrequirements(Rodegerdtsetal.,2010).Generally,theoperationalperformanceofaroundaboutisdeterminedbyitsgeometricdesign,alongwiththetrafficvolumeusingtheroundaboutatagiventime.Geometricelementsthatinfluenceoperationsincludeentrycurvesandwidth,circlediameter,circularroadwaywidth,exitcurves,centralandsplitterislands,stoppingandISD,bicycleprovisions,sidewalktreatments,parkingconsiderations,busstoplocations,andright‐turnbypasslanes(Rodegerdtsetal.,2010).Manyoftheaforementionedgeometricparametersdependonthedesignvehicleandtheaccommodationofheavyvehicles,bicyclesandpedestrians.However,allareessential,andsmallchangestoevenonecouldresultinsignificantchangestotheoverallroundaboutoperationperformance.Geometryalsodictatesthenumberoflanesthatarerequiredtofacilitatethetrafficdemandandaffectsdrivers’perceptionoftraveltime,theirenteringandcirculatingspeed,andthegapbetweenvehicles.2.4.3OperationalAnalysisofRoundaboutAccordingtotheHCM,thecapacityofafacilitycanbedefinedas“themaximumsustainablehourlyflowrateatwhichpersonsorvehiclesreasonablycanbeexpectedtotraverseapointorauniformsectionofalaneorroadwayduringagiventimeperiodunderprevailingroadway,trafficandcontrolconditions.”(TRB,2010a,p.4‐1).TheHCMdefinesspecificperformancemeasure(s)foreachhighwayfacilitytype.Controldelayisusedtodefinethelevelofservice(LOS)atalltypesofintersectionsincludingroundaboutsandsignalizedandunsignalized.Anotherperformancemeasureisgeometricdelay,i.e.,theadditionaldelaycausedbytheintersectiongeometry.Forroundabouts,thisadditionaldelayisexperiencedwhendriversslowdowntonegotiatetheroundabouts’curvature(TRB,2010a).Otherrelevantperformancemeasurementsincludedegreeofsaturationandqueuelength.Besidesroundaboutperformancemeasures,afewfeaturesarecommontothemodelingtechniquestocalculatecapacitythatisincorporatedintoallanalysistools(Rodegerdtsetal.,2010).Modernroundaboutsuseyieldcontrolatapproachlanesanddriversmustyieldtheright‐of‐waytocirculatingvehiclesandacceptgapsinthecirculatingtrafficstream.Therefore,theoperationalperformanceofaroundaboutisdirectlyinfluencedbytrafficpatternsandgapacceptancecharacteristics.Also,theoperationalperformanceofroundaboutsisinfluencedbytheirgeometricfeatures(Rodegerdtsetal.,2010,pp.4‐3to4‐4).Onewaytoconstructaroundaboutcapacitymodelisthroughempiricalmodeling,whichusesstatisticalmethodologytomodelcapacitybasedonobserveddata(Al‐MasaeidandFaddah,1997;PolusandShmueli,1997;Wei,GrenardandShah,2011).Typicallyaresearchprocessforcreatinganempiricalroundaboutcapacitymodelistouseregressiontofindtherelationshipbetweenvolumeperhourandthegeometriccharacteristicsofaroundabout.Mostoftheliteraturerelatedtoroundaboutcapacitymodelsconsistsofdescriptionsofanalyticalmethodsandtypesofmeasurement.Theanalyticalmodelisprimarilybasedondriverbehavior,measuredingapacceptance(Fisk,1991;Akçelik,ChungandBesley,1997;Al‐Masaeid,1999;FlanneryandDatta,1997;Polus,LazarandLivneh,2003;Hagring,Rouphail,andSorenson,2003).



2.4.3.1GapAcceptanceintheRoundaboutandCapacityModel.Akçeliketal.(1997)presentedamethodforestimatingthecapacityandperformanceofroundaboutentrylanes.Thismethodisbasedonmodelingthegapacceptanceprocessundertheadjustmentofthecharacteristicsoftheapproachflows.Theauthoralsopresentedacasestudythatisanapplicationofthemethod.Themodelinthispapercombinedtheconceptofoverflowqueueandsignalanalogytoanalyzethecapacityandperformancesothatitisagoodfitforheavyandunbalanceddemandcasesinreallife(Akçeliketal.,1997).2.4.3.2ComparisonbetweenDifferentModelsandApproachesforCapacityMeasurement.Roundaboutcapacitycanbemodeledbasedontwotypesofapproaches.Lane‐basedmodelsmeasureandpredictroundaboutcapacitylanebylane,andcanbeextremelyusefulinthecaseofmulti‐laneroundaboutswithdifferentlanecapacities.Incontrast,approach‐basedmodelscombinetheentrylanesasananalytical“lanegroup.”AstudybyHagringetal.(2003)showedthatalane‐basedmodelisbetterthantheapproach‐basedmodelincomparingobservedheadways.Theyfoundthecriticalgapsfortheleftandrightentrylanesweredifferentandtypicallylargerfortheleftlanes.However,forthecirculatinglanes,thecriticalgapswerefoundtobesimilar.Akçelik(2011)concludedthattheHCM2010modelisauniquelane‐basedmodelandifcalibratedwithdriverbehavior,couldbeaveryaccuratemodelforcapacityanalysis.Akçelik’sstudyalsoshowsthattheuseofVISSIMandSIDRAyieldedsimilarresultsforcontroldelayandqueuelength.However,otherstudiesshowthatVISSIMpredictedlargerdelayvaluesthanSIDRA(YinandQui,2011).2.4.4RoundaboutCapacityunderDifferentConditionsVariousresearchershavestudiedthecapacitymodelforroundaboutsunderdifferentcircumstances.Inthisresearch,thecontextusuallyaddressestheimportanceofthenumberoflanescirculatingandenteringtheroundabout,thepresenceofsliplanes,thespecificshapeofroundabouts(e.g.,turbo),andtheapproachingflowintotheroundabout.2.4.4.1UnconventionalRoundaboutCapacity.Roundaboutswithtwoormoreentrylanescanalsohavedifferentcapacity.Lindenmann(2006)concludedthatasmallroundaboutwithtwo‐laneentriesandasingle‐lanecirculatingroadwayhasacapacitymorethan20%greaterthanthosewithone‐laneentries.SisiopikuandOh(2001)determinedthatatwo‐laneroundaboutisthebestdesignforintersectionswithhighthroughandleft‐turningtraffic.Theirstudyalsoconcludedthatroundaboutscouldhaveahighercapacitythansignalizedintersections(SisiopikuandOh,2001).Anothertypeofconventionalroundaboutisaturboroundaboutwhichisatypeofmodernroundaboutwithspiralroadmarkings,designatedlanes,andraisedlanedividers.Thereforecapacityforturboroundaboutscanalsobedifferent.2.4.4.2RoundaboutswithUnbalancedFlow.Unbalancedtrafficoccurswhereoneapproachvolumedominatestheotherapproachvolume,orthereisasignificantdifferencebetweenapproachvolumes.Thecapacitymodelofroundaboutswithunbalancedflowconditionswasstudiedandresultsshowedthatthosewithunbalancedflowconditionsweresignificantlydifferentfromotherroundabouts(Akçelik,2004;SisiopikuandOh,2001;Valdez,CheuandDuran,2011).SisiopikuandOh(2001)foundthatfromanoperationalperspective,unbalancedtrafficpatternsinroundaboutscouldsometimescarryhighervolumesthantraditionalintersections.2.4.4.3RoundaboutCapacitywithSlipLanes.Asliplaneinaroundaboutfacilitatesright‐turningtraffictoreducedelayandincreasecapacityandsafety.Threetypesofsliplanesareincorporatedintoroundaboutdesigns:free‐flowsliplanes,yield‐controlsliplanesandstop‐controlsliplanes.Al‐Ghandour,etal.(2012)believedthatallsliplanetypescouldreduceaveragedelayinasingle‐laneroundaboutandthatafree‐flowstylesliplaneperformsthebest.Theresultsofthesestudiesshowedthattheaveragedelayisexponentiallyrelatedtosliplanevolumes.Allthreetypesofsliplaneshaveasignificantpositiveeffect



oncapacity,withthefree‐flowsliplanehavingthemostpositiveeffect,followedbyyieldandstop‐controlsliplanes.Howeverwhenpedestrianshavepriority,afree‐flowsliplanecanincreaseroundaboutdelaybyfivetimesifthepedestrianvolumeandright‐turnvolumearebothhigh(Al‐Ghandouretal.,2012).2.4.4.4Roundaboutsinseriesofsignalizedintersections.Thecapacityofroundaboutscanbedramaticallyaffectedbylocation,aswellasthetrafficprogressionbeforeandaftertheroundabout.Severalstudiesexaminetheimpactoncapacitythatroundaboutshaveonaseriesofsignalizedintersections.BaredandEdara(2005)foundthatifaroundaboutiswithinone‐quartermileofasignalizedintersection,itresultsindelayscomparabletoafullysignalizedarterial.Hallmark,Fitzsimmons,Isebrands,andGiese(2010)foundthattheuseofroundaboutsinasignalizedcorridordidnotappeartoadverselyaffecttrafficfloworoperations.2.4.5SummaryofRoundaboutOperationLiteratureReviewRodegerdtsetal.(2010)summarizedhowtoconductroundaboutoperationalanalysesasfollows:

Datacollectionandprocessing.Trafficdatacanbecollectedwithliverecordingsofturningmovementsinroundabouts,trafficflowinintersections,andorigin‐destinationpatterns.Fieldobservationisnecessaryformeasuringsomeoftheoperationalperformancemeasuressuchascontroldelay(Rodegerdtsetal.,2010).Dataprocessingincludesdeterminingroundaboutflowratesbyconvertingturn‐movementvolumestoroundaboutvolumesandadjustingforheavyvehicles.

Determinestudymethodsandtools.Avarietyofmethodologiesareavailableforstudyingroundaboutsdependinguponthestageinthedevelopmentoftheroundabout.Intheearlierstagesofanalysis,suchasplanning‐levelsizing,andpreliminarydesign,thepractitionerwillusedeterministicsoftwareortheHCM.Inlaterstages,suchastheanalysisoftheimpactoftheroundaboutonspecialusers,suchaspedestrians,oronthetransportationsystemandforcommunicatingtothepublic,simulationtoolsbecomemoreimportant.Thedecisiononwhichmethodtouseisbasedontherequiredoutputandtheavailabledata.Rodegerdtsetal.(2010)presentedatable(seeTable1)specifyingthemethodselectionstandard.



Table1.SelectionofAnalysisTool(Rodegerdtsetal.,2010)

2.5RoundaboutsandSafetySafetyisoneoftheprimaryreasonsfortheincreaseduseofroundaboutsintheUnitedStatesandaroundtheworld.Thevolumeofliteratureonroundaboutsafetyisquiteextensivecomparedwiththeavailableliteratureonroundaboutcapacityandaccessmanagement.NCHRPReport674CrossingSolutionsatRoundaboutsandChannelizedTurnLanesforPedestrianswithVisionDisabilitieshighlightsthecloserelationshipbetweensafetyandaccessibility,particularlyinthecaseofroundabouts(Schroederetal.,2011).AccordingtoSchroederetal.(2011),“afacilitycouldbeconsideredsafeifthecrashrateatthefacilityislow.”Consequently,crashrateisthemostfrequentlyusedmeasuretoestimatesafetyintrafficengineeringingeneral,andforroundaboutsaswell;however,theuseofthecrashcanbeachallengebecausethecrashrateisseldomalinearrelationship.Theliteraturethatexploressafetyasitpertainstomodernroundaboutsplacesemphasisondifferentareas:safetyeffectiveness,safetyofvehiclesandvulnerableusers(i.e.,bicyclistsandpedestrians),comparisonofthesafetyperformanceofroundaboutswithothercontrolledintersections,andotherfactorsrelatedtodriversafety.Crashratesbasedonbefore‐and‐afterorcross‐sectionalstudiesareoftenusedtoevaluatesafetyatroundabouts.Duetothelackofexposuredata,thesafetyofvulnerableroadusersisoftenestimatedusingdirectobservation.Despitedifferentviewsaboutsafetyandaccessibilityatroundabouts,mostoftheliteratureconfirmsthatmodernroundaboutshavesignificantsafetybenefitsforalltypesofroadusers.TheFHWASafetywebsiteonroundaboutshasconsiderableinformationregardingroundaboutsafety,includingseveralreportsandmanualsontheapplicationofbestsafetypracticesinroundaboutdesignandplanning.Themostcommonlyusedsafetyguidebooksinclude:



Roundabouts,AnInformationalGuide(Robinsonetal.,2000) PedestrianAccesstoModernRoundabouts:DesignandOperationalIssuesforPedestrianswhoare

Blind(USAB,2006) NCHRPReport672:Roundabouts:AnInformationalGuide,2ndEdition(Rodegerdtsetal.,2010). NCHRPReport572:RoundaboutsintheUnitedStates(Rodegerdtsetal.,2007). NCHRPReport674:CrossingSolutionsatRoundaboutsandChannelizedTurnLanesforPedestrians

withVisionDisabilities(Schroederetal.,2011)2.5.1OverallSafetyEffectsoftheRoundaboutsInthepastresearchershavestudiedthesafetyperformanceofroundaboutsandcomparedthefindingswithothertraffic‐controlledintersections,suchasstop‐controlledintersections,andsignalizedintersections.Mostresearchersusecross‐sectionalstudiesthatcomparetheroundaboutseitherwithpreviousmeansofintersectioncontrolorwiththosemeansoftrafficcontrolwithinanareaclosetotheroundabouts.Safetyperformancemeasuresorindicatorscommonlyusedarecrashfrequency,crashrate,crashseverity,andcrashtype(Isebrands,2009b).Specifically,differentlocationswithintheroundaboutmayaffectthesafetyperformanceofroundabout.AccordingtoArndtandTroutbeck(1998),crashescanbecategorizedassingle‐vehicleandmultiple‐vehiclecrashes.Formultiple‐vehiclecrashes,thefollowingcharacteristicsareincluded:wherethecrashoccurred;whetherthevehiclewasentering/circulatingtheroundabout;exiting/circulatingtheroundabout;whetheritwasitasideswipecrash;andotherlowfrequencytypesofcrashes.Thelocationsincludedepartureleg,exitpoint,approachingrearend,entering/circulatingcrash,entrypoint,andsideswipecrashes.Figure7illustratesthelocationsofthetypesofcrashesinroundabouts.

Figure7.CrashTypesonaTypicalRoundabout(ArndtandTroutbeck,1998,p.28‐3)

Previousstudiesfoundthemagnitudeofsafetyeffectsrangedfroma17to70%reductioninthenumberofcrashes.FlanneryandDatta(1996)foundanaverageofa60‐70%reductionincrashfrequencyforthe



safetyeffectof13roundaboutsinthreestates:Maryland,Florida,andNevada.Retting,Persaud,Garder,andLord(2001)foundthatachangeto24roundaboutinstallationsfrom20stop‐controlledintersectionsandfoursignalizedintersectionsledtoa38%reductionintotalcrashfrequencyanda76%reductionininjuryseverity.Similarly,Persaudetal.(2001)foundasafetyeffectforroundaboutsthatledtoa40%reductionintotalcrashfrequencyandan80%reductionininjuryseverity.Isebrands(2009b)foundthatroundaboutsreduceinjurycrashfrequencyandinjurycrashrateby84%and89%,respectively.She(Isebrands,2009b)alsofoundthatroundaboutsreducedtotalcrashfrequencyandtotalcrashrateby52%and67%,respectively.DeBrabander,Nuyts,andVereeck(2005)evaluatedthecrashfrequencyfor95roundaboutsand119comparableintersectionsinFlanders,Belgiumandfounda34%reductioninthenumberofinjurycrashes.Similarly,inanotherstudy,DeBrabanderandVereeck(2007)foundthatroundaboutsresultedina39%reductionininjurycrashes,a17%reductioninseriousinjurycrashes,anda38%reductioninminorinjurycrashes.Churchill,Stipdonk,andBijleveld(2010)concludedthatroundaboutsreducedthenumberoffatalandseriousinjurycrashesby76%and46%respectively.Elvik(2003)foundconversionfromanintersectiontoaroundaboutresultedina30‐50%reductioninthetotalcrashrate.Thefatalcrashratewasreducedby50‐70%.Despitethesegenerallypositiveresults,notallconversionofroundaboutssignificantlyreducesthenumberofcrashoccurrences.Forexample,Rodegerdts(2007)concludesthattheconversionfromfour‐waystopcontrolled(FWSC)intersectionstothemodernroundaboutsdonotappreciablyreducethetotalandinjurycrashrates.Thisstudyalsohighlightsdesignfeatures,suchasthenumberoflanes,whichwerefoundtoperformbetterthanmulti‐laneroundabouts,whicharemoresensitivetosuchcharacteristics.Theresultmayalsobedependentontheprevioustrafficcontroltype,priortoroundaboutconstruction,andthenumberofapproachlegs(Elvik,2003).Furthermore,placementrequirementsshouldbeconsideredbeforeroundaboutconversion.Forexample,roundaboutsareconsideredunfavorableforlocationswhentrafficflowonapproachlegsisunbalanced,atlocationswheregeometryislimited,andatlocationsnearapersistentbottleneck(Vlahosetal.,2008).

Incontrasttotheeffectsofroundaboutsonsingleormultipleautomobilecrashes,priorstudiesmakevariousargumentsregardingcrashesinvolvingvulnerableusers,i.e.pedestriansandbicyclists.First,theargumentisthatroundaboutinstallationsreducesafetyforvulnerableusers(DeBrabanderandVereeck,2007;Danielsetal.,2008).Intheirmeta‐analysisstudy,DeBrabanderandVereeck(2007)foundthatcrashesinvolvingvulnerableroadusersincreasedbyabout28%.Moreover,Danielsetal.(2008)concludedthatinbuilt‐upareas,crashesinvolvingbicyclistsincreasedby48%.Inbuilt‐upareas,bicycle‐vehiclecrashesatroundaboutsthatwereconvertedfromstop‐controlledandsignalizedintersectionsincreasedby55%and23%,respectively.Outsidebuilt‐upareas,thechangeinbicycle‐vehiclecrashesbeforeandafterroundaboutconstructionwasstatisticallyinsignificant.AstudyinSwedenreachedseveralconclusionsrelatedtocrashesinvolvingbicyclistsandpedestrians:(1)single‐laneroundaboutsaremuchsaferforbicyclistsandpedestriansthanformultilaneroundabouts;(2)forpedestrians,roundaboutsarenolesssafethanconventionalintersections;(3)issaferforbicyclisttobypassaroundaboutonabicyclecrossingthantotravelonacarriageway;and(4)fewercyclistcrashesoccurwhenthecentralislandisgreaterthan10m(33ft.)andwhenbicyclecrossingsareprovided(Rodegerdtset.al,2006).Otherresearcharguesthatnosignificantproblemswerefoundforpedestriansatroundabouts(HarkeyandCarter,2006).Thesedifferentresultsmaybecausedbydifferentareasofstudy,thenumberofvulnerableusers,andtypeofanalysis;attheveryleast,theyreinforcetheimportanceofconsideringthecontextoftheroundaboutintheanalysis. 2.5.2AspectsofSafetyPerformanceofRoundaboutsSeveraldesignaspects,suchasconflictpoints,roundaboutdesign,speed,geometry,sightdistance,andpavementmarkings,determinethesafetyperformanceofroundabouts.Theimportanceofeachoftheseaspectsisexploredbelow.



2.5.2.1ConflictPoints.Aconflictpointisdefinedasalocationwherethepathsoftwomotorvehicles,oravehicleandabicycleorapedestrianpath,diverge,merge,orcrosseachother(Rodegerdtsetal.,2010,p.5‐5).Thenumberofpotentialconflictpointscouldbeasurrogatemeasureofsafety;fewerconflictpointscouldresultinenhancedsafety.Roundaboutshavefewerconflictpointscomparedtoconventionalintersections,withtheresultingpotentialforimprovedsafety.Figure8showstheconflictpointsatatraditionalstop‐controlledorsignalizedintersectionandatasingle‐laneroundabout.Atraditionalstop‐controlledorsignalizedintersectionwithfourlegshas32conflictpoints,whilearoundaboutwithfourlegshasonlyeightconflictpoints(Bie,Lo,Wong,HungandLoo,2005;Rodegerdtsetal.,2010;Stoneetal.,2002).Byreducingthenumberofconflictpoints,roundaboutscanincreasesafetyatanintersection(Elvik,2003;HydenandVarhelyi,2000).

Figure8.VehicleConflictsandVehicle‐PedestrianConflictsatSignalizedIntersectionsandSingle‐LaneRoundabouts(Rodegerdtsetal.,2010,Exhibit5‐2,p.5‐7)

Theone‐waytrafficflowthroughroundaboutsgivesasenseofeasetodriverswhenobservingoncomingtraffic,andhasbeenshowntoimprovesafetybymakingdriversmorecautious(DanielsandWets,2005).

( ) V hi l



Certaincrashtypes,includingright‐turn,angle,andleft‐turncrashesareeliminatedasvehiclesmoveinonedirectionthroughtheroundabout.Further,crashesatroundaboutsareoftenlesssevere;mostcrashesresultinminorinjuriesorpropertydamageonly(Rodegerdtsetal.,2010).Adesirableroundaboutdesignestablishesahighpriorityonspeedreductionandspeedconsistency(Robinsonetal.,2000).Vehiclesmustbeabletonavigatetheroundaboutthroughaseriesofturningmovementsatlowerspeeds,usuallylessthan20mph.Geometricfeaturescanalsocontrolvehiclespeeds.Someofthesafetybenefitsforagoodroundaboutdesigninclude: Areductionincrashseverityforpedestriansandbicyclists; Moretimefordriversenteringtheroundabouttomakeproperdecisions,adjusttheirspeedandentera

gapincirculatingtraffic; Safermergesintocirculatingtraffic; Moretimefordriverstodetectandcorrecttheirmistakesorcompensateforthemistakesofothers; Makingintersectionssaferfornoviceusers;and Eliminatingleft‐turncrashes.Whenproperlydesigned,roundaboutsreducethespeedofvehiclesapproaching,circulating,andexitingtheroundabout.Lowertravelspeedsreducethespeeddifferentialsamongvehicles.Vehicleshavelowandhomogenousrelativespeedsinroundabouts,forcingtraffictoslowdownbecauseoflateraldisplacement(DanielsandWets,2005).Consequently,drivershavemoretimetoanticipateandreacttopotentialconflicts.Ingeneral,higherspeeddifferentialsyieldedhighercrashratesfortotalcrashesandentryrear‐endcrashtypes(Zirkel,Park,McFadden,AngelastroandMcCarthy,2013).Asaconsequence,speedstandardsontheroundaboutsarenecessary(Montella,Turner,Chiaradonna,andAldridge,2013).Studiesalsoshowuneventrafficflowisacontributingfactortospeedvariations(St‐Aubin,Saunier,Miranda‐Moreno,andIsmail,2013).ResearchatfiveroundaboutsinQuébec,Canadaalsoreportedthatlargeandinconsistentspeedvariationwasmainlyduetoregionaldifferencesindesignandroaduse(St‐Aubinetal.,2013).Insafetyperformancemodels,speedmayperformasasurrogatevariableindesigningroundabouts(Chen,PersaudandLyon,2011).Afteranalyzingcrashdataandapproachleveldatafor33approachesat14roundaboutsfromeightstates,theauthorsconcludedthatspeed‐basedmodelsperformedbetterthannon‐speedbasedmodels.Afterrelatingspeedtogeometricfeaturesusingcorrelationanalysisandcalibratingthemodel,theauthorsidentifiedtheinscribedcirclediameter(ICD),andentrywidthassignificantgeometricfeatures.Higherapproachspeedsresultinincreasedcrashratesatroundabouts(Mahdalová,etal.,2010)Furthermore,“relativespeedsamongadjacentgeometricelementsshouldbeminimizedforoptimumsafety”(ArndtandTroutbeck,1998,p.16).Vehiclespeedscouldbereducedby“reducingtheradiusoftheapproachcurve,minimizingtheentry,exit,andcirculatinglanewidth;betterpositioningoftheentryanddeparturelegs;andincreasingthecentralislanddiameter”(ArndtandTroutbeck,1998,p.13).Inthisstudy,otherrelevantconclusionsinclude:theidealdifferentialspeedbetweentheupstreamintersectionandtheroundaboutisabout20km/h;andlargerradiidecreasethefrequencyofsingle‐vehiclecrashes,butpotentiallyincreasemultiple‐vehiclecrashrate.Tokeepdriversfromcuttingintoanadjacentlane,thisstudysuggeststhattheapproachroadwayshiftlaterallyby7m.Theauthoralsosuggeststhatthe85thpercentilespeedsonalltheapproachlegsbelimitedtoabout60km/h.Thiscanhelpminimizerear‐endcrashes.Finally,theentering/circulatingvehiclecrashescouldbeminimizedbylimitingtherelativespeedbetweenvehiclesenteringandcirculatingintheroundabouttoabout35km/h.Thesizeoftheinscribedcirclediameter,theentry/exitradii,trafficflow,andgeometricallayoutinfluencesafetyatroundabouts(Mahdalová,etal.,2010).Speedlimitalsohasaneffectonsafety.Forexample,higherapproachspeedsresultedinrelativelyhighercrashrates,especiallyiftheapproachspeedwas



above70km/h.Furthermore,thecrashratewasfoundtoincreasewithanincreaseinthenumberofapproachlegs.Daniels,Brijs,Nuyts,andWets(2011)foundthatthree‐legroundaboutsperformedlesseffectivelythanfour‐legroundabouts.TheauthordevelopedPoissonandgamma‐modelstopredictcrashesusing148roundaboutsinFlanders,Belgium.Thestudyalsoconcludedthatroundaboutswithacyclepathhadfewercrashesthanthosewithotherbicyclefacilities,whilethosewithlargecentralislandshadmoresingle‐vehiclecrashes.2.5.2.3SightDistance.Indeterminingpropersightdistancesattheroundabouts,designersshouldconsidertheISD,upstreamapproachsightdistance,andcirculatingsightdistance.Whileaninadequatesightdistanceisconsideredunsafe,agreaterdistancemayincreasethepercentagesfortotalandrear‐endcrashfrequenciespossiblybecauselargersightdistancesencouragehigherspeeds(Angelastro,McFaddenandMehta,2012).Theauthorsdevelopedcrashpredictionmodelsasafunctionofaverageannualdailytraffic(AADT)andsightdistanceattributestopredicttotalandrear‐endentrycrashesperyearperroundaboutapproach.ThemodelsshowthatsightdistanceparameterscouldbetterexplainthevariationsofcrashfrequencieswhencomparedtobasemodelsthatuseAADTastheonlypredictor.Moreover,exceedingsightdistancethresholdsincreasedtheriskofcrashoccurrenceandyieldedgreaterspeeddifferentialsbetweentheapproachandtheentrytotheseroundabouts(Zirkeletal.,2013).2.5.2.4PavementMarkings.Severalstudiesexaminedtheimpactofdifferentpavementmarkingsonthesafetyoftheroundabouts(Bieetal.,2005;Fortuijn,2009).ThefirststudycomparedconventionalandAlberta‐typelanemarkingsinroundabouts(asshowninFigure9).Alberta‐typemarking,alsoknownasspiralmarkingsystem,isusedfortwoormorelaneroundaboutsandincludespavementmarkingstoindicatetodriversatwhichlanetheyneedtobetoexistfromtheroundabout.Asafetyanalysiswasperformedusingacell‐basedmodeltodeterminepotentialconflictswhentwoormorevehiclesareprojectedtocollideinthesamecellatthesametimeinterval.AlthoughAlberta‐typemarkingtendstocentralizetheconflictspotsandpotentiallyinfluencesafety,thisstudyfindsnostatisticallysignificantdifferenceinthesafetyofroundaboutswithconventionalandAlberta‐typemarkings.

Figure9.DifferentMarkingSystems(Bieetal.,2005)

Inthelaterstudy,Fortuijn(2009)reviewedraisedlanedividers,alsoknownasturbodividers,andevaluatedtheireffectivenessinminimizingsideswipecrashesattwo‐laneroundabouts.Fortuijin(2009)evaluatedthenewtypeofdesignatsevenroundaboutlocationsintheNetherlandsandfoundthatitreducedcrashesby72%.Theroundaboutswithturbodividersarecalledturboroundabouts.Turboroundaboutscanbedefinedasaspecifickindofspiralmarkingroundabout.2.5.2.5CrashTypes.Differenttypesofcrashoccurrencesdeterminetheemphasisofroundaboutgeometricdesign.Forexample,singlecrashesatroundaboutsmayoccurwhendriverslosecontroloftheirvehiclesandcollidewithapartoftheroundabout,orasaresultofweather‐relatedfactorsandroadconditions.Forinstance,wetroadconditionsresultinalowercoefficientoffrictionandcollisionswiththe



apronorcurbsofroundabouts.Also,visibilityisreducedatnightandduringfoggyconditions.Single‐vehiclecrashratesarefoundtobehigheratroundaboutswiththefollowinggeometry:highabsolutespeedsonaparticulargeometricelement,highdifferentialspeedsbetweenadjacentroadsandtheroundabouts,longcurves,andcurvesthatrequiredhighvaluesofsidefriction(ArndtandTroutbeck,1998).Thepredominanttypesofmultiple‐vehiclecrashesincluderear‐endcrashes,crashesinvolvingvehiclesentering/exiting/circulatingtheroundabout,andsideswipecrashes.Thesecrashesaremainlyduetohighdifferentialspeedsbetweenvehicles,orobstructiontodrivers’viewofothervehiclesortheroundabout(ArndtandTroutbeck,1998).Insingle‐laneroundabouts,safetycouldbeimprovedbyprovidingadequatevisibilityandsufficientright‐of‐wayforgooddeflectiononthecenterisland(Flannery,2001).Byobservingcrashstatisticsaftertheroundaboutconstructionofninesingle‐laneroundaboutsinMaryland,Nevada,andFlorida,theauthorfoundthat27.3%oftotalcrashesweresideswipes,24.2%wererear‐endcrasheswitharelativehighof45.5%oftotalcrashesduetoalossofcontrol.Thiscouldbeattributedtohighspeedsonentryapproachesandpossibledriverviolations.Specifically,safetycouldbeimprovedattheselocationsbyimprovingthegeometricdesignoftheapproaches.2.5.2.6Signing.Signageandclearinformationhavearoleinimprovingsafetyeffects.Lowsafetyeffectsintwo‐laneroundaboutsraisedstudyconcernsabouttheimpactofsignage(Inmanetal.,2006b).Thestudyshowsthatroundaboutuserseitherdonotuseordonotunderstandassociatedsignage.RichfieldandHourdos(2013)hadasimilarconcernaboutsafetyontwo‐laneroundaboutsandevaluatedtheimpactofchangesmadetostripingandsigningatatwo‐laneroundaboutinRichfield,Minnesotaondrivingbehavior.Thestudyfoundthatimproperturnsandfailuretoproperlyyieldwerethemaincausesofamajorityofcrashes.Changesinsignageandstripingresultedina55%reductioninimproperturnsanda59%reductionineventswheredriverschoseincorrectlanes.2.5.3SafetyforDifferentRoundaboutUsersandModesSafetyisalsorelatedtodifferenttypesofusers.Inthissection,literaturereviewforsafetyofvulnerableroadusers,pedestrians,bicyclists,andheavyvehiclesarediscussed.2.5.3.1VulnerableRoadUsers.Thesafetyperformanceofmodernroundaboutsforvulnerableroadusershaslongbeendebated.Althoughseveralstudieshavefoundnosignificantissues(HarkeyandCarter,2006;Schroederetal.,2006);vulnerableroadusers,particularlybicyclistsandvisually‐impairedpedestrians,couldencounterpotentiallyunsafesituationsatroundabouts.Researchresultsareextremelydependentonthelocationofthestudies.Forexample,studiesfromcountriesoutsidetheUnitedStates,particularlyBelgium(DeBrabanderandVereeck,2007)andDenmark(HelsandOrozova‐Bekkevold,2006;MøllerandHels,2008),concludethatthesafetyofbicyclistsandpedestriansworsenedafterroundaboutimplementation.Thiscouldbebecause,comparedwiththeUnitedStates,pedestrianandbicyclisttrafficissignificantlyhigherinthesecountries.Crashdataofvulnerableroadusersislimitedbecausefewercrashesarereported.Additionally,pedestriansandbicyclistsmaytendtoavoidroundabouts,resultinginlimitedexposure.Consequently,studiesconductedintheUnitedStatesonpedestrianandbicyclesafetyrelyprimarilyonobservational,ratherthanstatisticaltechniques.SafetystudiesintheUnitedStatestypicallyfindeithernosignificantissueswithroundaboutconversionsoranimprovementinsafetyforpedestriansandbicyclists(Stone,ChaeandPillalamarri,2002;HarkeyandCarter,2006;Schroederetal.,2006).Eventhoughdifferentargumentsexistonthesafetyeffectsofmodernroundabouts,amajorityoftheliteratureconcludesthattwo‐laneroundaboutsaremoredangerousforpedestriansandvisually‐impairedpedestriansthansingle‐laneroundabouts.Inman,DavisandSauerburger(2005)proposedadditionalcrossingtreatmentforvisually‐impairedpedestriansintwo‐laneroundabouts.Schroeder(2013)also



concludedthatadditionaltreatmentwasnecessary.However,Inmanetal.(2006a)foundthatsoundcuetreatmentsdonothelpandmayresultinnumerousfalsealarms.Unlikevehiclecrasheswhereroundaboutsresultedinfewerseriousinjuries,forvulnerableusers(i.e.,pedestrians,bicyclists,mopeddrivers,andmotorcyclists)thepercentagesgoup.Conversionfromasignalizedintersectiontoaroundaboutincreasedthenumberoffatalpedestrianandbicyclistscasualtiesperseriousinjuryratefrom0.03to0.17(DeBrabanderandVereeck,2007).Theirstudyfocusedonroundaboutintersectionswithapproachspeedsof50km/h(31mi/h).Conversionfromastop‐controlledintersectiontoaroundaboutresultedina14%reductionininjurycrashfrequency.Ontheotherhand,conversionfromasignalizedintersectiontoaroundaboutresultedina28%increaseininjurycrashfrequency.Similarly,conversionfromastop‐controlledintersectiontoaroundaboutincreasedthenumberoffatalcausalitiesperseriousinjuryratefrom0.12to0.19(DeBrabanderandVereeck,2007,p.588).Conversely,HarkeyandCarter(2006)havenotfoundsubstantialsafetyproblemsforpedestriansandbicyclists.Theauthorsuseddigitalvideoforobservationalanalysisatsevenroundabouts.Theyobservedthedigitalvideosandcodeddifferentreactionsfrompedestriansandbicyclistsas“normal,”“hesitant,”“retreat,”and“run.”Further,motorist‐yieldingbehaviorwascodedas“activeyield,”“passiveyield,”and“didnotyield.”Thestudyshowednosubstantialproblemsforpedestriansandbicyclists.Nonetheless,theresearchhighlightedtheneedforamorepedestrian‐friendlydesignofroundaboutsinexitlegsandtheneedtoprovideadditionaltreatmentsformulti‐laneroundabouts.2.5.3.1.1Bicyclists.Bicyclistsinroundaboutscanbetreatedaspedestriansorasdrivers;thisdistinctioninfluencesthenumberofconflictsexperiencedbycyclists.DanielsandWets(2005)addedthatthedetailsofroundaboutdesigninfluencethenumberofconflictpointsforbicyclists.Thenumberofconflictpointsincreasesifbicyclistsaretreatedasdriversduetothespeeddifferentialandthedifferenceinvisibilitybetweenbicyclistsandothermotorizedvehicles(Brown,1995;DanielsandWets,2005;Robinsonetal.,2000).Figure10showsfourtypesofalternativetreatmentsforbicyclistsatroundabouts:(1)mixedtrafficwithmotorizedtraffic,(2)adjacentbikelanes,(3)separatedbikelaneswithpriorityforbicyclistsatcrossings,and(4)separatedbikelaneswithoutpriorityforbicyclistsatcrossings.Alternative(3)wasfoundtobesaferthanAlternative(4)becausemotorizedvehiclesyieldtobicyclistswhenpriorityisgiventobicyclists(DanielsandWets,2005).Alternative(3)hadaslightlyhighernumberofseriousinjuriescomparedtoAlternative(4)(DanielsandWets,2005).Bothalternatives(i.e.,3and4)performedbetterthanAlternative(1)andAlternative(2)forinjurycrashes(DanielsandWets,2005).However,specificrecommendationswerenotmadeduetolackofsufficientevidence.



Figure10.(1)Mixedtraffic;(2)adjacentbikelanes;(3)separatedbikelaneswithpriorityforbicyclists;and(4)separatedbikelaneswithoutpriorityforbicyclists(DanielsandWets,2005,p.6‐8)

Stillonthesafetyperspectiveofbicyclists,roundaboutsinbuilt‐upareasperformedworsecomparedwiththoseoutsidebuilt‐upareasresultingina48%increaseinbicyclecrashfrequencyatroundaboutsconstructedinsideabuilt‐uparea.Noincreaseinbicyclecrasheswasfoundatroundaboutsconstructedoutsidebuilt‐upareas(Danielsetal.,2008).Furthermore,theauthorsestimateda15‐24%increaseinsevere‐injurybicyclecrashes.Despitethosefouralternatives,twootheralternativesthatwerenotdiscussedbytheauthorincludetreatingbicyclistsaspedestriansandprovidinggrade‐separatedcrossingsattunnelsandbridges.Incontrast,bicyclistsappearedtogainmorerespectfromdriversafterroundaboutconstructionasthepercentageofyieldingincreasedfrom13to77(HydenandVarhelyi,2000).Thisstudyconductedon‐siteobservationswiththeobjectiveofviewingtheinteractionsbetweenroadusersatjunctionsaftertheroundaboutconstruction.HydenandVarhelyi(2000)alsoperformedaconflictanalysisandfoundthatthefrequencyofbicycle‐vehicleconflictsdroppedfrom77to45,withtheexpectednumberofinjurycrashesperyeardownfrom4.2to1.7.Thebehaviorofviolenceinfluencedsafetyperformance.Forexample,usingobservationforallbicyclemovementsandanyobservedbicycle‐vehicleinteractionsonsingle‐laneroundaboutslocatedinMassachusetts,BerthaumeandKnodler(2013)foundthatwhenthenumberofbicyclesthatperformedunsafemaneuverswascomparedtothetotalnumberofbicyclesobservedtraversingtheroundabout,about3%oftotalbicyclemaneuverswerefoundtobeunsafe.Inaddition,bicycle‐vehiclecollisionsatroundaboutswerefoundtobemorefrequentwhenbicyclistsunderestimatedtheriskand/orhadlittleknowledgeoftherelevanttrafficrules(MøllerandHels,2008).Theperceivedlevelofriskataroundaboutwithoutabikefacilitywashigherthanthatforbicyclistsataroundaboutwithabikefacility.Additionally,theperceivedlevelofriskwasalsoinfluencedbyage,gender,involvementinanearcrash,trafficvolume,andwhetherthereisabikefacility.Apossiblecountermeasuretoincreasetheperceivedriskandtocorrectunsafepracticesistoimplementefficientsignageforbicyclists.Aftergeneratingamodelusingdata



collectedbetween1987and1993with1,385observationsandcomparingbicyclelanesinroundaboutswithandwithoutpedestriansignals,DabbourandEasa(2008)recommendusingpedestriansignalsatroundabouts.2.5.3.1.2Olderpopulation.Clearsignageinfluencessafetyforolderroadusers(i.e.,≥65years)usingaroundabout(Lord,Schalkwyk,ChryslerandStaplin,2007).ThestudywasconductedusingstructuredinterviewsandfocusgroupsinCollegeStation,TX,andTucson,AZ.Theparticipantsincluded14menand17women.Inthisstudy,designelementswerereviewed,includingadvancewarningsigns,lanecontrolsigns,directionalsigns,yieldtreatments,andexitsigntreatments.ALikert‐typescalewithsevenpointswasused.Researchersthenusedtheanalysisofvariance(ANOVA)tounderstandifthereweresignificantdifferencesbetweenthebasecondition,countermeasure#1,andcountermeasure#2.Table2providesadetaileddescriptionofthebaseconditionandtestedcountermeasures.Table2.DetailedCountermeasuresforDesignElements(Lordetal.,2007,p.429)

DesignElement BaseCondition Countermeasure#1 Countermeasure#2A.AdvanceWarningSigns

Theadvancewarningsigntemplate[W2‐6]wasusedaccordingtotheguidelinesproposedintheMUTCD(FHWA,2003).

TwochangesweremadecomparedtotheBaseCondition:(1)asolidblackcirclewasaddedinthemiddleofthesign,and(2)aplaquewiththetext"ROUNDABOUT"wasattachedbelowtheadvancewarningsign.

Aplaquewithanadvisoryspeedof30mphwasplacedbelowthewarningsignusedforcountermeasure#1(i.e.,thesignwiththesolidblackcircle).

B.RoundaboutLaneControlSigns

TheBaseConditionwasmodeledaftertheR3‐8seriesofadvanceinter‐sectionlanecontrolsigns(FHWA,2003).

Asolidblackcirclerepresentingthecentralislandwasaddedtotheleftlane'sroute,butnotfortherightlane'sroute.

Thetext"LEFTLANE"and"RIGHTLANE"underthecorrespondingrouteswereaddedtothesignusedfortheBaseCondition.

C.DirectionalSigns(one‐waysign)

TheBaseConditionshowsacentralislandwithoutanyguidesignsorspecialpavementmarkingtoguidetrafficcirculatinginsidetheroundabout,aspertheguidelinesproposedbytheMUTCD(FHWA,2003).

Aone‐waysign(templateR6‐1)wasplacedonthecentralisland,positionedtofacethecenterlineoftheapproachingroadwayata90ºangle.Inthisposition,driverswillseethesignastheyapproachtheroundabout.

Thesameone‐waysignwasplacedonthecentralisland,butdirectlyinfrontofthedriver'sentrypointatthegorearearatherthanfacingthecenterlineoftheapproachingroadway.Thisplacementputsthesignmoredirectlyinthedriver'slineofsightfromtheyieldline.

D.YieldTreatment

ThestandardR1‐2yieldsignwasprovidedonbothsidesoftheroadattheentranceoftheroundabout.ThisconditionrepresentsthestandardsetbySection2B.10oftheMUTCD(FHWA,2003).

AyieldlineconsistingofsolidwhiteIsoscelestriangleswasaddedtotheBaseCondition.

ThistreatmentincludedallofthecomponentsnotedforCountermeasure#1,butaddedaplaquereading"TOTRAFFICINCIRCLE"belowtheyieldsigns.

E.ExitTreatment TheBaseConditionconsistedofplacingastreetexitsign(basedontheD1series)priortoreachingtheexit;thesignwasplacedbetweentwointersectingstreetsfacinginwardtowardthetrafficinthecircle.

ThesamestreetexitsignfromtheBaseConditionwasused,butwasmovedontothesplitterislandoftheintendedstreetexit;thissignstillfacedinwardtowardthetrafficinthecircle.

Anarrowpointingtotheexitlegwasaddedonthestreetnamesignusedforcountermeasure#1.



Theresultsofthisstudyforeachdesignelementareasfollows.A“ROUNDABOUT”legendispreferredasanadvancewarningsignupstreamofaroundabout.Addingdirectionalsignsarefavored;however,theresultsforthisdesignfeaturewerenotstatisticallysignificant.Fortheyieldtreatmentelement,adding“TOTRAFFICINCIRCLE”undertheYIELDsignwasfoundtobestatisticallysignificant.Thearrowforexitsigntreatmentyieldedamorepositiveresponsefromparticipants.2.5.3.1.3Pedestrians.RoundaboutseliminateseveralpotentialconflictsforpedestriansasTable3shows.However,pedestrian‐vehicleconflicts,whentheyexist,involvehigh‐speed,right‐turning,andleft‐turningvehicles(DanielsandWets,2005).Theincreaseinpedestrian‐vehicleconflictshasbeenshownbyseveralstudies(Hyden,2000;Stone,ChaeandPillalamarri,2002).ThefirststudyexaminestheeffectofroundaboutinstallationatoneintersectioninRaleigh,NCbyconductingthreeanalyses:thepedestrian‐vehiclecrashhistorieswithandwithouttheproposedroundabout;astatisticalanalysisforpedestrian‐vehiclecrashesversusstreetandintersectioncharacteristics;andatrafficsimulation.TheresearchersusedParamicssoftwarebecauseitmodeledroundaboutsexplicitlyratherthanasone‐waystop‐controlledintersections.Thestudyconcludedthattheproposedroundaboutseemedpromisinginthatthereisa7%reductioninpedestrian‐vehiclecrashesintheroundaboutcomparedwiththoseonthestreetoratintersections.Inaddition,thesimulationshowedthattheproposedroundaboutwouldimprovepedestriansafetycomparedwithaFWSCintersection.Thisisduetofewerconflictpointsandlowerspeedsofvehicles.Thesecondstudyshowedthatthataftertheinstallationofroundabouts,theproportionofvehiclesyieldingtopedestriansincreasedfrom24%to51%,andthenumberofconflictswasreducedfrom19tofour.HydenandVarhelyi(2000)observedthenumberofpedestrian‐vehicleconflictsbeforeandafterinstallationofroundaboutsusingthe30‐hourobservationperiod.Additionally,theresultsalsoshowedthatroundaboutconstructionresultedinareductionintheexpectedfrequencyofinjurycrashesfrom0.6to0.1.Fordesign‐specificconcerns,Furtado(2004)foundthatroundaboutswithcentralislandsthathaveadiametergreaterthan10m.performbetterthanthosewithsmallerdiameters.Furthermore,theauthormadethefollowingrecommendations:(a)theminimumoffsetfromtheyieldlinetothecrosswalkshouldtobe7.5m.,(b)adetectablewarningsurfacedelineatingthetravellanefromtherefugeareashouldbeinstalled,and(c)signingandpavementmarkingtreatmentsforcrosswalkfacilitiesshouldbeprovided.Theythenpointouttheadvantagesanddisadvantagesofroundaboutsforpedestrians,asshowninTable3.Table3.AdvantagesandDisadvantagesofRoundaboutforPedestrians(Furtado,2004)

Advantages Disadvantages

Vehiclespeedisreducedascomparedtootherintersections

Pedestrianshavefewerconflictpointsthanatotherintersections

Splitterislandsandresultingpedestrianrefugeareasallowuserstofocusononedirectionoftrafficatatime

Crossingmovementcanbeaccomplishedwithlesswaittimethanatconventionalintersectionsthathavemultipleprotectedphases

Vehicletrafficisyieldcontrolled;therefore,trafficdoesnotnecessarilystopanditcouldcausepedestrianstohesitate

Maycauseanxietyinpedestrianswhoarenotconfidentaboutjudginggapsintraffic

Crossinglocationsandsetbacksfromtheyieldlineoftenresultinlongertraveldistancesforpedestrians

NotwidelyusedinNorthAmerica,providingsignificantchallengesforthevisuallyimpaired

Inevaluatingthesafetyofroundabouts,pedestrianswithvisualdisabilitiesrequirespecialconsiderationEventhoughissuesofvisually‐impairedpedestriansatroundaboutshavebeendiscussed,untilrecentlytherehadbeennoextensiveresearch.Tofillthisgap,Ashmead,etal.,(2005)conductedastudyto



comparesixnormal‐sightedpedestriansandsixcompletelyblindpedestriansastheycrossedatwo‐laneroundabout.Theyfoundthatvisually‐impairedpedestriansaremoresusceptibletodangerswhencrossingaroundabout.Also,visually‐impairedpedestrians’waittimewaslongerthanthatofsightedpedestrians.ThestudywassimulatedinNashville,TN.Participantswithnormalvisionwalkedaroundoncewithanexperimenterwhopointedoutthesamefeaturesthatweredescribedtothevisually‐impairedpedestrians.Theexperimenteronlyintervenedasasafetymeasure.Thestudyshowedthatthesightedparticipantsdidnotneedanyinterventionfromtheexperimenter.However,therewere10instanceswherethevisually‐impairedpedestriansneededinterventionbecausetheydidn’trealizetheywerewalkingintoapotentiallydangeroussituation.Also,outofthe144totalcrossings,therewere15instanceswherethevisually‐impairedpedestrianbegantocrossandthenabortedthecrossing.Visually‐impairedpedestriansmayhaveproblemsincrossingmodernroundaboutsbecausetheymayhavethefollowingdifficulties:locatingthecrosswalkwithintheroundabout;identifyingthedirectionofcrosswalkalignmentthatmightbeperpendiculartothesidewalk;decidingwhenthetrafficiscontinuous,andidentifyingwhetheravehicleisyielding;andfollowingthepathofcrossingalignmentsandcrossmultiplelanesthroughtheendofthecrosswalk(Schroederetal.,2006).Thecurvedgeometryofmodernroundaboutsoftenforcesvisually‐impairedpedestrianstobefamiliarwithhowtocrossinthesecircumstances,asopposedtotraditionalintersections.Sincemostroundaboutsdonothavetrafficsignals,thetaskofidentifyinggapsintrafficatroundaboutsisquitedifficultforvisually‐impairedpedestrians.Modernroundaboutshavecontinuoustrafficandhighnoiselevelsthataddtothedifficultyofvisually‐impairedpedestriansindeterminingwhetherthevehicleshaveyielded,stopped,orcontinued.Thetotalnumberofcrashesinvolvingpeoplewithdisabilitiesincreasedaftertheconstructionofroundabouts;however,crashseveritydramaticallydecreased(SingerandHicks,2000).SingerandHicks(2000)alsoreviewedthechallengesindesigningamodern,pedestrian‐friendlyroundaboutinTowson,MD.Thechallengesincludedtheunusuallayoutoftheroundabout;difficultyinaccommodatingpeoplewithdisabilitiesandcomplyingwiththeAmericanswithDisabilitiesAct(ADA);theavailabilityofalternateroutes,andliabilityissues.TheauthorsprovidedinsightsonhowtheMarylandStateHighwayAdministrationcouldaddressthesechallenges.Theyinvolvedvariousstakeholdersinthedevelopmentoftheroundabout,conducteddriverandpedestrianeducationprograms,andprovidedadditionalinformationtothepublic,suchasBraillemaps.Inresponsetothoseissues,Schroederetal.(2006)testedadditionaltreatmentsforsingle‐lanemodernroundaboutswhichincludedsoundstrips,apedestrian‐actuatedflashingbeacon,andacombinationofthetwotreatments.Fortwo‐laneroundabouts,theauthorstestedaraisedcrosswalkandpedestriansignalwithPedestrianHybridBeacon(PHB).Inthisstudy,Schroederetal.(2006)usedthedegreeofriskincrossingtheroundaboutasaperformancemeasure.Theyusedapre‐andapost‐within‐subjectexperimentaldesignwherethesamevisually‐impairedpedestrianscrossedtheroundaboutinbothpre‐testandpost‐testscenariosaftertheroundaboutconstruction.Inthebefore‐and‐afterstudy,theauthorsusedasimulationofcrossingtheroundaboutsinwhich16peopleparticipated.Thestudyfinallyconcludesthatasingle‐laneroundaboutdoesnotposesignificantdifficultiesforvisually‐impairedpedestrians.Thisisduetolowvehiclespeeds,yieldingfromamajorityofdrivers,properlyinstalleddetectablewarningsurfacesandtheavailabilityofO&Mspecialists.However,tosignificantlyreducepedestriandelayattwo‐laneroundabouts,additionalcrossingtreatmentsarerequired.Tofurtherunderstandspecifictreatmentsfortwo‐laneroundabouts,Inman,DavisandSauerburger(2005)testedwhetherrumblestrip‐likedevicesandpedestrianyieldingsignswouldencouragedriverstoyieldmoreforpedestrians.Inmanetal.(2006a)conductedtwoexperimentsonacontrolledandtreatedcoursewithsevenseverelyvisuallyimpairedindividuals.Dataforeachexperimentwascollectedfor1.5hourseveryafternoonforaperiodoftwoweeks.Performancemeasuressuchascorrectlydetectingastoppedvehicle,failuretodetectthestoppedvehicle,falsealarms,andthenumberofcorrectlydetecteddepartures



ofstoppedvehicleswererecorded.Theresultsofthestudysuggestedthatsoundcuesonthepavementincreasedtheproportionofdouble‐yieldingdriversanddecreasedthetimeforvisuallyimpairedpedestrianstodetectyields;however,falsealarmswerenotaffected.TheYieldtoPedestriansigns,onceinstalled,increaseddrivers’yieldingactsfrom11.5%to16.7%.However,sincefalsealarmsarestillaproblem,theauthorsconcludedthatthetwotreatmentsdidnothaveasufficientlevelofsafetyimprovementtobeimplementedintwo‐laneroundabouts:yet,theyremaineffectiveinthecaseofsingle‐laneroundabouts.2.5.3.2HeavyVehicles.Ifroundaboutshavenotbeendesignedproperlytheymayinhibitthesafeandefficientmovementoflargetrucksduetoroundaboutdesignconstraints(ParkandPierce,2013).Usinganonlinesurvey,theauthorssynthesizedtruckingindustryobservationsregardingthechallengesexperiencedbycommercialtruckdriverswhileapproachingroundabouts.Themainissuesidentifiedincludedtheneedforlargerroundaboutcircumferences,moreeducationfordriversofpassengervehicles,andareevaluationofroundaboutdesign.About73%ofrespondentsbelievedthatroundaboutsweremoreproblematicforlargetruckscomparedtoothertypesofcontrolledintersections.Motorcarrierscommentedonroundaboutnavigationproblemsthatareuniquetolargetrucks,specifically,smallroundaboutcircumferences,designfeaturesthatcausedamagetotrucks,andsafeinteractionwith passengercars.Whenaskedtoproposepotentialsolutions,motorcarrierswishedthatroundaboutscouldbetteraccommodatelargetruckswithoutsacrificingsafetyandoperationalefficiency.Daniels,Brijs,Nuyts,andWets(2010)conductedastudytoexplorethecrashseverityatroundaboutsusingdatafrom1,491crashesthatoccurredat148roundaboutsinFlanders,Belgium.Theanalysisperiodvariedfromlocationtolocationbasedondataavailability.Theminimumperiodwas3years,themaximum10yearsandtheaverageacrossalllocationswas8.03years.Theydevelopedamodelforheavyvehiclesthatincludedtrucks,trailers,buses,andtractors.Eachroundaboutexperiencedanaverageof1.22annualinjurycrashes;meanwhile,theheavyvehiclecrashratewasfoundtobe0.09annualcrashesperroundaboutwithavarianceof0.02.Furthermore,atotalof18single‐vehiclecrasheswerefoundbythis7yearsstudytoinvolveheavyvehicleswithonefatalityandtwosevereinjuriesperyear.Likewise,97multi‐vehiclecrashesinvolvedheavyvehicleswithnofatalorsevereinjuries.2.5.4MethodsinRoundaboutSafetyAnalysisCommonmethodsusedtoanalyzingthesafetyeffectsofroundaboutincludedescriptiveanalysisusingdescriptivestatisticsandchi‐squarestatistics,empiricalobservation,generalizedlinearmodel,odds‐ratioandmeta‐analysis,ESEprocess,andempiricalbefore‐afterstudy.2.5.4.1AverageMean(Descriptive).Safetyevaluationofroundaboutscanbeobtainedusingasimplebeforeandafterapproach.Isebrands(2009b)conductedabefore‐andafteranalysisfor17high‐speedruralintersectionsusingadescriptivemethodwhichcalculatingtotalcrashfrequency,crashrateandcrashseverityinfivestates:Kansas,Maryland,Minnesota,Oregon,andWashingtonState.Datawereobtainedfromcrashrecordsandaveragedailytraffic(ADT)atthestudylocations.Specificallyforcrashrate,crashespermillionenteringvehicles(MEV),wasusedasameasureofexposure.Figure11displaysthebefore‐and‐aftercrashfrequencystatisticsateachofthe17locations.



Figure11.CrashFrequenciesinRoundabouts(Isebrands,2009b)

2.5.4.2Chi‐SquareStatistic.Furthermore,thechi‐squarestatisticandanormalapproximationtestmaybeusedtoseetherelationshipbetweenretrofittedmodernroundaboutandtrafficcrashes(FlanneryandDatta,1996).Theauthorsconsideredcrashfrequencyandthemeanofcrashesasperformancemeasures.Theyusedcrashdatabeforeandaftertheretrofittedperiodsforeachlocation.Tounderstandwhetherthebeforeretrofittedconditionsaredifferentfromthoseoftheafterconditions,theauthorsusedaChi‐squaretestwith=0.05,sixlocations,andfivedegreesoffreedom.Theresultindicatedthat,ata95%levelofconfidence,thereisasignificantdifferencebeforeandaftertheconstructionofroundabouts.Figure12givesthedatausedintheChi‐squareanalysis.

Figure12.DataRequiredforChi‐SquareAnalysis(FlanneryandDatta,1996,p.6)

Theauthorsusedanormalapproximationtesttoprovethatthebefore‐and‐aftergroupdataareneithercorrelatednorstatisticallyindependent.Sincethistestrequiressimilartimeperiodsforbothbefore‐and‐afterconditions,theyuseddatafromtwoyearspriortotheconstructionoftheroundaboutanddatafromoneyearaftertheroundaboutinstallation(FlanneryandDatta,1996,p.107).TheauthorsfoundthatX*=(8.93)andis>X.Thus,the“[r]eductioninthemeanofcrashesforbeforeandafterperiodofroundaboutconstructionissignificantata99%levelofconfidence”(FlanneryandDatta,1996,p.108).However,resultsfromIsebrands(2009b)andFlanneryandDatta(1996)shouldbeusedwithcaution.First,thenumberofcrashesalwaysfluctuatesinastochasticprocess(DanielsandWets,2005).Second,othergeneraltrendsmayinfluencethenumberofcrashes,includingpolicies,law,andchangesintrafficvolume.Third,theinstallationofroundaboutsissometimestheresultofhighcrashratesthatcanhavearegression‐to‐the‐mean(RTM)affectthatisnotaccountedforinasimplebefore‐and‐afterstudy.



2.5.4.3EmpiricalObservation(ConflictStudies).InSweden,HydenandVarhelyi(2000)usedabefore‐and‐afterstudytotestthelong‐termeffectsofsmallroundabouts.Theyattemptedtoanswersevenquestionspertainingtoroundabouts;dothey:(1)reducespeed,(2)resultinloweredriskofinjury,(3)promoteuserinteractions,(4)havenoeffectonredistributionoftraffic,(5)increasetimeconsumptionwhennogiveawayregulationoccursordecreasetimeconsumptionwithnosignalization,(6)increaseemissionswhennogiveawayregulationoccursanddecreaseemissionwithsignalization,(7)havenochangeinnoiselevel?Twooftheabovementionedareas,rateofspeedandriskofinjury,arerelatedtosafety.Crashdatawascollectedatthestudylocationssixmonthsafterconstructionandwascomparedtocrashesinthebeforeperiod(1983‐1990).Theauthorsusedconflicttechnique,i.e.,relatingconflictstocrashes.Theseverityoftheconflictwasbasedontimetoaccident(TA)andconflictingspeed(CS).Trainedobserversvideorecordedeachofthe12intersectionsfor30hours.Additionally,theauthorscalculatedthenumberofexpectedinjurycrashesperyearbymultiplyingtheratioofseriousconflictsandinjurycrashesdependingonthetypeofroadusersinvolved.Abehavioralstudywasalsoconductedtoseetheinteractionsamongtheroundaboutusers.Conflictsbetweenmultiplevehicles,bicyclesandvehicles,andpedestriansandvehicleswereexamined.Theresultsshowedthatseriousconflictsbetweenvehiclesandvehiclesincreasedwhilepedestrian‐vehicleandbicycle‐vehicleconflictsdecreased.Thisbefore‐and‐afterstudyisslightlybiasedbecausetheintersectionsselectedforthisstudywerechosenbecausetheyhadahighfrequencyofcrashespriortotheconstructionofroundabouts.2.5.4.4GeneralizedLinearModels.Churchilletal.,(2010)conductedbothacross‐sectionalstudyandabefore‐and‐afterstudytounderstandtheoverallsafetyeffectofroundabouts.CrashdatafromallroundaboutsbuiltintheNetherlandsfrom1999to2005wasanalyzed.Theauthorswerelimitedintermsofthetotalnumberofconventionalintersectionsandthetrafficvolumesrelatedtobothconventionalintersectionsandroundabouts.Asaresult,theyexaminedtheaggregatefatalcrashdataandfoundthatwhilethenumberoffatalitiesatconventionalintersectionsdecreased,thenumberoffatalitiesatroundaboutsincreased.However,thismaybeduetothefactthatthefatalcrashfrequencywasnotnormalized(i.e.,totalnumberofroundaboutswasnotincludedintheanalysis).Theresultsmaynotrepresentactualconditionsforeitherroundaboutsorconventionalintersectionsbecausethecross‐sectionalanalysisinthisstudywasfoundtobebiased.Forthebefore‐and‐afterstudy,datawasobtainedfromtheDutchNationalroadsdatabaseandtheDutchdatabaseofregisteredcrashes.ArcGISwasusedtogeocodethedataintoamap.Theresearchersassumedabufferof40metersaroundtheroundaboutforcrashes.Thisproceduremightinducesomebiasbecausethepreciselocationoftheintersectionsisunknown.Ageneralizedlinearmodelwasbuiltwiththeassumptionthat“thecountspercrashyearandperreconstructionyeararelinearlydependentonthenumberoflocationsretrofittedinthatyear”(Churchilletal.,2010,p.38).

2.5.4.5Odds‐ratioandMeta‐Analysis.Branbander,Nuyts,andVereeck(2005)conductedanotherbefore‐and‐afterstudythatincludedacomprehensiveanalysisofthesafetyofexistingroundaboutstoothercontrolledintersections.Usingodds‐ratiomatching,theauthorsfirstmadesurethecomparisongroups(intersections)hadthesamecharacteristics(i.e.,speedlimit)astheroundabouts.Anodds‐ratiomatchingisdefinedas“theratioofthechangeinthenumberofcrashesattheroundaboutlocationsbeforeimplementationandthechangeinthenumberofcrashesinthecomparisongroup”(Branbander,NuytsandVereeck,2005,p.290).Theodds‐ratioforoneyeariscomparedtothepreviousyear.Sincethenumberofcrashesataspecificlocationfluctuatesaroundanunknownaverage,theexpectednumberofcrashesataroundabout,takingintoaccountthereversiontomean(RTM)affectcanbecalculatedusingtheexpectednumberofcrashesatthelocationwheretheroundaboutwastobebuilt,aftercorrectionforRTMeffect,theaveragenumberofcrashesperyearforthecomparisongroup,includingthecrashesatthelocationwheretheroundaboutisimplemented;(beforetheconstructionoftheroundabout,



thelocationisconsideredcomparabletothecomparisongroupandcouldbeincluded).Nextthenumberofyearsisconsidered,thenumberofcrashesinyeart,atthelocationwhereroundaboutswereconstructed,andtheweightgiventotheaveragenumberofcrashesofthegroup(forthecomparisongroup)arecalculated.Then,theeffectivenessratioiscalculatedandfinally,theoverallsafetyeffectivenessisdefinedas"theweightedaverageoftheresultsoverthedifferentyears,wheretheweightassignedtothegroupofroundaboutsistheinverseofthevariance"(Branbander,NuytsandVereeck,2005,p.292).SimilartoBranbanderetal.,(2005),Elvik(2003)performedthelog‐oddsmethodofmeta‐analysis.Theauthorestimatedthesafetyeffectforroundaboutinstallationbycomparingthenumberofcrashesaftertheconversiontothenumberofcrashesbefore,andthencomparingthisratiototheratioofthenumberofcrashesafterandbeforeinacomparisongroupofintersections.Inthisstudy,Elvik(2003)reviewed28studiesthatevaluatedsafetyonroundabouts.Thestudyalsoconductedtraditionalmeta‐analysis,wherethedataweregroupedbasedonnumberofapproachlegsandcrashseveritytoexplorethesourceofvariation.Additionally,meta‐regressionanalysiswasusedtosupplementthetraditionalmeta‐analysis.Danielsetal.,(2008)alsousedodds‐ratiomatchingandmeta‐analysistoevaluatebicyclists’safetyatroundabouts.Takingasampleof91roundaboutsinFlanders,Belgium,andcrashdatafrom1991to2001,theygroupedtheroundaboutsarounddifferentspeedlimits,andtheirlocations(i.e.,insideoroutsidebuilt‐upareas).Theyalsotookthecomparisongroupofothercontrolledintersections,76forinsidebuilt‐upareas,and96intersectionsforoutsidebuilt‐upareas,andthenprioritizedthenearbyintersectionsbasedonapproachspeeds.Meta‐analysishastwobasicweaknesses.First,meta‐analysiscannotimprovethequalityoftheevaluationofthestudy(Elvik,2003).Forexample,afterevaluatingdifferentstudydesigns,Elvik(2003),statedthatthequalityofsimplerstudydesignsmightweakenthequalityofmoreadvancedstudies.Anotherpotentialweaknessofmeta‐analysisisthatitcanbebiased.Thebiasmayoccurwhenpreviousstudies’findingsgoagainstconventionalwisdomsotheyareregardedashavinglittlevalue.Therefore,thisstudyadoptsthetrim‐and‐fillmethodtohelpconvertthebias,whichisdefinedas“anon‐parametricmethodfordiagnosingandcorrectingforpublicationbias,basedontheassumptionthatafunnelplotofresultsshouldbesymmetricaroundthemeanintheabsenceofpublicationbias.”(Elvik,2003,p.5)2.5.4.6ESEProcess.TurnerandBrown(2013)usedtheESEprocesstoassessthesafetyimprovementsofroundabouts.“ThethreekeyelementsoftheESE(orEASY)processare:1.estimationofexpectedcrashesusingthebestavailablebase(crash)model;2.safetyobservationbasedonexperience;and3.evidencefromnationalandinternationalroadsafetyresearch.Togiveconfidenceintheresults,theESEprocessincludescheckingthroughouttheprocessbyreviewingandcomparingwithotheravailableinformationsources.”(TurnerandBrown,2013,p.2).2.5.4.7EmpiricalBasedBefore‐and‐AfterStudies.AccordingtoPersaudetal.,(2001),asimplebefore‐and‐afterstudymaybebiasedduetotheRTMeffectbecauseroundaboutsareusuallyconstructedwhenanintersectionhassafetyproblems.Consequently,ifthestudyfailstocontrolthiseffect,thestudyislikelytooverestimatethesafetyeffectoftheroundaboutconversion.TorespondtotheneedtoaddresstheRTMeffect,Persaudetal.,(2001)employedtheempiricalBayesbefore‐and‐afterprocedure.Rettingetal.,(2001)andRodegerdtsetal.,(2007)alsousethisprocedure.Rodegerdts(2007)evaluated310roundaboutsintheUnitedStateswithdifferentcharacteristics,suchasurban‐suburban‐ruralsetting,numberoflegs,numberofcirculatinglanes,previousintersectiontype,ageofroundabout,andgeographiclocations.Theauthorsanalyzed90roundaboutsbasedondataavailability,geometricinformationandenteringdailytrafficvolumes.Roundabout‐levelcrashpredictionmodelsasa



functionofnumberoflanes,numberofapproachlegs,andAADTweredeveloped.Similarly,approach‐levelcrashpredictionmodelsrelatedcommontypesofcrashestoAADT,includingkeygeometricfactors.2.5.5RoundaboutsandSafety:ConclusionThissectionreviewedtheexistingliteratureonroundaboutsandsafety.Manystudiesshowedthatroundaboutshaveincreasedsafetyperformance,withsafetyeffectsrangingbetween17to70%forcrashreductions.However,theseresultscouldnotbefullytakenastheeffectofroundaboutconversionbecausethereareothercontextsandissues,suchastheargumentthatconversionsfromFWSCintersectionstothemodernroundaboutsdonotsignificantlyreducethetotalandinjurycrashrates(Rodegerdts,2007).Asaconsequence,howtheretrofittedprocessesandlocationselectionsweremademayinfluencethesafetyeffectcalculation.Furthermore,theliteraturereviewfoundnumerousconcernsfromresearchersabouttheeffectofretrofittedroundaboutsforvarioususersandmodes.Safetyperformancesofroundaboutsmaybereducedforvulnerableuserssuchasbicyclists,pedestrians,peoplewhoarevisually‐impairedorwithdisabilities,andelderlyroadusers.Theconcernisalsohighlightedforbigtrucksthatrequirespecialtreatmentsanddesignontheroundabout.Manymethodsareavailableforperformingsafetyanalysis:descriptiveanalysis,chi‐squarestatistics,empiricalobservation,generalizedlinearmodel,odds‐ratioandmeta‐analysis,ESEprocess,andempiricalbefore‐afterstudy.



2.6EvaluationofGapsinRoundaboutLiterature Anevaluationofexistingliteratureonroundabouts,accessmanagement,safety,andcapacityshowedseveralgapsinknowledge.Gapsareidentifiedbaseduponavailableliteratureregardingtheuseofroundabouts,particularlyastheyapplytoaccess,operationsandroadwaycapacity,andsafety.2.6.1LiteratureGapsinAccessManagementBasedontheliteraturereviewonaccessmanagement,majorgapsintheliteraturewereidentified.Littleliteratureexistsaboutaccessmanagementasitspecificallyappliestoroundabouts.Aswasdescribedearlierinthischapter,manystudieshavebeencompletedabouttheuseofaccessmanagementstrategiesatintersectiontypes(stop‐controlled,signalizedintersections,un‐signalizedintersections)astheyrelatetovariousdesignandplanningelementconsiderations.However,fewsuchstudieshavebeencompletedrelatedtoroundaboutsandaccessmanagement.2.6.2LiteratureGapsinRoundaboutOperationsandCapacityBasedonthereviewofliteratureonroundaboutoperationsandcapacity,severalgapsintheliteraturewereidentified:

Theanalyticalapproachseemstobethemostcommonmethodologyinroundaboutcapacityanalysis;thereisalackofstudiesthatusestatisticalapproaches.Itismoredifficulttousestatisticalapproachesbecausetherearefewerroundaboutsthatreachcapacity.Theanalyticalapproachdoesnothavethatrequirement;itisbasedongapacceptance.

o Theanalyticalapproachneedstoincorporatethecalibrationofdriverbehaviortomatchspecificlocalconditions.

o Amorestreamlinedprocessofcollectingthedatafromlocalroundaboutscouldalsobeconsideredtostandardizethedatacollectionprocess.

Onlyafewstudiesfocusontheimpactofbicyclesandpedestriansonroundaboutcapacity.o Forstudiesspecificallyrelatedtoaccessmanagement,moreinformationisneeded

examininghowslowtrafficinfluencesroundaboutcapacitymodels,particularlyasrelatedtodriverbehavior.

o However,thisinformationwouldbedifficulttoacquire,sinceeachroundabouthasuniquegeometricandpedestriancrossingdesigns.

o Thereiscurrentlynotareliablesimulationtoolforpedestrianmovementatroundabouts. Studiesonunbalancedtrafficatroundaboutentrieshaveincompletedata.

o Sinceaccessmanagementistheprimarygoalofthisresearchproject,unbalancedtrafficissuesshouldbeaddressedwithcare,sinceexistingstudiesshowunbalancedtrafficcouldhaveagreatimpactonroundaboutperformanceandcanindirectlyaffectaccesstobusinessesnearroundabouts.However,thedegreeoftheimpactisnotyetclear.

Althoughsomestudiesconsidertheimpactofheavyvehiclesonroundaboutcapacity,thisimpactisheavilydependentonlocalconditions,especiallythegeometricdesignoftheroundabouts.

o Theuseofastandardizeddesignguiderelatingvehiclecharacteristicstoroundaboutgeometricdesignwouldpresentreliablestandardsforengineerstodesignroundabouts.

Overall,therearefewstudiesexploringtheimpactsofroundaboutsoncorridors.Existingliteraturesuggeststhatroundaboutsdonotperformsignificantlybetterthansignalizedintersectionsinacorridor.Roundaboutsseemedtohavehigherperformancewhenthecorridorhasirregularintersectionspacing(KittelsonandAssociates,Inc.2013).Butwhetheracorridorofroundaboutsissuperiortoothertypesofintersectionsreallydependsonsite‐specificoperationalconditions



(KittelsonandAssociates,Inc.2013).Ofevenmoreinteresttoourresearch,wouldbestudiesalongcorridorswithunbalancedtrafficconditions,orhighlevelsofpedestrianorbicycletraffic,andabefore‐and‐afterstudyoftheconversionfromsignalizedintersectionsintoacorridorofroundabouts.

2.6.3LiteratureGapsinRoundaboutSafetyThereissubstantialagreementintheliteraturereviewedthatmodernroundaboutshavesignificantsafetyimpactswhencomparedtotraditionaltrafficintersectiontreatments.Whilethesesafetyimprovementshavebeenobservedandstudiedinternationallyusingseveraldifferentmethods,gapsinthisresearchstillexist.Basedonthereviewofliteratureonroundaboutsafety,severalgeneralgapsintheliteraturewereidentified:

Longitudinalsafetystudiesgenerallyincludelessthantwoyearsofdata.o Studiesshouldbemadeoverperiodslongerthantwoyears,becausethenthesafetyeffects

canbemoreclearlyidentified.o Inthefirsttwoyearsofimplementationoradaptationperiod,usersarestilllearningthe

rulesandguidelines. Collectivelythelongitudinalsafetystudieslacklocationvariation.Roundaboutsinagreater

diversityofcontextsneedtobeanalyzedinlongitudinalstudies. Insomestudies,thelocationofmodernroundaboutsseemstohavebeenchosenbecausethose

intersectionshavehighcrashfrequencies.Thisselectionbiasweakenstheconclusionsbecauseitcanbedifficulttoknowiftheimprovementsareduetotheunsafeconditionsbeforetheconversiontoaroundabout,changesindriverbehaviorduetotheconversiontoaroundabout(i.e.,thetreatmenteffect)orwhetherthelackofimprovementisduetothedifficultyofdesigningasolutioninahigh‐crashlocation.

o Studiesshouldincorporatedifferentlocationswithdifferentcharacteristics. Moststudiesusedsmallsamplesizes.

o Studiesshoulduselargersamplesizes,togiveadditionalstatisticalsignificanceandaccuracy.

Simplemethodsofbefore‐and‐afterstudiesdonotcomparetheeffectivenessofmodernroundaboutstootherintersectionswithoutroundabouts.Inotherwords,morecarefullydesignedcontrolstudiesneedtobedeveloped.

Twomethodsthatacknowledgebothbefore‐afterandcrosssectionalconditionsareodd‐ratioandempiricalBayes.Thesemethodshavebeendeployedindifferentcontexts,whichmaylimittheirgeneralizabilitytoothercontexts.

o Theodd‐ratiomethodwasusedbyBranbanderetal.,(2005),Danielsetal.,(2008),andElvik(2003)instudiesthattookplaceinEurope.

o TheempiricalBayesmethodwasusedbyPersaudetal.,(2001),Retting,etal.,(2001),andRodegerdtsetal.,(2007)intheanalysisofroundaboutsintheUnitedStates.

o Bothmethodsusedthemeta‐analysistoenablethegroupsofcontexts:forexample,suburbanandurban,thenumberoflegs,andtrafficflow.However,thelattermethodincorporatesthecharacteristicsofmodernroundaboutsorothercontrolled‐intersectionsinthepredictionmodel.Inotherwords,empiricalBayesgivesamorecompletepictureofthevariablesthatinfluencethecrashrate.

Rodegerdtsetal.,(2007)isthemostcomprehensivestudyusingthelargestnumberofroundaboutinthesample(310roundabouts).However,theevaluationofsafetyforagroupoflocationsthatsharesimilarusers’characteristics,roundaboutdesign,anddriverbehavior,forexampleinonestate,maybeimportanttoenhancetheknowledgeofthesafetyofroundabouts.

Someoftheliteratureproposesadditionaldifferentgeometriesontheroundabouts;additionalstudytoaccommodatetheneedsofotherusersisanothergapinknowledge.Althoughtheresultof



thegeometryispromising,itmayaffectothergroupsofusersthatmightfindmorechallengingconditionsincrossingtheroundabouts.

SpecificallyinFlorida,theClearwaterBeachroundabouthasbeenevaluatedintensivelytounderstandpedestriansafety(Rodegerdtsetal.,2007).Althoughthislocationmaybeagoodlocationtounderstandpedestrianbehaviorandsafety,itisnotnecessarilyrepresentativeofroundaboutlocations.Additionalresearchisnecessarytodeterminehowrepresentativethislocationisofthepedestrianconditionsatroundabouts.

Basedonthereviewofliteratureonroundaboutpedestriansafety,severalgapsintheliteratureareidentified:

StudiesontheeffectivenessofmodernroundaboutsintheUnitedStatesexamineveryfewlocations,andthosesamelocationsareexaminedrepeatedly.Assuch,agreaternumberofsamplelocationsshouldbeincorporatedintoroundaboutresearch,andagreaterdiversityofbothpedestrian,bicyclistandlargevehicleconditionsshouldbeincorporatedintothisanalysis.

CrashreportsandthepotentialforlocationbiasbydisabledpedestriansforcestudiestorelyuponobservationalresearchintheUnitedStates.Observationalresearchshouldbefurtherincorporatedwithstatisticalresearchatlocationswithhighnumbersofpedestriansorbicyclists.

Althoughperceivedriskandactualriskmayleadtodifferentconsequencesinthemodernroundaboutdevelopment,knowledgeaboutperceivedriskforeachgroupofvulnerableusersisimportantforenhancingthebalanceofusers’needs.

Understandingtheperceptionsofvulnerableusersmayhelpdesignersofthemodernroundaboutaddresstheneedsofthoseusers.

Treatmentofvulnerableusers,includingbicyclistsandpedestrians,isinconsistentthroughoutthedifferentstates.Nationaltransportationorganizationsshouldprovidegeneralguidelinesregardinghowtoincorporateallusers’needs,especiallyvulnerableusers.

Basedonthereviewofliteratureonroundaboutdesignandsafetymeasures,severalgapsintheliteraturewereidentified:

ArndtandTroutbeck(1998)showtheimportanceofunderstandingdriverbehavior,trafficconditions,androundaboutgeometryinonespecificlocation,andtheycompareAustraliaandtheUnitedKingdom.Consequently,thisimpliesthatthoseconditionsaredifferentintheUnitedStates.TheenhancementofpreviousmodelsavailabletobeappliedintheUnitedStatesorotherspecificlocationsmaybethegapofknowledge.

Eventhoughitisacknowledgedthatmulti‐laneroundaboutsarelesssafethansingle‐laneroundabouts,multi‐laneroundaboutsneedadditionalattentionbecausetheyareoftenusedforcapacityreasons.Additionalresearchshouldexploretheeffectsofmulti‐laneandcomplexroundaboutsonbothsafetyandcapacity.

Althoughthesestudiesshowseveraldesign‐relatedinfluencesonsafetylevels,theroundaboutdesignshouldbalanceotherfactors,suchas,capacityandconstructioncost.Optimumbalancesbetweensafety,capacity,access,andcostshouldbefurtherexplored.

Chapter4ReviewofFederalandStatePractices


ChapterThree:MethodologyThisresearchusedmultiplemethodstounderstandthestateofpracticeinroundaboutsandaccessmanagementinthestateofFlorida.Theyincludeareviewofstateaccessmanagementandroundaboutguides,thecollectionandanalysisofcrashinformationatallroundaboutsinthestate,andtheselectionofasamplingofroundaboutsinthestateandthecollectionofandanalysisofthefieldoperationsofthesesites.Inaddition,areviewandanalysisofFlorida‐specificsoftwaretoanalyzethecapacityandoperationsofroundaboutswithinthestatewillbeconducted.AsdescribedintheLiteratureReview,theanalysisofthisinformationforFloridaiscomplicatedbythelackofpreviousresearchthatspecificallyaddressesaccessmanagementnearroundaboutsandtheabsenceofstandardmethodsofprovidingguidanceonaccessmanagementandroundaboutsbystatedepartmentsoftransportation.3.1AccessManagementandRoundaboutGuides’Selection.Thereviewofnationalandstateguidancewascompletedbyreviewingtwotypesofguidance:accessmanagementguidesandroundaboutguides.Severalsourcesofnationalguidanceonaccessmanagementwereidentified.Documentsthatcontainaccessmanagementelementswerefoundinthefollowingtypesofdocuments:roadwayorhighwaydesign/manuals;accessmanagementmanuals;anddrivewaymanuals.NCHRPSynthesis404,StateofPracticeinHighwayAccessManagement(GluckandLorenz,2010)isparticularlyusefulforthisresearchbecauseitincludesdataonwheretofindinformationonaccessmanagementforeachstate;theinformationinthatreportisupdatedwithareviewofstatedepartmentoftransportationwebsites.Twenty‐oneDOTsincludeaccessmanagementinformationontheirwebsite.Table4summarizesthevarioustypesofdocumentsthatstateDOTsuseasapartoftheiraccessmanagementprogram.Mostwebpagescontaininformationabouttheintroductionofaccessmanagement,theaspectsthatshouldbeconsideredinanalyzingaccessneedsofnewdevelopment,andlinkstodesignmanualsandotherrelateddocumentsusedbyDOTstaff.Forty‐threestateshaveincorporatedaccessand/oraccessmanagementontheirplanninganddesignpolicies.Morespecifically,nineteenstateshaveaccessmanagementmanuals,separatefromgeneraldesignmanuals.ElevenstateDOTsmentionaccessmanagementondesignmanuals;whilesixteenotherDOTshaveadditionaldocumentswithvariousnames.ThecompletelistandlinkstoDOTwebsitescanbefoundinAppendixB.



Table4.MainDocumentsonAccessManagement–RelatedStateDOTGuidebooks

AccessManagementManual/Guidebook

Roadway/HighwayDesignManual

OtherRelatedDocuments

Alabama(2013)Florida(2009)Idaho(2001)Indiana(2009)Iowa(2012)Kansas(2013)Michigan(2001)Minnesota(2008)Mississippi(2012)Missouri(2003)Nevada(1999)NewJersey(2013)NewMexico(2001)Ohio(2001)Oregon(2012)SouthCarolina(2008)Texas(2011)Vermont(1999)Virginia(2007)

Arizona(2012)California(2012)Connecticut(2012)Illinois(2010)Massachusetts(2006)Montana(2007)NewYork(2002)Utah(2007);NorthDakota(2009)SouthDakota(web,2013)Washington(2012)

StateHighwayAccessCode/Manual:Colorado(1998)Delaware(2011)DistrictofColumbia(2010)Maryland(2004)Wyoming(2005)DrivewayManualor/andEncroachmentControl:Georgia(2009)WestVirginia(2004)AccessConnectionPolicy/Rules:Louisiana(2012)Maine(2005)AccessControlPolicy:Nebraska(2006)Washington(2009)Wisconsin(FDM,2011)RightofWayManual:Utah(2006)Montana(2007)DrivewayPermit/Access:NewHampshire(2000)NorthCarolina(2003)

Source:DOTwebsitesThereviewofmanualsandguidebooksforthisresearchissimilartothatcompletedinNCHRPSynthesis404StateofPracticeinHighwayAccessManagement(GluckandLorenz,2010),butthisresearchreviewedagreatervarietyofaccessmanagementdocuments;assuch,itupdatesthatreport.Oftheforty‐threestatesandtheDistrictofColumbiawithaccessmanagement‐relateddocuments,sixteenstatesandtheDistrictofColumbiaupdatedtheirguidelinesafter2009.Asahighlight,theNCHRPSynthesis404‐StateofPracticeconductedsurveysforallfiftystatesandobtainedcomprehensiveinformationaboutaccessmanagementprogramelementsbeingdevelopedbystateDOTs,suchasguidelines,generaldepartmentpolicies,anddrivewaypermitmanuals,andstandards.Furthermore,thisreviewspecifiestheaccessmanagementtechniquesandgeometricdesignelementsthathavebeenadoptedbymanystates.Oncethestateguidancedocumentswereidentified,theanalysisusesthesixteencategoriesoftypicalaccessmanagementtechniquesthatareusedintheNCHRPSynthesis404:StateofPracticeanalysis(GluckandLorenz,2010,p.49‐50):

1. Installationofthemedians2. Spacingformedianopenings/breaks3. Spacingforun‐signalizedpubicstreetintersections4. Spacingforun‐signalizedprivatedriveways5. Spacingfortrafficsignals6. Prohibitionofcertainturningmovements,7. Cornerclearance,and8. Spacingforcross‐streetinthevicinityofinterchanges9. SetbackandISD10. Geometricdesignstandardsfordriveways



11. Provisionsforright‐turnandleft‐turnlanes12. Purchaseofaccessrights13. Internalconnectionofparkinglotsbetweenadjacentparcels14. Subdivisionrestrictionsforlargeparcels15. Requirementsfortrafficimpactstudies16. Requirementfortrafficimpactfees

Amongthesetechniques,thesynthesisreportedthat80%ofthestatesappliedthefirsttenaccessmanagementtechniquesandrequirementsfortrafficimpactstudiesoftechniques(number15).Thepurchaseofaccessrights(number12),wasusedby66%ofstateDOTs.Internalconnectionofparkinglotsbetweenadjacentparcels(number13)andsubdivisionrestrictionsforlargeparcels(number14)areusedby48%and30%respectivelyofstateDOTs,andonly16%ofstateDOTshaveincorporatedtrafficimpactfees(number16).AsummaryoftheuseoftheaccessmanagementelementsandtechniquesbythestatescanbefoundonAppendixC.Nationalguidanceonroundabouts,accessmanagement,safety,andcapacity,supplementedbyahandfulofstates,whoareleadingthewayinprovidingstatewideroundaboutguidance.ThoseDOTsincludedroundaboutguidanceinvarioustypesofdocuments.Forexample,somestatesincluderoundaboutdesignstandardsintheroadwaymanual.Somestatesprovidespecificlinkstoinformationaboutroundaboutdesign.TheVirginiaDOT(VDOT)placestheroundaboutdesigninformationintheaccessmanagementdesignstandards;thisistheonlystatethatdirectlyprovidesthisinformationinasingleplace.Overall,26stateshavevariouslevelsofinformationaboutroundaboutsontheirwebsites.MoststateDOTwebsitescontaininformationfordriversabouthowtousearoundabout.Somestatesalsolinktotheroundaboutwebsiteofotherstatesandthenationalguidance.Oncetheroundaboutinformationforthe26statesandtheDistrictofColumbiawerereviewed,16statesthatrefertoaccessmanagementinthecontextofroundaboutsintheirguidebookswereselectedforfurtherexaminationonroundabouts:Arizona,Florida,Kansas,Indiana,Iowa,Kentucky,Maryland,Michigan,Minnesota,NewHampshire,Pennsylvania,California,Washington,andWisconsin.SeeTable5forinformationonthelocationofstateinformationonroundabouts.

Table5.TheSourcesofRoundaboutStates’DesignGuidebooks

RoundaboutGuideDocument

FacilityDevelopmentManual

AccessManagementDesignStandard

RoadwayorHighwayDesignManual

Florida(1996,2000,2012)Arizona*(2003)Kansas(2003)Pennsylvania(2007)California(2007)Iowa(2008)Michigan(2011)Maryland(2012)

Wisconsin(2011) Virginia(2007)

NewHampshire(2007)Iowa(2009)Minnesota(2009)Kentucky(2010)Maryland(2011)Washington(2011)Arizona(2012)

*–cannotbeaccessedonline3.2SiteIdentificationThefirststepinboththeoperationalanalysisandsafetyanalysiswastheidentificationofthelocationofallroundaboutsinthestateofFlorida.TheFDOT’sRCIdatabaseincludesanelementcalled“ROTARY,”whichincludesthefollowingthreecodes:roundabout,trafficcircleandmini‐roundabout.Atotalof219roadwaysegmentscodedas"roundabout"wereidentifiedfromthe2011RCIdatabase.Onlyfourofthoseroundaboutswerelocatedontheon‐system(i.e.,state)roads,whiletheremaining215werelocatedonthe



off‐systemroads.SincetheRCIdatabasedoesnotincludealltheoff‐systemroads,anextraeffortwasmadeusingGoogleEarthtovisuallyidentifyadditionalroundaboutsontheoff‐systemroadsthatarenotcoveredintheRCIdatabase.Thisnettedanadditional64locations,foratotalof283roundaboutsforthisstudy.Foroperationalanalysis,226roundaboutsintheStateofFloridawereanalyzedbyviewingthemapusingGoogleMap,andfinally13siteswereselectedforadetailedanalysis.Thesummaryofthe226sitesareoutlinedinthefollowingtable.Table6.SummaryofRoundaboutsinFloridabyDesignandContext

Category Aspects NumberofRoundaboutsNumberoflegs Two 3

Three 85Four 122Five+ 16

Numberofcirculatinglanes

Singlelane 164Multi‐lane 53Turbo/Spiral 9

LocationofDriveway Atapproachlane 24Ategresslane 33Drivewaydirectlylinktoroundabout 10Morethanonedriveway 128Nodriveway 31

Surroundinglanduse Residential 100Commercial 63Mixed‐use 54Other 9

3.3SafetyAnalysisThissectiondescribesthemethodologyusedtoconductsafetyanalysis.ItincludeshowtheroundaboutlocationsinFloridaarecategorized,howcrashdataincludingbothcrashrecordsandpolicereportsforthelocationsidentifiedwereextracted,howcrashlocationstoimprovedataqualitywerecorrected,andhowpolicereportsforin‐depthsafetyanalysiswerereviewed.3.3.1CategorizeRoundaboutLocationsAfterthe283roundaboutsinthestatewereidentified,additionalinformationsuchaslanduse(i.e.,commercialorresidential),roundabouttype(i.e.,singleormulti‐lane),presenceofotherroundaboutsinthevicinity,numberofapproachlegs,numberofcommercialandresidentialdriveways,presenceandtypeofmedian,presenceofon‐streetparking,presenceofbikelanesandpedestriancrosswalksonroundaboutapproachlegswascollected.Forsafetyanalysis,roundaboutswereclassifiedaseithercommercialorresidential.Commercialroundaboutsarethosethatarelocatedincommercialareasthatservemostlycommercialtraffic.Locationswithamixoflanduses,includingbothcommercialandresidential,arere‐classifiedascommercial.Residentialroundaboutsarethosethatarelocatedinmostlyresidentialareas.Figure13givesanexampleofeachoftwolandusetypes,respectively.



(a)CommercialLandUse (b)ResidentialLandUse

(Location:PierParkDrive.,PanamaCityBeach) (Location:SW77Avenue.,Alachua)Figure13.ExamplesofRoundaboutsLocatedinEachLandUse Type

3.3.2ExtractCrashDataFiveyearsofcrashdatafrom2007‐2011wereusedinthisanalysis.CrashesthatoccurredinthevicinityoftheroundaboutswerespatiallyidentifiedinArcGIS10.0.Thelocationsofthe219roundaboutsidentifiedusingtheRCIdatabasewereimportedintoArcGISusingtheirroadwayIDsandbeginandendmileposts.Theremaining64roundaboutsthatwerevisuallyidentifiedwereimportedintoArcGISusingtheirlatitudeandlongitudecoordinatesobtainedfromGoogleEarth.Shapefilesofthecrashdatafortheyears2007‐2011weredownloadedfromtheFDOTUnifiedBasemapRepository(UBR)forbothon‐systemandoff‐systemroads.ThesefileswereseparatelyimportedintoArcGIS.A500ft.bufferwasthencreatedaroundeachofthe283roundabouts.Allthecrashesthatoccurredwithinthe500ft.bufferwerespatiallyidentified.Aninfluenceareaof500ft.waschosentoincludeallthecrashesthatcouldhavebeenpotentiallyaffectedbythepresenceofroundabouts.Atotalof2,941crasheswerefoundtohaveoccurredwithin500ft.oftheroundabouts.PolicereportsofallthesecrashesweredownloadedfromtheHummingbirdwebsystemhostedonFDOT’sIntranet.3.3.3CorrectCrashLocationsandReviewPoliceReportsAnexistingin‐houseweb‐basedtoolwasadaptedforthisstudytofacilitatetheprocessofreviewingthepolicereports.Thetoolhasthecapabilitytovisuallydisplaycrashesbycrashtypeandcrashseverity,asshowninFigure14andFigure15,respectively.Thetoolhelpstoquicklynavigatefromonepolicereporttothenextbyeitherclickingthe“Next”and“Previous”buttons,orbyclickingonthecrashiconintheaerialmap.Thetoolalsohasthecapabilitytomovefromoneroundaboutlocationtothenext,andtonavigatetoaspecificroundaboutbasedonroadwayname.



Figure14.CrashesDisplayedbyCrashTypeataRoundabout

Figure15.CrashesDisplayedbyCrashSeverityataRoundabout



Afewroundaboutsdidnotexistfortheentirestudyperiodastheywereconstructedafter2006;however,theexactconstructionperiodwasunknown.Basedontheillustrativesketchesinthepolicereports,crashesthatoccurredatthestudylocationspriortotheconstructionoftheroundaboutswereexcludedfromtheanalysis.Sincetheanalysisfocusesonevaluatingtheinfluenceofaccessfeaturessuchasdriveways,medianopenings,etc.,onthesafetyperformanceofroundabouts,accuratecrashlocationsarecrucial.Aquickreviewofthepolicereportsrevealedthatthecrashlocationsareapproximate,andinsomecases,thelocationsareoffbyseveralhundredft.Toaddressthisissue,crashlocationsofall2,941crashesweremanuallyverified.Locationsof1,191crashes(40.5%)werefoundtobeincorrectandwereupdated.Foreachcrash,thecrashlocationwasverifiedandupdatedusingthefollowingsteps:

1. IdentifytheroundaboutlocationonGoogleEarth.2. Reviewpolicereport(s)ofthecrashtopinpointtheactuallocationwherethecrashoccurred.This

stepmightrequirereviewingboththecrashdiagramandthedescriptionfromthepolicereports.3. ObtainlatitudeandlongitudecoordinatesofthecorrectcrashlocationfromGoogleEarth.4. Recordthecorrectcoordinatesintheweb‐basedtool.

Oncethelocationsofallcrasheswereverifiedandrecorded,thecrashfileintheweb‐basedtoolwasupdatedbasedonthenewcoordinates.Next,allthecrashesthatdidnotoccurontheroundaboutoronanapproachlegleadingtoaroundaboutwereexcludedfromfurtheranalysis.Forexample,Figure16showsacrashthatoccurredwithin500ft.fromtheroundabout,butdidnotoccurontheroundaboutanditsapproachlegs.Atotalof1,059crasheswerenotfoundtobedirectlyrelatedtotheroundaboutsandwereremoved.Thisresultedinatotalof1,882crashesthatwereincludedinthedetailedanalysis.

Figure16.AnExampleofaCrashThatWasNotDirectlyRelatedtotheRoundabout

Forthepreliminarysafetyanalysis,potentialsafetyissuespertainingtoroundaboutsandaccessfeatureswerefirstidentifiedfromtheliteraturereview.Accordingly,thesafetyanalysisfocusedonthefollowingfourpotentialsafetyareasassociatedwithroundabouts:



1. Impactofdrivewaycornerclearancesonroundaboutsafety.2. Safetyimpactofmedianopeningsinthevicinityofroundabouts.3. Safetyatroundaboutsthatprovidedirectaccesstoactivitycenters.4. Safetyofvulnerableroadusersincludingpedestriansandbicyclists.

Oncethecrashlocationswerecorrected,theillustrativesketchesanddescriptionsinthepolicereportswerereviewedindetailtocategorizecrashesintotheaforementionedcategoriesfordetailedanalysis.Theweb‐basedtoolwascustomizedtofacilitatethisprocess.Figure17givesthescreenshotofthetool’sinterfaceusedfordatacollection.Inaddition,datafromthepolicereportswereusedtoobtaincrashseverityusingthefollowingcodes:

K – Fatal Injury A – Incapacitating Injury B – Non-Incapacitating Injury C – Possible Injury O – Property Damage Only

Figure17.DataCollectionusingWeb‐basedTool

3.4OperationalAnalysisThepurposeoftheoperationalanalysisistoevaluatetheperformanceofroundaboutsandidentifythepotentialissuesrelatedtoaccessmanagement.Thissectionexplainsthemethodforselectionofstudysites,thecollectionofdataonroundaboutoperations(videoandsiteobservations),andtheanalysisofthedatacollectedateachofthesites.AnevaluationofFDOT‐utilizedsoftwareisalsoincludedtoassessthesuitabilityofthesesoftwarepackagesonanalyzingroundaboutandaccessmanagementissues.3.4.1DataCollectionSiteSelectionUsingGoogleEarth,wevisuallyinspectedeachofthe283roundaboutstounderstandthedesign,regionalcontext,andaccesscharacteristicsofeachroundaboutusingthecategoriesshowninTable7.



Table7.CriteriaforSelectingRoundaboutsforOperationalAnalysis

Category Aspect Definition

Designofroundabout

Type—numberoflegsNumberofapproachinglegs:Arangefrom3to6legs

Type—numberoflanes

Numberofcirculatinglanes:Singlelane;Multilane;ComplexRoundabout(Spiral,turbo)

Geometricconsideration

Thegeometriccharacteristicsoftheroundaboutincludes:Mediansonapproachinglane;SlipLanes;Stub‐out.

Regionalcontext

Regionallocationcontext Relativelocationtonearesttown

Whetherinurbanarea Urban,suburban,rural

Transportationcontext

Whetherornotonastatehighway;Within1mileofinterstate;Nearstatehighway;Nohighwaynearby.

Access

Drivewayplacement

Inthemiddleofroundabout;Ontheaccessapproachofroundabout;Ontheegressapproachofroundabout;Onbothaccessandegressapproachofroundabout;Nodrivewaynearby.

Landusetypearoundroundabout

Residentialsingle‐familyhousing;Residentialmulti‐familyhousing;Commercial;Mixed‐use.

Onceallsiteswereevaluated,asmallersetofsiteswereselectedfortheoperationalanalysisbasedonthefollowingcriteria:(1)modernroundaboutwithsplitterisland;(2)locatedinanurbanareawithsignificantamountoftraffic;(3)havepotentialforaccessmanagementissues,e.g.,adjacentdrivewayandintersectionnearby;(4)eitheronelaneormulti‐lane;and(4)couldhaveon‐streetparkingorbeapartofaseriesofroundabouts.Fortheoperationalanalysis,theroundaboutlistwasnarroweddowninthreestages.First,100siteswereselectedfromtheentirelistbymerelylookingatroundaboutgeometricdesignfeaturesandthelandusecontextaroundtheroundabout.Then,severalteammatesfurthernarrowedthenumberdowntothirty‐fourbasedonmorestringentcriteria,suchasselectingsiteswithlargertrafficvolume.Afterthat,eachresearcherintheteamvotedfortensites,andthehighestrankedeighteensiteswerechosenforactualvisitsthroughareviewprocessthatinvolvedinternalteammeetings,discussions,andasiteselectionmeetinginthestateofFloridawiththeFDOTProjectTeam.Finally,theeighteensiteswerevisited,fromwhichthirteensiteswereconsideredsuitablefordatacollectionbasedonthetrafficvolumeandgeometricdesignofthesites.Thefivesitesthatwereinitiallyselected,butforwhichwedidnotcollectdata,wereeliminatedbecausethereisnodrivewayincloseproximitytotheroundabout,ortheyarelocatedinalow‐densityareawherethereisnotenoughtraffictocreatesignificantdelayandqueuingneartheroundabout.



Amongthethirteenselectedsites,onlyoneislocatedonastatehighway.Table8showsthesummaryofroundaboutselectionprocess.Videowascollectedfromthosethirteensites.Figure18showsthelocationsofbothselectedroundaboutsandthepoolofroundabouts.DetailsaboutthethirteenselectedsitesareincludedinAppendixD.

Table8.SummaryofRoundaboutSelectionProcess

StepsinSelection Number

AllRoundabouts 283

ConsideringContextofRoundabouts(e.g.,geometricdesign,landusecontext) 100

DetailedAnalysisbyprojectteam(e.g.,locationofdriveways,leveloftraffic) 34

Rankingbyeachteammemberandreviewbyprojectmanagers 18

Siteobservation‐datacollection 13

Figure18.RoundaboutsitesinFloridaSelectedforOperationalAnalysis



3.4.2DataCollectionInadditiontothecontextdatacollectedasapartoftheselectionprocess,theoperationalanalysisofroundaboutsrequiredthecollectionoffielddataonvehicleturningmovements,conflicts,andviolations.Duringthedatacollection,twotechniqueswereusedtogatherinformationrequiredforoperationalanalysis:siteobservationoftheflowoftrafficneartheroundabout,andvideorecordingoftheentireintersectionfollowedbymanualextractionofvideoclipswithaccessmanagementissues.Table9summarizesthefeaturesandtimeofdatacollectionfortheselectedsites.Table9.SummaryofFeaturesandSurveyTimeofSelectedRoundaboutsofThirteenRoundaboutsand

DataCollectionTimesforOperationalAnalysis

County SiteName DataCollectionDateandTime

NumberofCirculatingLanes

NumberofLegs

PresenceofDriveway

Alachua SW 2nd Ave. and SW 6th St.

4/5/13: 3:00 pm – 5:30 pm

1 4 On both access and egress approaches

Broward Margate Blvd. and NW 58th Ave

5/23/13: 7:40 am – 9:40 am

Spiral 4 On both access and egress approaches

Holmberg Rd. & Parkside Dr.

5/16/13: 3:25 pm – 5:30 pm


Duval Independent Dr. and S. Laura St.

4/23/13: 11:00 am – 2:00 pm


Miami-Dade

Biltmore Way and Sagonia St.

5/15/13: 4:50 pm – 7:15 pm


Greenway Dr. and Sagovia St.

5/14/13: 4:50 pm – 7:10 pm


NE 10th Ct. & SW 152nd Ave.

5/13/13: 5 pm – 7:20 pm


Ponce De Leon Blvd. and Ruiz Ave.

5/21/13: 4:50 pm – 7:05 pm


Orange Eagle’s Reserve Blvd. and Dyer Blvd.

4/14/13: 12:00 pm – 1:00 pm

2 4 On the access approach

Osceola MLK Blvd. and N. Central Ave.

4/5/13: 11:00 am – 12:00 pm

Spiral 4 On the egress approach

Pinellas Causeway Blvd. and Mandalay Ave.

3/22/13: 3:00 pm – 5:30 pm

2 6 In the middle of roundabout

St. Johns CR-210 and Mickler Rd.

5/9/13: 1:00 pm – 3:00 pm

1 4 In the middle of roundabout

St. Lucie CR-707 and Ave A 5/9/13: 1:00 pm – 3:00 pm


Duringthesitevisits,fiveactivitiestookplace.First,weverifiedthegeometricconditionsintheroundaboutdiagrams.Next,wereviewedthetrafficoperationsapproachingandexitingtheroundaboutbycollectingtwotofourhoursofvideodataatthepeakoperatingtimeofeachsite.Informationwascollectedonlandusesassociatedwithadjacentdrivewaysandontrafficvolumeatthelocationofaccesspointsduringthesitevisit.Trafficmovementwasvideotapedatall13selectedsites,andusefulvideoclipswithaccessmanagementissueswereextractedfortheoperationalanalysis.Thecamerasforthedatacollectionateachroundaboutwereplacedbasedonthegeometricdesignanddrivewaylocationsofeachroundabout.Figure19showsanexampleofthecameralocationforfielddatacollection.Undersomecircumstances,asshownbyCamera1



inFigure19,inordertorecorddrivewaymovementoneachsitecameraswereputfurtherawayfromtheroundabouttocapturetheinteractionbetweenadrivewayandtheapproachinglane.Cameras2and3areplacedinordertorecordthepedestrianflowandvehicleconflictsontheothertwoapproachlegsoftheroundabout.

Figure19.CameraLocationofVideoRecordingforIndependentDriveandSouthLauraStreetinJacksonville

In order to collect enough information, data collection took place during the busiest hours of operation(peakhours)ateachroundabout.Forexample,ifaroundaboutislocatedonamajorarterialsection,datawere collected during the usual peak hour. For roundabouts located near shopping centers, data werecollectedslightlylaterthanthepeakhouroronweekends.3.4.3DataAnalysisTheoperationalanalysisaimedatfindingaccessissuesrelatedtoroundabouts.Morespecifically,inthedataanalysis,weconsideredtheconflictpointsattheintersectionofdrivewaysandtheapproachinglaneoftheroundabouts,theimpactofthequeueontheoperationofnearbystop‐controlleddriveways,theconflictsbetweenvehiclesandotherroadwaysusers,e.g.,bicyclistsandpedestrians,andtheimpactofdrivingviolationsontheoperationswiththeroundabouts,e.g.,pickupanddropoffinactivedrivinglanes.Thisanalysisincludestheimpactofmedianopeningsattheapproachinglaneontheoperationoftheentireroundabout,andthequeuingassociatedwithadrivewaythatislocatedneararoundaboutwhichmaydisrupttheoperationofeitherthedrivewayortheroundabout.Thevideoscollectedduringthesitevisitswerecarefullyreviewedtoidentifythetypesofaccessissues.



3.4.4AssessmentofFDOTSoftwareforRoundaboutEvaluationSoftwarepackagesusedbytheFDOTwereevaluatedtounderstandtheircapabilitytoanalyzeroundaboutoperationsandcapacityand,inparticular,toaddress,issuesrelatedtoaccessmanagement.Bothdeterministicsoftwareandsimulationpackageswereevaluated.SoftwarepackagescurrentlyusedbyFDOT,includingHCS2010,SYNCRO,andCORSIM,arecomparedwithothersoftwarepackagestounderstandthesuitabilityofthesetoolstoevaluateaccessissues.Examplesofanalysisofroundaboutscapacity,delayandqueue,aregivenintheanalysisinordertoevaluateitseffectivenessinassessingroundaboutoperations.Wherethesetoolsmaybedeficient,recommendationsaremadeonhowtoimprovethemtomakethemmoreeffectivefortheevaluationofroundaboutsandaccessmanagement.



ChapterFour:ReviewofNationalandStatePracticesThischapterisorganizedintosixsections.First,thenationalandstateguidebooksforaccessmanagementandroundaboutsarereviewed.Second,Florida’sguidebooksaresummarized.Third,nationalandstateguidebooksthathavetakenaccessmanagementintoconsiderationinthecontextofroundaboutsarepresented.Then,roundaboutlocationconsiderationguidelinesandgeometricdesignfromthenationalandstateguidebooksarebrieflymentioned.Next,thefindingsofsafetyandoperationalanalysisofroundaboutsarepresented.Accessmanagementissuesarediscussedwithconsiderationofsafetyandoperationalaspectsofroundabouts.ThischapteralsoincludesadetaileddiscussionofthelimitationsofFlorida’sroundaboutguidebooks.4.1NationalandStateGuidebooksforRoundaboutsandAccessManagementTounderstandthestate'sroleinroundaboutdesignandaccessmanagement,weidentifiedexistingroundaboutpoliciesandguidanceatthenationallevelaswellasinall50statesandtheDistrictofColumbia.Inthissection,thenationalandstatereportsandguidesforroundaboutsandaccessmanagementidentifiedinthemethodologysectionareanalyzed.4.1.1NationalGuidanceforAccessManagementTheprimaryauthorityonaccessmanagementintheUnitedStatesistheTRBAccessManagementCommittee(AHB70).TheTRBAccessManagementCommitteealongwithFHWAandFDOTpublishedtheAccessManagementManualin2003asacomprehensiveresourceonstate‐of‐the‐artpracticesfortheuseofpractitionersandstakeholdersaffectedbyaccessmanagementactions.BesidestheAccessManagementManual,alimitednumberofguidesorinformationalreportsexistatboththenationalandstatelevelsthatincludeaccessmanagementprinciples;evenfeweraddressaccessmanagementprinciplesinthecontextofroundabouts.BasedupontheirlistingontheFHWAwebsite,thedocumentsbelowarereviewed.Thedocumentsarepresentedinreversechronologicalorder.

APolicyonGeometricDesignofHighwaysandStreets(GreenBook),6thEdition,AASHTO,2011. NCHRPSynthesis404:StateofPracticeinHighwayAccessManagement(GluckandLorenz,2010). NCHRPReport548:AGuidebookforIncludingAccessManagementinTransportationPlanning

(Roseetal.,2005). NCHRPSynthesis351:Accessrights:asynthesisofhighwaypractice.(HuntingtonandWen,2005). NCHRPReport524:SafetyofU‐turnsatUnsignalizedMedianOpenings(Potts,2004). NCHRPSynthesis337:CooperativeAgreementsforCorridorManagement(Williams,2004). TRBAccessManagementManual(TRB,2003). NCHRPSynthesisofHighwayPractice332:AccessManagementonCrossroadsintheVicinityof

Interchanges(ButoracandWen,2002). NCHRPSynthesis304:DrivewayRegulationPractices(Williams,2002). NCHRPReport420:ImpactsofAccessManagementTechniques(Gluck,Levinson,andStover,

1999). NCHRPReport395:CapacityandOperationalEffectsofMidblockLeft‐TurnLanes(Bonnesonand

McCoy,1997). NCHRPReport348:AccessManagementGuidelinesforActivityCenters(KoepkeandLevinson,

1992).4.1.1.1APolicyonGeometricDesignofHighwaysandStreets(GreenBook),6thEdition,AASHTO,2011.Thisbookcontainstenchapters:highwayfunctions,designcontrolsandcriteria,elementsofdesign,cross‐sectionelements,localroadsandstreets,collectorroadsandstreets,ruralandurbanarterials,



freeways,intersections,andgradeseparationsandinterchanges.Sectionsthatdiscussaccessmanagementarethehighwayfunctions(chapter1),accesscontrolandaccessmanagement(section2.5),elementsofdesign(chapter3),ruralandurbanarterials(chapter7),typesandexamplesofintersections(section9.3),androundaboutdesign(section9.10).Roundaboutsandthetypesofroundaboutsaredefinedinsection9.3.Section9.10includesaspectsofroundaboutgeometry,sizeandspaceneeds,andfundamentalprinciples(speeds,lanebalanceandcontinuity,appropriatenaturalpathalignment,designvehicle,non‐motorizedusers,andsightdistanceandvisibility).Sightdistance,asoneoftheaccessmanagementaspects,coverstwotypes,SSDandISD.Thisdocumentprovidesgeneralinformationontheuseofaccessmanagementmeasuresforalltypesofroadwaysforallcontextsincludingroundabouts,butitdoesnotspecifyanymeasurethatisappliedonlytoroundabouts.Detaileddesignstandardsareprovidedforlocalruralroads,localurbanstreets,special‐purposestreetssuchasrecreationalroadsandresourcerecoveryroads,collectors,arterials,andfreeways(Chapters5through8).Geometricdesignelementsincludesightdistance,vertical,andhorizontalalignment.Sightdistancefeaturesaredescribedfordifferenttypesofintersections,includingthree‐legandfour‐legwithandwithoutchannelization,androundabouts.Frontageroadsarealsoexploredbecausetheyimpactadjacentpropertiesaturbanarterialsorfreewaysthatdonothavedirectaccessduetoaccesscontrols.4.1.1.2NCHRPSynthesis404:StateofPracticeinHighwayAccessManagement(GluckandLorenz,2010).Thisdocumentprovidesacompletereviewofaccessmanagement,withtheaimofreviewingcurrentadministrationandpracticesinall50states.Surveyswereconductedatall50stateagencieswitha100percentresponserate.Thesurveyscoverthecontentofpoliciesandprograms,programimplementation,anditsreportedeffectiveness.ThereviewincludedaccessmanagementprogramsinthestatesofVirginia,NorthCarolina,Indiana,Minnesota,Oregon,Louisiana,California,andNewJersey,asspecificexamplesofcurrentpractices.Basedonthesurveyresults,moststateshaveutilizedaccessmanagementpractices,withtwo‐thirdsofthosekeepingtheformalprograms.Accessmanagementprogramsarecommonlyusedonthedrivewaypermitlevel(92%),theprojectlevel(78%),thecorridorlevel(64%)andthestatewidelevel(60%).Themostimportantaspectofimplementingaccessmanagementprogramsincludeastrongorganizationalcommitment.Meanwhile,thebarrierstoimplementationarepoliticalresistance,humanandfundingresources,andorganizationalandinstitutionallimitations.“Othercommonbarrierscitedincludedalackofeducationandtrainingopportunities,resistancebythedevelopmentcommunity,limitedcoordinationwithlocalgovernments,legalissues,andalackofvision”(pp.106,GluckandLorenz,2010).Inaddition,thissynthesisgivescompletelinkstoallaccessmanagementdocumentsmaintainedbythestateDOTsandindividualresearchers.Inconclusion,thisresearchpresentsaspectsofaccessmanagementthatmaycontributetoprogramsuccess.Theseelementsincludeastrongaccessmanagementauthority,aframeworkforanaccessclassificationsystem,anaccesscommittee,anaccountableanddedicatedstaffforaccessmanagement,accesschampions,alegalcasehistory,casestudies,educationandtraining,outreachtotheaffectedparties,stakeholderscooperation,astatewidemasterplan,andhavingmonitoringandevaluationprogramsinplace.4.1.1.3NCHRPReport548:AGuidebookforIncludingAccessManagementinTransportationPlanning(Roseetal.,2005).Thisreportdescribesbestaccessmanagementpracticesforhighwaysystemsacrossthecountry,andoffersguidanceonincludingaccessmanagementintransportationplanning.Thereportidentifiesseveralbenefitsofaccessmanagement,suchasincreasedsafetyforvehiclesandpedestrians,environmentalefficiency,accesstoproperties,protectionofphysicalintegrity,coordinationbetweenlanduseandtransportation,andprotectionoftheintendedaccessfunctionstateandregionalroadways.Itisaguidancedocumentfortheimplementationofaccessmanagementelementsonageneralscalefortransportationplanninganditrecognizesdifferentformsandstylesofaccess



managementacrossthecountry.Thereportisorganizedroundthetypeoftransportationplan;forexample:overallplanning,long‐rangeplans,andcorridorandsub‐areaplanning.Therefore,itisevidentthatthebroadrangeofvariablesandthecontext‐dependentnatureofaccessmanagementhaveresultedinfewinvestigationsatalocallevelorcasestudieswithspecificexamples.4.1.1.4NCHRPSynthesis351:Accessrights:asynthesisofhighwaypractice.(HuntingtonandWen,2005).Thepurposeofthisstudyistounderstandon‐goingpracticesofhighwayaccessmanagement.Anationalsurveywasconductedwithfollow‐upinterviewstoexplorethreespecificconcernsaboutaccessrights:acquisition,management,anddisposal.ThreecasestudieswereselectedinMontana,Ohio,andOregontoexploretheon‐goingpracticeofaccessmanagement.Whiletheacquisitionofcompleteaccesscontrolhasbeenasuccessfulmethodinreducingcurrentandfutureaccesstoaroadway,effortstoimplementpartialaccesscontrolhavenothadsimilarsuccessinsomeagencies.Inthatregard,engineeringandplanninganalysisisrequiredtoplaceboththedrivewaysandtheattachedaccesscontrolforthosedriveways.4.1.1.5NCHRPReport524:SafetyofU‐turnsatUnsignalizedMedianOpenings(Potts,2004).Thisreportcontainstheguidelinesforevaluatingvariousdesignsofunsignalizedmedianopeningsbasedonsafetyandoperationalperformance.Withthefocusonurban/suburbanarterials,thisresearchcategorizesmedianopeningsinto17typesofmedianopeningdesignsandperformsfieldstudiesat26urbansitesand12medianopeningsonruralarterials.Inaddition,thisreportpresentsthecurrentdesignpoliciesandpracticesofhighwayagenciesobtainedfrommailsurveysof35stateand30localhighwayagencies.CrashratesatU‐turnandleft‐turnmaneuversatunsignalizedmedianopeningsarelow.Morespecifically,theaverageofU‐turnplusleft‐turnaccidentspermedianopeningperyearaturbanarterialcorridorsis0.41,andthesameaverageatruralarterialcorridorsis0.20.Thisstudyrecommendsthatthemidblockmedianopeningsbetakenintoaccountasanoptionforeitherthreeorfour‐legintersections.Also,thecombinationofdirectionalmedianopeningsanddirectionalmidblockmedianopening(s)maybeconsideredasanoptiontoconventionalmedianopeningsatthreeorfour‐legintersections.4.1.1.6NCHRPSynthesis337:CooperativeAgreementsforCorridorManagement(Williams,2004).Thisresearchfocusesoncooperativeagreementsbetweentwoormoreagenciesforcorridormanagement.Theresearchexaminesongoingpracticesincooperativeagreementsbylookingatsurveysfrom22agenciesatbothstateandprovinciallevels.Fivecaseswereselected:Arkansas,Wyoming,Colorado,FloridaandCalifornia.Reviewsofthesecooperativeagreementsinclude:resolutions,memorandumsofunderstanding,intergovernmentalagreements,public‐privateagreements,andelementsofcorridor‐managementagreements.Issuesfoundoncooperativeagreementsforcorridormanagementincludetheagencies’lackofunderstandingaboutcorridormanagement,alackofagencyleadershipincorridormanagement,andoppositionfromthelocalcommunityornopublicacceptance.Intermsofimplementation,theproblemsarelocalcommitment,legalandpoliticalconcerns,andcallsfortechnicalassistance.Toreacheffectiveagreements,everyaffectedstakeholdershouldcompromiseandinteractwithothersasequalpartnersandconsiderinputfromallagenciesontheprocessesneededtoimplementthesuggestedagreement.Commonvision,anintegratedpointofviewforcorridormanagement,andthewillingnessofthosestakeholderstoworktogethertowardsthesamevision,maybuildthefoundationforeffectivecorridormanagement.4.1.1.7TRBAccessManagementManual(TRB,2003).Thismanualexploresthegeneralbenefitsofmanagingaccesstoroadways,explaininghowaccessmanagementcanbeachieved,itsaspectsandprinciples,aswellastherolesofvariousinstitutionsinaccessmanagement.



Accessmanagementaffectssafety,operations,economicfactorsrelatedtotheretailorcommercialmarketandpropertyvalues,landuse,andtheenvironment.Severalstudiesmentionedinthisreportshowedthatthecrashrateisreducedasthenumberofaccesspointspermileisreduced,whenthereisaraisedmedian,andwhenU‐turnsareaccommodatedinsteadofdirectleftturns.ThismanualalsoincludesasummaryofresearchonthesafetyandoperationaleffectsofAccessManagementTechniques(TRB,2003,p.19).Furthermore,itshowsthatbusinessowners’concernsabouteconomicdownturnareinsignificant,sinceleft‐turnrestrictionsinTexasandmedianchangesinFloridadidnotaffectthebehaviorofregularcustomers.Accessmanagementmayinfluencethesurroundingmarketareasandpropertyvalues.Evencommercialstripswithoutproperaccessmanagementmayincreaseinpropertyvalue.Furthermore,accessmanagementmayhelptosustaineconomicdevelopmentinanarea.Nevertheless,thesameareamayexperienceeconomicdeclineifpooraccessmanagementisemployed.Lastly,landuseandenvironmentaleffectsofaccessmanagementincludeaesthetics,unificationofactivitycenters,maintainingthecapacityofavailableroadways,minimizingtheenvironmentalimpactofindividualaccessroads,andmoreefficientfuelconsumption.Threebasicstepsinimplementingaccessmanagementtoaroadwayaredefiningaccesscategories,establishingaccessmanagementstandards,andassigningcategoriestotheroadwaysorroadwaysegments.Initialfactorstobeconsideredarethedegreeofroadwayimportance,roadwaycharacteristics,landuseandgrowthmanagementobjectives;andthecurrentandpredictedflowsofgeneraltransitaswellaspedestrianandbicycletraffic.Fourgeneralaspectsofdevelopingaccessmanagementstandardsincludemedians,degreeofurbanization,speed,andsafety(TRB,2003,p.71).Finally,theassignmentofcategoriesinroadwaysystemsneedstotakeintoaccountthefollowingfactors(p.77):

Theintendedfunctionoftheroadwayasacomponentofacompletetransportationsystemnetwork;

Theroadwaysegment’senvironment(ruralandundeveloped,urbanfringe,sub‐urban,urban,anddenselydevelopedorurbancore);

Theavailabilityofasupportingroadwaysystemtosupplyalternativeaccess;and Thedesiredorappropriatebalancebetweensafetyandfrequencyofaccess.

4.1.1.8NCHRPSynthesisofHighwayPractice332:AccessManagementonCrossroadsintheVicinityofInterchanges(ButoracandWen,2002).Thisdocumentreviewscurrentpracticesinaccesslocationanddesignofcrossroadsinthevicinityofinterchanges.Eightcasestudieswereselected—threefornewinterchangesandfiveforretrofitinterchanges.Varyingdegreesofaccessmanagementonthecrossroadsinthevicinityofinterchangesareemployedbystateandprovincialagencies.Therespondingagenciesinnineoutof36stateshavelegislativesupportfortheaccessspacingstandard,byadoptingthoseintoregulations.Inthisdocument,itismentionedthateventhoughagenciescouldusedifferentfactorsindeterminingaccessspacingrequirements,anumberofthemwereestablishingaspacingof100ft.forurbanand300ft.forruralinterchangesfollowingthe1991AASHTOrecommendations.Inpractice,theaccessspacingstandardsforcrossroadsrangefromzeroto1,320ft.,withonlyhalfoftheagencieshavingdetailedmethodologyforcalculatingtheactualdistance.Agenciesusefourdifferentreferencepointstomeasuretheaccessspacingdistancetothenearestdownstreamintersection.Importantfactorsthatcontributetothespacingdistanceandappropriatecrossroadlocationsare:turningmovementcomplexity,designspeed,surroundinglanduseandenvironment,crossroadclassification,andlevelofinterchange.Otherfindingsarerelatedtoissuesonputtingaccessmanagementintopractice.Barrierstoaccessmanagementimplementationcouldbeconqueredbyhavingconsistentaccessmanagementpolicies,integratingtheprocessofplanning,designing,andoperating,aswellasreservingtheinterchangefacilitiesandthedownstreamaccesslocationpointsonthecrossroads.4.1.1.9NCHRPSynthesis304:DrivewayRegulationPractices(Williams,2002).Thisresearchexaminesstateandlocalagencies’surveysfortheirdrivewaypolicies.Alongwithaliteraturereviewabout



driveways,thefollowingobjectivesarepresented:(1)reviewthecurrentpracticeofdrivewaysregulations,(2)presentstateandlocalpracticeregardingdrivewayregulations,(3)determinetheimpactofthedrivewayregulations,and(4)findtheissuesandlessonslearnedfromthecases.Suggestionsforeffectivedrivewayregulationsincludehavingconsistentdecisionsandenforcement,apre‐applicationprocess,strongstatutoryauthority,up‐to‐datedesignstandards,andfieldreviews.Otherimportantaspectsarestakeholders’activecommunicationsandcoordination,competentstaffs,andpubliceducationofdrivewayregulations.InNCHRPSynthesis304,specificdistancesfordrivewaysareprovidedforSouthCarolina.Morespecifically,atSouthCarolina,theaccessspacingstandardsdependontheoperatingspeed.Thespacebetweentwodrivewaysissettoaminimumof100ft.foroperatingspeedsof30mphorlessandtoaminimumof350ft.betweendrivewaysonroadswithspeedsof55mphormore.Thesestandardsmaybemodifiedtoaccommodateuniquecasesbutspacelessthan40ft.betweentwoone‐waydrivewaysisnowhereallowed.ThisdocumentreferstodrivewaywidthfortheWashingtoncountyinOregonwherearesidentialdrivewaymustbebetween12and24ft.wide,unlessspecialpermissionisobtainedforincreasingthewidthandacommercialdrivewayshouldbebetween15and40ft.wide.4.1.1.10 NCHRPReport420:ImpactsofAccessManagementTechniques(Gluck,Levinson,andStover,1999).Thisreportfocusesonthemethodsforevaluatingparticularaccessmanagementtechniquesintermsofsafetyandtrafficoperations.Thisresearchidentifiesavailabletechniques,andcollectsandanalyzesthemethodsanddatafromvarioussources.Theprioritiesforaccessmanagementanalysisare:

1. Trafficsignalspacing2. Unsignalizedaccessspacing3. Cornerclearancecriteria4. Accessseparationatinterchanges5. Medianalternatives6. Left‐turnlanes7. U‐Turnsasalternativestodirectleftturns8. Right‐turnlanes9. Typesofdriveways10. Frontageroads

Thisreportreachesseveralconclusions.Crashratesarehigherwheresignaldensityishigher,orwhereun‐signalizedintersectionsaremorecloselyspaced.Safetyandoperationsaspectsarebetterifthereismorecornerclearance.Safetyisalsoassociatedwithraisedmedians.Left‐turnstoragelanesupgradesafetyandcapacitybyprovidingspacesforturningvehicles.Indirectleft‐turnsorU‐turnsmayimprovesafety,capacityandtraveltime.Problemscanexistiffrontageroadsarelocatedtooclosetotherampterminal.Frontageroadsalongfreewaysmayneedtobeallocatedproperlytodecreasearterialleftturns,weavingmovements,andenhancetheaccess.4.1.1.11NCHRPReport395:CapacityandOperationalEffectsofMidblockLeft‐TurnLanes(BonnesonandMcCoy,1997).Thisresearchprovidesamethodologytoevaluatemidblockleft‐turntreatmentsandtheguidelinestoselecttheappropriateraised‐curbmedians,two‐wayleft‐turnlanes,andundividedcrosssectionsalternativesforintersections.Threemodelswereevaluated:theoperationmodel,safetymodel,andaccessimpactmodel.Datatobuildthemodelscamefrom32fieldstudiesineightcitiesandfourstates,alongwithinformationobtainedfromtheinterviewsof165businessownersandmanagerswithbusinessesalongfourarterialsinfourcitiesandthreestatesand117additionaltrafficsimulationrunstoobtainmoretrafficdata.Whilethisresearchwascompletedneartraditionalsignalizedandunsignalized



intersections,theconcernsraisedheremaybeapplicabletomid‐blockleft‐turntreatmentsnearroundabouts.Thisresearchfocusesonthetwotreatments—anundividedcrosssectionandtwo‐way‐left‐turnlanes(TWLTL).Importantfindingsfromthisresearchinclude:(1)decreasingperformanceofunsignalizedintersectionswhentheproximitybetweenintersectionsiscloser,(2)anundividedcrosssectionmaygivemoredelaythantheraised‐curbmedianandTWLTL,(3)whenthedemandis40,000vehiclesperdayorless,anyoftheleft‐turntreatmenttypesperformswithoutcongestion,(4)safetyanalysisshowshigherfrequencyofcrashesonstreetsegmentswithhighertrafficdemandsanddenserdrivewaysandpublicstreets,(5)fieldstudiesshownochangeintheprovidedaccesstoadjacentpropertiesaftertheretrofitofleft‐turntreatment,(6)businessownersbelievethatchangingfromanundividedcrosssectiontoeither330‐ft‐openingsofraised‐curbmedianorTWLTLmayenhancebusinessandtrafficconditions;meanwhile,theyalsobelievethat660‐ft‐openingsmaynotimprovethoseconditionsifthechangingoccursfrom330‐ft‐openingsofraised‐curbmedianorTWLTL,and(7)businessownersconsiderthatcustomersholdserviceorqualitytobemoreimportantthanpropertyaccess.4.1.1.12NCHRPReport348:AccessManagementGuidelinesforActivityCenters(KoepkeandLevinson,1992).Thisreportprovidestheaccessmanagementguidelinesforactivitycenter.Althoughitfocusesonaccessmanagementnearactivitycenters,theprinciplesdiscussedinthisdocumentcanbemoregenerallyapplicabletotheuseofaccessmanagementinothercontexts.Overall,thepurposeofaccessmanagementis“topreservethefunctionalintegrityandoperationalviabilityoftheroadsystem(p.1)”.Takingthemaindefinitionofaccessmanagementas“theprocessthatprovidesormanagesaccesstolanddevelopmentwhilesimultaneouslypreservingtheflowoftrafficonthesurroundingroadsystemintermsofsafety,capacityneeds,andspeed”(KoepkeandLevinson,1992,p.1),thisdocumentconsidersthreekeyelementsforaccessmanagement:(1)specifyingthecontrolaccesswithvariousroadwayclassifications,(2)identifyingamethodtohavespecialpermissiononceitwasdeterminedthatproperaccesscouldnotbebuilt,and(3)findingwaystoimplementthestandards.Thedocumentpresentstherevisedguidelinesformanagingaccessonstreetsandhighwaysinthevicinityofactivitycenters.Theinformationprovidedwasobtainedbyinterviewingstateandlocalgovernmentofficials,aswellasactivitycenterdevelopersandmanagers.Thisreportdiscussesthebenefitsofaccessmanagementincludingreducingdevelopmentcostsandincreasingsafety.Thetenchaptersofthisdocumentfocusonthebroadguidelinesforbuildingupaccessmanagementprograms.Intheend,thisdocumentproposesthatprogramsshouldhaveproperaccessmanagementcodesthatincludeaccesscontrolandspacingcriteria;designstandards;andtrafficpermitproceduresandrequirements.4.1.2States’GuidanceforAccessManagementStateDocumentsthatrefertoaccesselementsareroadwayorhighwaydesign/manuals,accessmanagementmanualsanddrivewaymanuals.ThelisteddocumentscanbefoundanddownloadedfromstateDOTwebsitesaboutAccessManagementandfromNCHRPSynthesis404,StateofPracticeinHighwayAccessManagement(GluckandLorenz,2010),whichincludesinformationonwheretofindeachstatedocumentonaccessmanagement.Twenty‐oneDOTsincludeaccessmanagementdocumentsontheirwebsites.ThecompletelistandstateDOTwebsitelinkscanbefoundinAppendixB.Mostwebpagescontaininformationaboutaccessmanagement,andtheaspectsthatshouldbeconsidered.Thewebsitesalsoincludelinkstodesignmanualsandotherrelateddocuments.



4.1.2.1 AccessManagementGuidelines.Table10showsthatstateDOTshavevarioustypesofdocumentsmentioningaccessmanagement.Forty‐threestates,includingtheDistrictofColumbia,haveincorporatedaccessand/oraccessmanagementintotheirplanninganddesignpolicies.Morespecifically,19stateshaveaccessmanagementmanuals,separatefromgeneraldesignmanuals.ElevenstateDOTsmentionaccessmanagementordesignmanuals,whileanother16DOTshaveotherrelateddocumentswithothernames.ThelinkstothosedocumentscanbefoundinAppendixB.Table10.MainDocumentsoftheAccessManagement‐RelatedStateDOTsGuidebooks

AccessManagementManual/Guidebook

Roadway/HighwayDesignManual

OtherRelatedDocuments

Alabama(2013)Florida(2009)Idaho(2001)Indiana(2009)Iowa(IowaDOT,2012)Kansas(2013)Michigan(2001)Minnesota(2008)Mississippi(2012)Missouri(2003)Nevada(1999)NewJersey(2013)NewMexico(2001)Ohio(2001)Oregon(2012)SouthCarolina(2008)Texas(2011)Vermont(1999)Virginia(2007)

Arizona(2012)California(2012)Connecticut(2012)Illinois(2010)Massachusetts(2006)Montana(2007)NewYork(2002)Utah(2007);NorthDakota(2009)SouthDakota(web,2013)Washington(2012)

StateHighwayAccessCode/Manual:Colorado(1998)Delaware(2011)DistrictofColumbia(2010)Maryland(2004)Wyoming(2005)DrivewayManualor/andEncroachmentControl:Georgia(2009)WestVirginia(2004)AccessConnectionPolicy/Rules:Louisiana(2012)Maine(2005)AccessControlPolicy:Nebraska(2006)Washington(2009)Wisconsin(FDM,2011)RightofWayManual:Utah(2006)Montana(2007)DrivewayPermit/Access:NewHampshire(2000)NorthCarolina(2003)

Source:CompilationfromDOTwebsitesTheformatofthesemanualsandguidebooksissimilartotheNCHRPSynthesis404,StateofPracticeinHighwayAccessManagement.However,thisreportupdatestheNCHRPSynthesisreport,whichwascompletedin2010,becausemanystatespreparedorrevisedtheirguidelinesaftertheNCHRPstudy.Ofthe43statesthathaveaccessmanagement‐relateddocuments,16stateguidelines,includingWashingtonDC,weredevelopedduringorafter2009.Asahighlight,StateofPracticeconductedsurveysofall50statesandobtainedcomprehensiveinformationaboutthestateDOTprogramelements.ThesurveyresponsesareshowninAppendixC(GluckandLorenz,2010,p.47).Incontrast,thisresearchexploresDOTwebsitesandlocatesaccessmanagementdocumentsandresourcesonthosesites.4.1.3NationalandStateGuidebooksforRoundaboutsNationalGuidebooks.Severalnationalguidebookswerewrittenaboutroundaboutsastheybecamemorepopularandgainedsupportfromdesignersandcommunitiesaroundthecountry.ThefirsthighwayguideforroundaboutswaswrittenbyFHWAinthelate1990s.BoththeAASHTOPolicyonGeometricDesignofHighwaysandStreets(2011)andtheFHWARoundabouts,AnInformationalGuide(Robinsonetal.,2000)providethecurrentnationalstandardondesignguidelinesforroundabouts,aswellasallothertraffic



engineeringanddesignaspectsacrossthecountry.OthernationalguidebooksandreportsthatgovernroundaboutdesignintheUnitedStatesincludethefollowingNCHRPreports: NCHRPReport672:Roundabouts:aninformationalguide.Vol.672,(Rodegerdtsetal.,2010). NCHRPReport674:CrossingSolutionsatRoundaboutsandChannelizedTurnLanesforPedestrianswithVisionDisabilities,(Schroederetal.,2008).

NCHRPReport572:RoundaboutsintheUnitedStates.Report572,(Rodegerdtsetal.,2007). NCHRPSynthesis264:ModernroundaboutpracticeintheUnitedStates,(Jacquemart,1998).

4.1.3.1NCHRPReport672:Roundabouts:aninformationalguide.Vol.672,(Rodegerdtsetal.,2010).Thissecondeditionoftheroundaboutguideiscomprehensive,coveringplanning,operation,safety,geometricdesign,trafficdesignlandscaping,andsystemconsiderations.Inonesectiononplanning,thisdocumentcomparesoperationalperformancefromtheroundaboutswithintersectioncontrols,suchasTWSC,AWSC,andsignalcontrol.Theoperationsectionincludescapacityandperformanceanalysisoftrafficoperation,e.g.degreeofsaturation,delay,queuelength,andfieldobservation.Specificallyforgeometricdesign,thisdocumentexplainshowtodesignroundaboutswith: Designspeed; Vehiclepaths; Inscribedcirclediameter; Designvehicle; Non‐motorizeddesignusers,entrywidth(tapperlength,additionallanelength,andflarelength); Circulatoryroadwaywidth; Centralisland; Entrycurvesandexitcurves; Pedestriancrossinglocationandtreatment; Splitterisland; Stoppingsightdistance(SSD); Intersectionssightdistance; Verticalconsideration(profiles,super‐elevation,anddrainage); Bicycleprovisions; Parkingandbusstoplocations;and Right‐turnbypasslanes.

Thesedesignstandardsarespecifiedfordouble‐laneroundaboutsandruralroundabouts.Specificdesignsincludeentrycurves,andexitcurvestoavoidpathoverlapindouble‐laneroundabouts;visibility,curbing,splitterisland,andapproachcurvesforruralroundabouts.Additionally,theseguidelinesexploremini‐roundabouts,whicharenotincludedinthisresearch.Inthesafetysection,thisdocumentreviewsconflictpointsfordifferentusers,andcommoncrashtypesinroundabouts.Signage,pavementmarkings,illumination,workzonetrafficcontrol,andlandscapingareexploredinthesectionontrafficdesignandlandscaping.Inthelastsection,systemconsiderationsfocusontrafficsignalsatroundabouts,at‐graderailcrossings,closelyspacedroundabouts,roundaboutinterchanges,roundaboutsinanarterialnetwork,andmicroscopicsimulation.However,thisdocumentdoesnotexplorehowroundaboutscanaccommodatelargevehiclesorhowtodesignthemwithmorethantwoentrylanes.Itdoesnotincludeinformationaboutspecific“legalorpolicyrequirementsandlanguage.”ThisreportistheonemostfrequentlyadoptedbystateDOTsfortheirroundaboutdesignorguidedocuments.4.1.3.2NCHRPReport674:CrossingSolutionsatRoundaboutsandChannelizedTurnLanesforPedestrianswithVisionDisabilities,(Schroederetal.,2008).Thisdocumentdiscussesthesafetyof



roundaboutsandchannelizedintersectionsforpedestrianswithvisiondisability.Theauthorsconductedthestudyusinganexperimentaldesign(beforeandafter)fortreatmentinstallations,pedestrianmodels,andsimulation.Treatmentsforpedestriansincludedthepedestrian‐actuated,flashing‐yellowbeacon,andon‐pavementsoundstripsforvisually‐impairedpedestrians.Thestudytookplaceonsingle‐laneanddouble‐laneroundabouts.TheformerwereinCharlotte,NC;Raleigh,NC;andGolden,CO,andthelatterinGolden,CO.Thestudyincludesmeasuresforcrossingopportunity,utilizationofcrossingopportunity,delay,andsafety.Oneoftheconclusionsisthatdelayisreducedafterthetreatmentforsingle‐laneroundabouts.Inotherwords,accessibilityforpedestriansisimproved.However,thetwo‐laneroundaboutischallengingandmaynotbeaccessibleforpedestrianswithvisiondisability.4.1.3.3NCHRPReport572:RoundaboutsintheUnitedStates.Report572,(Rodegerdtsetal.,2007).Themainpurposeofthisresearchwastodescribethemethodsofpredictingsafetyandoperationalaspectsofroundabouts.Inaddition,thisreportalsomodifiedthedesigncriteriarelatedtothesafetyandoperationsofroundabouts.Thedocumentincludesfourmainsections:safetyperformance,operationalperformance,geometricdesign,andpedestrianandbicyclistobservation.InadditiontoanalyzingtheapplicabilityofvariouscrashpredictionmodelstotheUnitedStates,thisdocumentinvestigatessafetyperformanceofroundaboutsusinganempiricalBayesbefore‐afterprocedure.ThisstudyfoundlargesafetyimprovementsfromconvertingTWSCandsignalizedintersectionsintoroundabouts,butfoundnosafetyimprovementcomparedtoAWSCintersections.Additionally,safetyimprovementsforsinglelaneroundaboutsweregreaterthanmulti‐laneroundabouts.ThisstudyalsofoundthatruralroundaboutshadgreatersafetyperformancethanurbanorsuburbaninstillationsandthatanysafetybenefitdeclinedwithincreasesinAADT.Next,theoperationalperformancereviewincludedentrycapacityandcontroldelaymodelsforone‐laneandmultilaneroundabouts.Ingeneral,thisstudyfoundthatexistingmodelsdoapoorjobofestimatingthecapacityforroundabouts.Tocorrectfortheseerrors,theauthorsproposeaseriesofcapacitymodelsthataremoreeffectivethanexistingmodelswithcalibration.However,controldelaymodelswerefoundtobeeffective.ThisstudyconcludesthatLOScriteriaforroundaboutsaresimilartothoseatunsignalizedintersections.Furthermore,aspectsofdesignthatmaybeimportanttoconsiderare:accelerationanddecelerationeffectsonspeeds,ISD,anddesigndetailonmultilaneroundaboutssuchasvehiclepathalignment,lanewidth,anddriverinformationregardinghowtouselanemarkings.Moreover,thisstudydidnotfindanysignificanteffectsofsafetyforpedestriansandbicyclists.Inaddition,thereisconcernaboutthedesignofexitlanestoincreasetheawarenessofpedestriansincrosswalks.Multilaneroundaboutdesignshouldcarefullyavoidpathoverlap,andcrosswalkvisibilityneedstobecarefullydesignedtoaddressthereducedtendencyofdriversinmultilaneroundaboutstoyieldtopedestrians.4.1.3.4NCHRPSynthesis264:ModernroundaboutpracticeintheUnitedStates,(Jacquemart,1998).Thisreportpre‐datesothernationalresearchonroundabouts.ThereportexploredNorthAmerican(i.e.,U.S.andCanadian)practicesatthetimeitwasdeveloped(1998).ItalsoprovidesexamplesofguidelinesfromAustralia,theUnitedKingdom,France,SwitzerlandandGermany.Specifictopicsaddressedincludesafety,capacityanddelay,issuesofroundaboutsforvarioususers,locationcriteriaforroundabouts,andexamplesoftheuseofroundaboutsintheUnitedStates.ThissynthesisincludestheresultsofasurveyconductedamongallstateDOTsintheUnitedStatesaswellastheircounterpartsintheCanadianprovinces.ThesurveyincorporatedtheresponsesofthosestateDOTsregardingthewillingnesstobuildmoreroundaboutsintheirjurisdiction,anddesignguidelinesfromothercountriesorstatesthattheyusedasprecedence.Specifically,formakingasafetyanalysisfieldstudy,thisresearchincludedasafetyanalysisthatexaminedbeforeandafterscenariosof11roundabout



sitesintheUnitedStates.Afterroundaboutswereinstalled,thetotalnumberofcrasheswasreducedby37%atthese11sites.Theauthorsfoundthatthesizeofroundaboutdiametersaffectthenumberoftotalcrashesandinjurycrashes,assmallerdiametersof37m.or121ft.showa53%decreaseintotalcrashesanda73%dropininjurycrashes.Overall,thesamplesofthisstudyshowedadecreaseindelaysofabout75%withtheroundaboutscomparedtopriortrafficcontrolmethodsatintersections.Issuesconcerningpedestriansandbicyclistswererelatedto“theabsenceofclearright‐of‐waycontrol(p.2).Inthecaseofone‐laneandlow‐speedroundabouts,itwassuggestedthebicyclelaneshouldmergeintotheroundaboutandthebicyclistshouldsharethelanewiththecars.Formulti‐laneroundabouts,itwasrecommendedthatbicyclistsshouldhaveseparatebikepaths,beassignedtoasharedpathwithpedestrians,orbererouted.Thissynthesisshowsthemarkedbenefitsofroundaboutsregardingsafety,delay,andcapacity.Inaddition,thisresearchagreesthatroundaboutsprovideaestheticandurbandesignbenefits.4.1.4StateGuidanceforRoundaboutsThestateguidebooksareusuallymentionedonstateDOTwebsites.Twenty‐sixstateshaveroundaboutwebsiteswithvaryingdegreesofinformation.Linkstootherstates’roundaboutwebsitesandnationalguidelinesarealsofoundonmostofthosewebsites.Inadditiontonationalguidanceonroundabouts,accessmanagement,safety,andcapacity,ahandfulofstatesareleadingthewayinprovidingstatewideguidancethatsupplementsthenationalguidance.Thosestatessupplementthenationalguidancewithvarioustypesofstate‐leveldocuments.Forexample,manyincludedtheroundaboutdesignontheroadwaymanual.Somestateshavespecificlinkstothedesignofroundabouts.Furthermore,VirginiaDOTplacedtheroundaboutdesignintheaccessmanagementguidance,whichrelatestothepurposeofthisproject.TheactivitiesoffourteenstatesincludingArizona,California,Iowa,Kansas,Kentucky,Maryland,Michigan,Minnesota,NewHampshire,Pennsylvania,Virginia,Washington,andWisconsinwereselectedforfurtherexaminationbecausetheyhaveadditionalguidancebeyondthatprovidedinnationaldocuments.Thesearedescribedindetailbelow.RoundaboutguidanceinFloridaisalsoreviewedingreatdetaillaterinthischapter.Thisreviewincludestheextentofroundaboutinformation,roundaboutusers’guide(s),existingroundaboutdesignguidance,accessmanagementguidance,anddrivewayspacinganddesignguidance.SeveralofthestateguidebooksbasetheirguidanceontheFHWARoundabouts:AnInformationalGuide(Robinsonetal.,2000)andNCHRPReport672:Roundabouts,AnInformationalGuide,SecondEdition,(Rodegerdtsetal.,2010).Particularattentionisgiventostateguidanceonaccessmanagement,driveways,safety,androundaboutcapacityastheyapplytoroundabouts.

Table11.RoundaboutStates’DesignGuidebooksReviewedinthisDocumentRoundaboutGuideDocument

FacilityDevelopmentManual

AccessManagementDesignStandard

RoadwayorHighwayDesignManual

Florida(1996,2000,2012)Arizona*(2003)Kansas(2003)Pennsylvania(2007)California(2007)Iowa(2008)Michigan(2011)Maryland(2012)

Wisconsin(2011) Virginia(2007)

NewHampshire(2007)Iowa(2009)Minnesota(2009)Kentucky(2010)Maryland(2011)Washington(2011)Arizona(2012)

*‐cannotbeaccessedonline



Arizona.Roundabouts:AnArizonaCaseStudyandDesignGuidelines(Leeetal.,2003)andRoadwayDesignGuidelines,Section403(AzDOT,2012)aretwodocumentsfromArizonaDOT(AzDOT).Thefirstisa260‐pagedocumentthatdiscussesthecasestudiesofroundaboutsinArizona.Thesecondincludesasix‐pagesectiononroundaboutdesign.BothdesignmanualsfollowthenationalguidelinesaboutroundaboutsCalifornia.ThemaindocumentaboutroundaboutsinCaliforniaisRoundaboutGeometricDesignGuidance(Caltrans,2007).This113‐pagedocumenthasthreemainchapters:vehicleoperationsassessment,pedestrianandbicycleconsiderations,andgeometricdesignconsiderations.TheresearchestablishespoliciesandstandardsforCaltransroundabouts.Theresearchfoundthatthesuccessfulperformanceofaroundaboutismorearesultofoutputs(operationalandsafetyperformance,andaccommodationofusers)thaninputs(individualdesigndimensions).ThisdocumentrecommendedmodificationofRoundabouts:AnInformationalGuide(Robinsonetal.,2000)inregardtoaccelerationanddecelerationeffects.Iowa.ThePlanning‐LevelGuidelinesforModernRoundabouts,TechnicalMemorandum(HallmarkandIsebrands,2008)andDesignManualChapter6,GeometricDesign,6A‐3ModernRoundabout‐GeneralGuidance(IowaDOT,2009)arethetwoguidancedocumentsusedforroundaboutsinIowa.Thefirstisa32‐pagedocumentthatprovidestheIowaDOTwithinformationandguidanceonroundaboutpolicies,designguidelines,andpubliceducation.Itdevelopsaroundabouttaskforce,documentsbestpracticesofstateswithsuccessfulroundaboutprograms,developsimplementationguidelines,developsdraftroundaboutpolicies,andassistsinpubliceducationaboutroundabouts.Theseconddocument,writtenbytheIowaDOT,isaseparatechapteroftheGeometricDesignmanual.Asectionofthechapter(16pageslong)focusesonmodernroundaboutsforIowa.Kansas.KansasRoundaboutGuide,ASupplementtoFHWA’sRoundabouts,AnInformationalGuide(Kittelson&Associates,andTransystemCorporation,2003)isa176‐pagedocumentthatshowssupplementalaspects,suchasdifferentiatingtrafficcirclesfromroundabouts,anddetailingroundaboutselectioncriteria.Thisincludesaddingroundaboutcategoriesonthedesigncharacteristictable(whetherurbanorruralroundaboutsandwhethersingleordoublelane),aswellasdetailsofthedesignprocess.TheguidehighlightsfiveprojectsinKansaswithrespecttocurbandpavementdesign,signageonurban,suburban,multilaneroundabouts,luminanceforintersectionsbasedonpavementclassification(thePortlandcementconcretesurfaceandtypicalasphaltsurface),androadwayclassification.Kentucky.KentuckyTransportationCabinet(KYTC)hasDesignGuidanceforRoundaboutIntersection(KYTC,2010)toprovidespecificexplanationsofhowKentuckymayreviewandapproveroundabouts.Thisdocumentalsolooksatwarrantanalysisandoperationalanalysisfortrafficdynamics.Theoperationalanalysistakesintoaccounttheaspectsthatimpactroundaboutcapacity,suchasgeometricdesign,andcriticalheadway.Maryland.TwodocumentsfromMarylandDOTare:Chapter3C—RoundaboutMarkings(RoundaboutDesignGuidelines,2011),andRoundaboutDesignGuidelines(MarylandStateHighwayAdministration,2012).Thefirstdocumentincludesmarkingsforone‐,two‐,andthree‐laneroundabouts,aswellascrosswalk,pedestrian,andbicyclistmarkingsinroundabouts.Theseconddocumentcoversdesignandoperationsaspectsforroundabouts.Michigan.ThefirstdocumentaboutroundaboutsinthestateofMichiganisEvaluatingthePerformanceandSafetyEffectivenessofRoundabouts(Bagdade,etal.MichiganDepartmentofTransportation,2011).ThisdocumentcompilesthegeometricfeaturesandcrashhistoryofroundaboutswithinMichiganandalsopresentstheSafetyPerformanceFunctions(SPFs)andCrashModificationFactors(CMFs)forroundaboutsinthestate.



Minnesota.MnDOThasroundaboutdesignguidelinesintheRoadDesignManual:Chapter12—DesignGuidelinesforModernRoundabouts(MnDOT,2009).Itshowstheenhancementtableoftypicalinscribedcirclediameterswithdailyservicevolumes,intersectioncontrolevaluationpolicy,asiterequirementsection,andspecialdesignfeaturestoaccommodatespecificlanduses.Additionally,thisdocumentsuggestsRODELandAssessmentofRoundaboutCapacityandDelay(ARCADY)astoolsforintersectioncontrolevaluations.NewHampshire.NHDOThasSupplementalDesignCriteria(NHDOT,2009).Thisisafive‐pagedocumentthatsupplementstheFHWARoundabouts:AnInformationalGuide(Robinsonetal.,2000)guidelinesforroundaboutdesignonNewHampshirestate‐maintainedroadways.Itmentionsconsiderationsforroundaboutdesign,includingoperations(withattachedcapacityworksheet,andRODELsetting),andgeometricdesign.Pennsylvania.ThemaindocumentaboutroundaboutsinPennsylvaniaistheGuidetoRoundabouts:PublicationNo.414(PennDOT,2007).This236‐pagedocumentsupplementsthepedestrianprovisionsofFHWA'sRoundabouts,AnInformationalGuide(Robinsonetal.,2000)andprovidesconsistentinformationregardingtheplanning,design,construction,maintenanceandoperationofroundaboutsinPennsylvania.Thisdocumentalsopresentsdetailedrequirementsfordetectablewarningsurfacesandotherpedestrianfeatures.Virginia.Virginia’saccessmanagementdocument,AccessManagementDesignStandardsforEntrancesandIntersection(VDOT,2007),includesinformationaboutroundaboutinChapterF‐40Section2,IntersectionDesign;SpacingStandard.This115‐pagedocumentexplainstheprocessofroundaboutdesigninVirginia,accessmanagementforhighways,andpedestrian/bicyclistsafety,bymanagingthenumberofentrancesandrestrictingaccessfromoneormoredirections.Thestatehasadoptedapolicyonintersectiondesignthatincludesthefollowingprinciples:limitthenumberofconflictpoints,coordinatedesignandtrafficcontrol,avoidcomplexmaneuvers,separateconflictpoints,favormajorflows,segregatemovements,accommodatepedestriansandbicyclists,considerthedesignvehicle,andconsideraroundaboutdesign.Washington.TheWSDOTDesignManual—Chapter1320Roundabout(WSDOT,2011)istheprincipaldocumentaboutroundabouts.A50‐pagesectiongivesinformationaboutprocedurestodesignaroundaboutinthestateofWashington.Section1320.11referstoaccess,parking,andtransitfacilitiesaround.Roundabouts.Morespecifically,thechapterincludesinformationrelatedtocornerclearance,parallelroundabouts,U‐turns,parking,andtransitstopsinthevicinityofroundabouts.Thisguidanceindicatedthatnoroadapproachconnectionstothecirculatingroadwayareallowedatroundaboutsunlesstheyaredesignedaslegstotheroundabout(WSDOT,2011).Fordrivewaysclosetoroundabout,thisguidancesuggestedthatitisdesirablethatroadapproachesnotbelocatedontheapproachordeparturelegswithinthelengthofthesplitterisland(WSDOT,2011).Theminimumdistancefromthecirculatingroadwaytoaroadapproachiscontrolledbycornerclearanceusingtheoutsideedgeofthecirculatingroadwayasthecrossroad(WSDOT,2011).Right‐in/right‐outdrivewaysarealsopreferredwhendesigningdrivewayclosetoroundabout.Wisconsin.ThemaindocumentforroundaboutguidelinesinWisconsinisChapter11,Section26:Roundabouts(WisDOT,2013).This79‐pagereportprovidesthegeneralguidelinefordesignandconstructionofroundabouts.Italsoprovidesthefirstsupplementaryguidanceforshared‐usepathsforbicyclists.Thisguidelineconsidersthreeaspectsrelatedtothelocationofdrivewaysontheroundaboutentryorexit:volumeofdriveways,operationalimpact,andsightdistancebetweenusers.Inaddition,thechapterexplainstheRODELsoftwareindetail.ThischapteriscurrentlybeingupdatedandHCM2010,usinglocallydevelopedgapparameters,willreplaceRODELasthesoftwaretooltoanalyzeroundaboutcapacityandoperations(PatrickFlemming,PersonalCommunication,June25,2013).



4.2StateofFloridaGuidance4.2.1AccessManagementGuidanceinFlorida.TheFDOTSystemsPlanningwebsite(FDOT,2014)doesnotspecificallyaddressplanningforroundabouts.However,when‘roundabout’wasusedasthekeywordonthesearchengine,severalinformationaldocumentsappear.TheFloridaDOT’sAccessManagementsiteprovidesdefinitionsandcontainsinformationaboutpermits,training,anddocumentsforaccessmanagement,butdoesnotprovidespecificguidanceonaccessmanagementnearroundabouts.Floridahastwomajorhandbooksrelatedtoaccessmanagement.Thefirst,theFDOTMedianHandbook(2006)isan81‐pagereportthataddressesseveraldesignconsiderationsrelatedtoroundabouts.However,itdoesnotexplicitlydetailanythingaboutroundaboutdesignoraccessmanagement.TheFDOTDrivewayInformationGuide(2008)isa94‐pagereportthataddressesseveralguidelinesfordrivewaydesigninFlorida,suchassightdistanceatdriveways,drivewaylocation,andpedestrianfactors,butdoesnotmakeanyreferencetoroundabouts.ThefollowingsectionsreviewaccessmanagementtechniquesinFlorida.Theseincluderoadwayclassification,drivewaydesignandspacing,cornerclearance,medianopeningdesign/spacing,sightdistance,turn‐lanelocationanddesign,andauxiliarylaneanddesign.RoadwayClassification.FDOT’sStateHighwayAccessManagementClassificationSystemandStandards(FDOT,2010)containsroadwayclassificationsbasedonaccessclass,segmentlocationandapplicablespacingstandards.FDOTsegmentsaccessintosevenclasses:(1)Accessclass1isforlimitedaccessfacilitiesthataredesignedforhighspeedandhighvolumetraffic(e.g.,interstatehighwaysandFlorida’sTurnpike;(2)accessclass2roadwaysarehighlycontrolledaccessfacilitiesdistinguishedbytheabilitytoservehighspeedandhighvolumetrafficoverlongdistancesinasafeandefficientmanner;(3)accessclass3roadwaysarecontrolledaccessfacilitieswheredirectaccesstoabuttinglandiscontrolledtomaximizetheoperationofthethroughtrafficmovement;(4)accessclass4roadwaysarecontrolledaccessfacilitieswheredirectaccesstoabuttinglandiscontrolledtomaximizetheoperationofthethroughtrafficmovement;(5)accessclass5roadwaysarecontrolledaccessfacilitieswhereadjacentlandhasbeenextensivelydevelopedandwheretheprobabilityofmajorlandusechangeisnothigh;(6)accessclass6roadwaysarecontrolledaccessfacilitieswhereadjacentlandhasbeenextensivelydeveloped,andtheprobabilityofmajorlandusechangeisnothigh;and(7)accessclass7roadwaysarecontrolledaccessfacilitieswhereadjacentlandisgenerallydevelopedtothemaximumfeasibleintensityandroadwaywideningpotentialislimited. AvisualdepictionofhowFlorida’sroadwaysystemfitsinwiththeaccessmanagementclassificationsisshowninFigure20:



Figure20.RoadwayFunctionClassificationinFlorida(FDOT,2010,p.24)Eachoftheseroadwayclassificationshasasetofspacingstandardsandotherassociatedaccessmanagementcategories.Forclass1roadways,decisionsonspacingarebaseduponwhetherasegmentislocatedwithinaCentralBusinessDistrict(CBD)orCBDfringeforcitiesinurbanizedareas.Thespacingisonemileintheexistingurbanizedareasotherthantype1;2milesinthetransitioningurbanizedareas;3milesinurbanareasotherthanareas1and2;and6milesinruralareas,respectively.Otherclasseshaveconnectionspacingstandardsbasedonthepostedspeedlimit.Class2toClass7aredefinedasfollows,accordingtotheirrestrictivenessfromthemosttotheleastrespectively(FDOT,2010,p.67).Accessclass2isfurtherdistinguishedbyahighlycontrolled,limitednumberofconnectionsandmedianopenings,andinfrequenttrafficsignals.Thelandadjacenttoaccessclass3and4roadwaysisgenerallynotextensivelydevelopedand/ortheprobabilityofsignificantlandusechangeexists.Theseroadwaysaredistinguishedbyexistingorplannedrestrictivemedians.Accessclass5roadwaysarealsodistinguishedbyexistingorplannedrestrictivemedians.Accessclass6roadwaysaredistinguishedbyexistingorplannednon‐restrictivemediansorcenterlines.Accessclass7includesonlyroadwaysegmentswherethereislittleintentoropportunitytoprovidehigh‐speedtravel.Exceptionstoaccessmanagementstandardsinthisaccessclassmaybeallowedifthelandownersubstantiallyreducesthenumberofconnectionscomparedtoexistingconditions.Theseroadwayscanhaveeitherrestrictiveornon‐restrictivemedians(FDOT,2010).DrivewayDesignandSpacing.Inexplainingthedrivewaydesign,FDOTprovidesthefollowingfiguretounderstandtheelementsofdrivewaylocation.

Figure21.DrivewayDesignandSpacing(FDOT,2008,p.9)



ThedrivewayfeaturesillustratedinFigure21aredescribedindetailinpage9oftheDrivewayInformationGuide(FDOT,2008)andarealsoprovidedbelow:

Radius(R)–sizeofcurvedapproach/exitofdriveway Flare(F)–sizeofangledapproach/exitofdriveway Width(W)–spaceforvehiclesoperatingondriveway DrivewayDistance(D)–orspacingbetweendriveways CornerClearance(C)–similarto(D)butmeasuredfromamajorintersection Angle(Y)–angleofdriveway Setback(G)–distancefrompublicrightofwaytothecloseststructure SightDistance–lengthofroadvisibletothedriverrequiredforvehiclestomakesafemovements DrivewayLocation–positionofdrivewayinrelationtoothertrafficfeaturessuchasintersections,

neighboringdriveways,andmedianopenings DrivewayLength–(alsocalled“throatlength”)distanceneededintositetotransitionvehiclesto

theinternalcirculationsystemofthesite Grade–slopeofdriveway DrivewayTrafficSeparators/ChannelizingIslands–sizeandpositionofbarrierseparating

trafficmovementsonthedriveway RightTurnLanes–separatelanesonroadwaytofacilitaterightturnsintodriveway Structure–Building,GasIsland,Gate,etc.

FollowingNCHRPReport548AGuidebookforIncludingAccessManagementinTransportationPlanning(Roseetal.,2005,p.40),FDOT’sDesignStandardsclassifiesdrivewaysbasedontheexpectedvolumeandthetypeoftraffic.ThedesignstandardsfordrivewaysarefoundinStandardIndex515(FDOT,2010).Additionally,FDOTgiveslanduseexamplesofeachcategory.Forinstance:thefirstcategoryhasexamplesofoneortwosingle‐familyhomes;thesecondcategoryhasthreeto60housingorapartmentunits,smallofficesinconvertedhomes,or“momandpop”businesses;thethirdcategoryhassmallstripshoppingcenters,andgasstation/conveniencemarkets;andthelastcategoryhasanexampleofa150,000‐ftshoppingcenter,grocery/drugstorewithtento15smallerstores.FDOTshowstheconstructiondesignsfortwoprimaryshapes:“curbedflareddrivewayorthedroppedcurb”andthe“radialreturn.”Unlessthedrivewaysarehighervolume,thestandardsfor“curbedflareddriveway”arepredominantinurbanroadways.However,afewruralroadwaysmayhavecurbsandgutters.Forruralroadways,FDOTsuggestsfollowingtheroundedradialreturndesign.TheDrivewayInformationGuidealsoexplainshowthedrivewayshouldintersectwithon‐streetparkingorbike‐lanes,andwheretheeffectiveturningradiusshouldbeincreasedfromaround6to14ft.ThecurbandeffectiveradiusaredisplayedinFigure22.

Figure22.EffectiveRadiusandCurbRadius(FDOT,2008)



Additionally,thedrivewaydesigncriteriaforseverallanduses,suchasshoppingcenter,officecomplex,andconveniencestoresaresuggested.ThisstandardisadaptedfromTransportationandLandDevelopment(2002)(seeAppendixB,otherstaterelateddocuments,andFloridaMedianHandbook(FDOT,2006)).Ramp design spacing is also explained in this document. It is based on area types, such as urbanized,transitioning,andrural,aswellasassumedpostedspeed.FDOThastherecommendedminimumspacing.Thedimensionoframpdesignspacingiscalculatedfromonoroff‐ramp,asdisplayedinFigure23.FDOTrefers to the NCHRP Report 420 ImpactsofAccessManagementTechniques for minimum ramp spacing(FDOT, 2008, p. 78). Under the circumstances when roundabouts are located close to highwayinterchanges, ramp design spacing must be considered. Small spacing between roundabout andinterchangescouldpotentially compromise theoperationofbothroundabout functional areaandrampsthatenter/exitroundabout.

Figure23.RampSpacing(FDOT,2008,p.78)



Figure24.RoundaboutatanInterchange(FHWA,2006,p.8)

CornerClearance.AccordingtotheAASHTOGreenBook,cornerclearancemeansproperdrivewayplacementsothatadrivewayisnotwithintheinfluencingareaofanotherdriveway.FDOT’sDrivewayInformationGuidedisplaysthefigure(Figure25)ofadrivewaywithanimproperlocationtoillustratecornerclearance.

Figure25.CornerClearance(FDOT,2008,p.73)Roadwayclassificationdeterminesthespacingforcornerclearance,alongwiththespeedlimitontheroadway.FDOTalsodetailsthedownstreamcornerclearancestandardforaminorsidestreet.Figure26illustratesthedownstreamcornerclearance.



Figure26.CornerClearanceforDownstream(FDOT,2008,p.76)Thestandardfordownstreamcornerclearanceisalsodefinedbywhethertheintersectionischannelized,(witharadiusof50ft).Foraradiusofmorethan50ft,thestandardappliesforchannelizationdownstream.MedianOpeningDesign/Spacing.FDOTappliesthemedianopeningstandardbasedonthepostedspeedsandonthephysicalcharacteristics—whethertheopeningisfullordirectional.Medianopeningdistancesrangefrom330to2,640ft.dependingonopeningtype,designspeedandroadwayclassification,asseeninTable12.AccessManagementStandardsfromRule14‐97(FDOT,2006,p.15).Table12.AccessManagementStandardsfromRule14‐97(FDOT,2006,p.15)

Class Medians MedianOpenings Signal ConnectionFull Directional Morethan

45mphPostedSpeed

45mph andlessPostedSpeed

2 Restrictivew/ServiceRoads

2,640 1,320 2,640 1,320 660

3 Restrictive 2,640 1,320 2,640 660 4404 Non‐Restrictive 2,640 660 4405 Restrictive 2,640

atgreaterthan45mphPosted

Speed

660 2,640atgreaterthan45

mphPostedSpeed

440 245

1,320at45mphorlessPostedSpeed

1,320at45mphorlessPostedSpeed

6 Non‐Restrictive 1,320 440 2457 BothMedianTypes 660 330 1,320 125 125

SightDistance.Thisguidanceisneededtoimprovesafety.ThesightdistancestandardsincludetheSSD,thedistancenecessarytostop,andISD.FDOTsets14.5ft.astheminimumdrivereyesetback.Fornewdevelopments,thedistanceforSSDshouldfollowthestandardbasedonthedesignspeedoftheroadway.



Figure27.SightDistanceandDriverEyeSetbackDrivewayInformationGuide(FDOT,2008,p.62)OtherthanSSDandISD,FDOThassightdistancestandardsforroadwaysupstreamanddownstreamthathaveon‐streetparking.Foraspeedof0to30mph,itissuggestedthattheupstreamlanesbeatleast85ft.andthedownstreamtwolanes,atleast60ft.Withfourlanesthedistanceshouldbe45ft.Foraspeedof35mph,upstreamisatleast100ft.downstreamfortwolanes,andatleast70ft.andfourlanesat50ft.Turn‐LaneLocationandDesign.FDOTsuggeststhestandardforaradialreturndesignisusedforanexclusiveright‐turnlane.Meanwhile,theflaredrivewaystandardisforlowvolumedriveways.Theguidelinegivesclassificationofroadwaysbasedonthepostedspeedlimit,andthenumberofrightturnsperhour,i.e.45mphorlesswith80‐125vehicles,andover45mphwith35‐55vehicles.FDOTsuggestshavingnomedianopeningsacrosstheleft‐turnlane(FDOT,2008,p.77).Thedrivewayshouldbelocatedatleast100ft.fromtheoppositemedianopening.ThisdocumentalsosuggestshavinganadditionalpavementacrossthemedianopeningbecauseitmaysupporttheU‐turnmovement.FDOTsuggestspermittingleft‐turnsacrosshighvolumeroads,whenjointandcrossaccessexist.Figure28showsanexampleofjointandcrossaccess.

Figure28.JointandCrossAccess(FDOT,2008,p.86)Foranotherjointandcrossaccess,theFDOTreferstothedocumentManagingCorridorDevelopment,AMunicipalHandbook(WilliamsandMarshall,1996),forthefollowinginformation.



Auxiliary‐LaneLocationandDesign.FDOThasthestandardforintersectionchannelizationdesign.First,thestandardchannelizesdivisionalislands,includingpedestrianrefugeislands,trafficseparation,andtrafficflowseparation.AccordingtoStandardIndex515,theminimumwidthforadrivewaydivisionalislandis4ft.andthemaximumis22ft.However,ifthedrivewaysarenotincludedinthestandardindex,theminimumis6ft.andthemaximumis16ft.TheDrivewayInformationGuiderecommendsthelengthfordrivewaysthathaveparkingmovementsshouldbeatleast50ft.togivespaceforonevehicletoenter(fromthesidewalk).Thepreferreddistanceforparkingmovementsisequaltoorgreaterthan30ft.fromtheroadway,andmorethanorequalto20ft.fromthesidewalk.Thislengthisdifferentforlanduseswithadrive‐through.Thisdocumentalsosuggeststhespacesallowvehiclequeuesatfast‐foodestablishments,banks,carwashes,daycarefacilities,drycleaners,anddrive‐throughstand‐alonedrugstores.FDOTalsosuggestsmaximumqueuesforschoolbusstops,anddrivewaysforstaff,parentsandstudents.Thisstandardisbasedoncriticalpeakmorningandafternoonhours.Inadditiontothosestandards,FDOTalsomakessuggestionsfordrivewaysnearbusstopsandtransitfacilities.Theoppositesidesofaroadwaymayresultinjogmaneuvers(forundividedroadwaysorthosewithtwo‐wayleft‐turnlanes(TWLTL)(FDOT,2008,p79).Asaconsequence,FDOTrecommendstheroadwayoffsetdistancesadaptedfromDOT.4.2.2RoundaboutsGuidanceforFloridaSeveraldocumentsareidentifiedasroundaboutguidelinesatFDOT.TheseincludeFloridaRoundaboutGuide(FDOT,1996),RoundaboutJustificationStudy(Chapter16inManualonUniformTrafficStudies,FDOT,2000),FloridaIntersectionDesignGuide2013(FDOT,2007)andBicycleandPedestrianConsiderationsatRoundabouts(Shenetal.,2000).The109‐pageFloridaRoundaboutGuide(FDOT,1996),whichdetailsroundaboutdesignandguidanceinthestate,waspublishedearlierthanFHWA'sRoundabouts,AnInformationalGuide(Robinsonetal.,2000).TheFloridaguideincludesprocedurestojustifytheneedtobuildaroundabout,whiletheFHWAdocumentdoesnot.Thisguideisintheprocessofbeingreplaced,withadditionalguidancebeingincorporatedintootherguidancedocuments;thestatehasofficiallyadoptedNCHRP672,Roundabouts,AnInformationalGuide(BansenandSullivan,2013).OthersupplementalaspectsoftheFloridaguideareexplanationsforusingtheSIDRAsoftware.Inaddition,thisdocumentalsoconsidersothersoftware,suchasARCADY,andRODEL.TheFloridaguideincludesformstodeterminecapacityandotherrequiredmaterialstojustifytheuseofaroundabout;muchofthisguidancehasbeensunsettedwiththeadoptionofNCHRP672,Roundabouts,AnInformationalGuideandtheinclusionofChapter7intotheState’sIntersectionDesignGuide2013.ThesecondroundaboutdocumentistheManualonUniformTrafficStudies,Chapter16‐RoundaboutJustificationStudy(2000).WrittenbyFDOTandpublishedin2000,this16‐pagereportisthelastchapterintheFDOTManualonUniformTrafficStudies(MUTS).TheMUTSestablishesminimumstandardsforconductingtraffic‐engineeringstudiesonroadsunderthejurisdictionoftheFDOT.ThechapteronroundaboutsjustifiestheiruseintheStateofFlorida,andcomparesthemtothreeotheralternativestointersectioncontrols–trafficsignals,TWSC,andAWSC.Thischaptercitesthe1996FDOTFloridaRoundaboutGuideforspecificguidelinesonroundaboutlocation,design,andoperation.ThethirddocumentthatprovidesinformationonroundaboutsistheFloridaIntersectionDesignGuide,2013ForNewConstructionandMajorReconstructionofAt‐GradeIntersectionsontheStateHighwaySystem.This226‐pagedocumentincludeschaptersonintersectiondesignconcepts,geometricdesign,



signalization,signsandmarkings,objectsandamenities,androundabouts.Itstatesthatmodernroundaboutsshouldbeconsideredforanynewroadorreconstructionprojectastheyseemtoprovidesafetyandoperationaladvantages.Consistentwithotherstateguidance,theIntersectionDesignGuideadoptsNCHRP672,Roundabouts,AnInformationalGuide(2010)asthemainguidefordesigningroundaboutsinFlorida.Itmentionsthatroundaboutscontrolright‐of‐waysimilartosignalizationbutoffermoreadvantagesthansignalizedintersections,suchasreducingtheconflictpointswithintheintersection,reducingdelay,norequiredpowerortimingsuchaswithsignals,lesseningthenumberorturnlanes,eliminatingtheneedforextraqueuingspace,andothers.Roundaboutscanalsoreduceright‐anglecrashes.FDOTgenerallyrecommendsuptotwolanesinroundaboutsunlesstherearespecificneedsinaccommodatingmovementsinspiralor“Turbo”roundabouts.Inaddition,drivewaysshouldnotbeallowedinthecirculatoryroadwayunlessthereisenoughdemandtosupporttheirconstructionasadditionallegsoftheroundabout.Regardingroundaboutsandaccessmanagement,thisdocumentacceptsthatroundaboutscanbeusedaspartofanaccessmanagementplanastheycontributetoreducingdownstreamleftturnsbecausevehiclescanperformU‐Turnswithintheroundaboutsandthenaccessanareabyturningright.Bicyclescanaccessaroundaboutasvehiclesusingthecirculatoryroadwayoraspedestriantrafficusingthesidewalks.Bicyclelanesshouldendatbypassrampstoallowbicyclestousethesidewalkiftheyprefer,alwaysyieldingtopedestrians.Pedestriantreatmentsatroundaboutsarethesameasinotherintersectiontypes.Incaseofbusroutespassingthroughroundabouts,busbaysshouldbeplacedcarefullyonthenearsideoftheroundaboutapproachsothatwillnotcreatevehiclequeuesthatspillbackintothecirculatoryroadway.Busstopslocatedonthefarsideoftheroundaboutshouldhavepulloutsorbemovedfurtherdownstreamtothesplitterislandinordertoavoidinterruptingregulartraffic.Asmentionedearlierinthisreport,adequateSSDhastobeprovidedatroundabouts.FloridaIntersectionDesignGuideadaptstheSSDformulaandtheISDrequirementsfromNCHRP672,Roundabouts,AnInformationalGuide(Equations6‐5‐6‐7,pp.6‐61‐6‐63inRodegerdtsetal.,2010).ThefourthroundaboutdocumentisBicycleandPedestrianConsiderationsatRoundabouts(2000).WrittenbyFDOTandpublishedin2000,thisreportexaminestopicsofspecificconcerntobicyclistsandpedestriansatroundabouts.Theconclusionsofthisstudyarethatifnotproperlydesigned,roundaboutscanhavehigherbicyclecrashratesthanthoseofvehiclesandpedestrians,andthemulti‐laneroundaboutscreatemoretensionandarelesssafeforbicyclistsandpedestriansthanone‐laneroundabouts.Thereportrecommendstheuseofadditionalbicyclefacilitiesoutsidearoundaboutifspaceisavailable.Alsorecommendedarecrossingprovisions,andpropersignage.Inadditiontotheabovedocuments,FDOTpresentedaPowerPointpresentation—Roundabouts,Florida’sImplementationStrategy(PrytykaandSullivan,2012)atthe2012DesignTrainingExpo.ThispresentationcapturessupplementalaspectsfromFHWA'sRoundabouts,AnInformationalGuide(Robinsonetal.,2000),especiallyonpedestrians,trucks,andpavementmarkinginformation.4.3NationalGuidanceonAccessManagementintheContextofRoundaboutsAmongallthenationalguidancedocumentsonroundaboutsandthedocumentsonaccessmanagement,onlyNCHRPReport672:Roundabouts,AnInformationalGuide(Rodegerdtsetal.,2010)referstotheaccessmanagementinthecontextofroundabouts.NCHRPReport672.Roundabouts,AnInformationalGuide(Rodegerdtsetal.,2010).Thisinformationalguideonroundaboutsincludesaccessmanagementinformationinthecontextofroundaboutsunderthegeneralcharacteristicsofroundaboutsaspartofthegeometricprocess(Sections



2.2.5p.2.9and6.11,pp.6‐95to6‐98).TheinformationonaccessmanagementbuildsupontheinformationprovidedintheKansasRoundaboutsGuide(Kittelson&Associates,Inc.andTranSystemCorporation,2003).Animportantfactmentionedinthisdocumentisthat“Mostoftheprinciplesusedforaccessmanagementatconventionalintersectionscanalsobeappliedatroundabouts”(p.2‐9).Thereportalsostatesthat“Accessmanagementatroundaboutsfollowsmanyoftheprinciplesusedforaccessmanagementatconventionalintersections”(p.6‐95).However,roundaboutsaredifferentfromothertypesofintersectionsbecausetheycanprovideU‐turnopportunitiesallowingforareductionoffullaccesspointsalongaroadwaysegmentandthereforeenhanceaccessmanagement.Publicandprivatepropertyaccesswithinthevicinityofaroundaboutshouldbecarefullyevaluatedandthecasesof“accessintotheroundaboutitself”and“accessneartheroundabout”shouldbetakenintoaccount.Drivewayslocatedintheroundaboutshouldbeavoidedbecausetheycancreateconflictsinthecirculatoryroadway,includingaccelerationanddeceleration,eventhoughtherearecaseswheredirectaccessisgiventoresidencies.Inordertohaveadrivewaytakingdirectaccesstothecirculatoryroadwayofaroundabout,noalternativeaccesspointsshouldbeavailable,lowtrafficvolumesshouldbepresentatthedriveway,alownumberofunfamiliardriversshouldusetheroundabout,thedrivewayshouldbeproperlydesignedtoallowvehiclestoturnaroundandexitfacingforward,andtheroundaboutshouldprovideadequatesightdistanceandSSD.Wheredrivewaysarelocatedinorneararoundabout,thedesignshouldgiveaclearvisualindicationthatprivatedrivewaysareadjacenttotheroundaboutandarenotforpublicuse.Theabilitytoprovidepublicandprivateaccesspointsneararoundaboutisinfluencedbyanumberoffactorssuchasthecapacityoftheminormovementsattheaccesspoints,theneedtoprovideleft‐turnstorageonthemajorstreettoservetheaccesspoint,theavailablespacebetweentheaccesspointandtheroundabout,andthesightdistanceneeds.Figure29showsthetypicaldimensionsforleft‐turnaccessnearroundabouts.Theyincludeaminimumof50ft.toclearthemedian,aminimumof75ft.toallowfortheleftturningmovement,and90ft.fordecelerating(oraccelerating)maneuveringandqueuingintheleftturnlane.

Figure29.TypicalDimensionsforLeft‐turnAccessnearRoundabouts(Rodegerdtsetal.,2010,p.6‐98)

4.4States’GuidanceonAccessManagementintheContextofRoundaboutsAsmallnumberofstatesrefertoaccessmanagementwithinthecontextofroundabouts.Someincludesuchinformationintheirroundaboutsmanualsandsomeintheiraccessmanagementmanuals.Fromtheseven



statesthatrefertoaccessmanagementinthecontextofroundabouts,onlythreeofthemsubstantiallysupplementinformationfromthenationalguidance.ThesestatesincludeKansas,Virginia,andWisconsin.AdditionalinformationisprovidedaboutaccessmanagementinCalifornia,Iowa,Michigan,andPennsylvaniaaccessmanagementguidancedocuments.Stateinformationisdescribedforthesesevenstatesinthefollowingsection.California.TheCaltransRoundaboutGeometricDesignGuidance(Caltrans,2007)mentionsthatattentionshouldbepaidtoprovidingaccesstopedestrianswithvisualimpairmentsatroundaboutsand,moreparticularly,atmultilaneroundabouts,asoften,conventionaldesignmaynotbesufficient.Also,CaltransDivisionofDesignandOfficeofGeometricDesignStandardsdevelopedtheDesignInformationBulletinNumber80‐01asasupplementtotheFHWAReport,Roundabouts:AnInformationalGuide.Oneoftheadditionsregardingaccessmanagementwasaccommodatingbicyclistsonthestatehighwaysystembyprovidingrampstoentertheshared‐usepathforthosewhodonotwanttousethecirculatoryroadway.Anotheradditionwastherecommendationofcrosswalkswith“zebra”longitudinallines,transverselines,anduseofdetectablewarningsurfacesatallpedestriancrossings.Iowa.IowaDOThassponsoreditsstateuniversitytodevelopPlanning‐LevelGuidelinesforModernRoundabouts(HallmarkandIsebrands,2008).Thatguidebookstatesthataccesstopedestriansisonlyallowedacrosstheapproachlegs,andparkingisnotallowedwithinthecirculatingroadway,andthatroundaboutscanbeconsideredincaseswherethereisneedforU‐turnsandwhereright‐in‐right‐outrestrictionsexist.Anoteinthedocumentmentionsthat“[a]ccessmanagementprinciplesalignwithhowroundaboutsfunctionandoperate.Corridorsthatarehamperedwithnumerousaccesses,especiallythosetobusinesses,canbenefitfromroundabouts.RoundaboutsfacilitatetheuseofU‐turnsatintersectionsandallowforrightturnsintodrivewaysandparkinglotsratherthanleftturnsacrosstraffic.Theimpactsofright‐in‐right‐outrestrictionsandclosedmediansbecomereducedwhenroundaboutsprovideanaturalU‐turnatanadjacentintersection”(HallmarkandIsebrands,2008,p.17).Kansas.AccessmanagementinthecontextofroundaboutsisreferredtointwoKansasDOT(KsDOT)documents:KansasRoundaboutGuide:ASupplementtoFHWA’sRoundabouts(Kittelson&AssociatesandTransystemCorporation,2003);andKsDOTAccessManagementPolicy(KsDOT,2013).ThefirstdocumentincludesalltheinformationonaccessmanagementthatNCHRPReport672,Roundabouts,AnInformationalGuideadapted.Thisinformationisdescribedabove,inSection4.3.Intheseconddocument,theaccessspacingfromroundaboutintersectionsisdiscussed.KsDOT’sroundaboutaccessspacingtoanaccesspointonthehighwayisconsistentwithKsDOT’sunsignalizedaccessspacing.Thatspacingshouldbemeasuredfromtheendofthesplitterisland,leavingtheroundaboutasshowninFigure30.Theappropriatecornerclearanceisthenprovidedbetweentheendofthesplitterislandandthefirstaccesspointalongthelocalintersectingroadway.



Figure30.MeasuredDistancefromSplitterIslandtoFirstAccessPoint(KsDOT,2013,p.4‐26)

Michigan.MDOThastwoguidebooksthatfocusonaccessmanagementwithinandnearroundabouts.InEvaluatingthePerformanceandSafetyEffectivenessofRoundabouts(Bagdadeetal.,2011)theresearchreportmentionsthatadditionalprovisionssuchaspedestrianhybridbeacons,flashingpedestrianbeacons,andraisedsidewalksmaybeincludedintwo‐laneroundaboutstoenhancethesafetyofvisuallyimpairedpedestrians.TheAccessManagementGuidebookstates(MDOT,2008)that“Drivewaysneedtobelocatedasafedistancefromaroundaboutwithadequatesignage.Drivewaysshouldnotbelocatedwithinaroundabout”(MDOT,2008,p.3‐29).Pennsylvania.Pennsylvania’sGuidetoRoundaboutsnotes(PennDOT,2007)thataccessiblepedestriancrossingshouldbeprovidedatallroundaboutsexceptruralroundaboutswithnonexistentpedestrianactivity.Pedestriancrossingsshouldbelocatedbackfromthecirculatoryroadwayandthesplitterislandshouldbecuttoallowpedestrians,wheelchairs,strollers,andbicyclestopassthrough.Bicyclesshouldbegiventheoptionoftravelingthroughtheroundabouteitherasavehicleorasapedestrian,basedonthebicyclist’slevelofcomfort.Inthecasewherebicyclistschoosetosharethesidewalkandtravelaspedestrians,theyarerequiredtodismounttheirbikeandwalkwithit.PennDOT’sGuidetoRoundabouts(PennDOT,2007)wasdevelopedbasedonKsDOT'sRoundaboutGuide,(Kittelson&AssociatesandTransystemCorporation,2003)anditincludesexactlythesameinformationonaccessmanagementinthecontextofroundaboutsastheKsDOT’sRoundaboutGuide.Virginia.AccessManagementDesignStandardsforEntrancesandIntersection(VirginiaDOT,2007,revised2011),includesinformationaboutroundaboutsinAppendixF,Section2(VirginiaDOT,2007).Inthatguide,roundaboutsareseparatedfromsignalizedandunsignalizedintersections/crossoversbytheunsignalizedintersectionspacingstandard(e.g.,secondcolumninFigure31).Theyarealsoseparatedfromotherroundaboutsbythepartialaccessentrancespacingstandard(i.e.,thelastcolumninFigure31);partialaccessentrancereferstoroadwaysthathaveaccessmanagementtechniquestopreventleft‐turningressandegressmovementsandfacilitateright‐inandright‐outmovements.Thespacingismeasuredfromtheouteredgeofthenearestinscribeddiameter,notthecenterline.ThespacingstandardsusedareshowninFigure31.Inaddition,designguidelinesregardingpedestrianandbicycletreatmentsshouldfollowNCHRPReport672,Roundabouts,AnInformationalGuide.



Figure31.MinimumSpacingStandardsforCommercialEntrances,Intersections,andCrossovers(VDOT,2007,p.F‐23)

Wisconsin.Wisconsin’sRoundaboutGuide(WisDOT,2011)includesinformationaboutaccesscontrolinChapter11,Section26.Thatchapterwasrecently(March4,2013)updated.Basedonthatguide,roundaboutswouldfacilitateleftturnsandU‐turnstoaccesspropertiesontheoppositesideofthehighway.Also,thepedestriancrossinglocationshouldbesetbackfromtheyieldline,typicallyonecarlength.Inaddition,connectingtworoundaboutswitharaisedmedianprecludesleftsin/outfromthesidestreetorbusinessaccesstoprotectmain‐linecapacity,althoughmajorcommercialdrivewaysmaybeallowedasonelegoftheroundabout.Minorcommercialandresidentialdrivewaysarenotrecommendedalongthecirculatingroadwayexceptiftheyaredesignedasalegoftheroundabout,anddrivewaysshouldbesetbacktopreventinterferencewithpedestrianmovementsincrosswalks(WisDOT,2011).Whenitcomestoaccessmanagement,theguidestates:Retrofitofsuburbancommercialstripdevelopmenttoaccomplishaccessmanagementobjectivesofminimizingconflictscanbeaparticularlygoodapplicationforroundabouts.RaisedmediansareoftendesignedforStatearterialstominimizeleftturnconflicts;androundaboutsaccommodateU‐turns.Left‐turnexitsfromdrivewaysontoanarterialthatmaycurrentlyexperiencelongdelaysandrequiretwo‐stageleft‐turnmovementscouldbereplacedwithasimplerrightturn,followedbyaU‐turnatthenextroundabout.Again,apackageofimprovementswithdrivewayconsolidation,reversefrontage,andinterconnectedparkinglots,shouldbeplannedanddesignedwithcloselocalcollaboration.Also,aroundaboutcanprovideeasyaccesstocornerpropertiesfromalldirections.(WisDOT,2011).4.5RoundaboutLocationGuidelinesKansasDOTmentionedsiteswhereroundaboutsbringadvantages,andwheretheroundaboutshouldbebuiltcautiously.Intersectionsthatmayhavebenefitsinconvertingintoroundaboutsaretheoneswith(Kittelson&AssociatesandTranSystemCorporation,2003,p.38):



Historicalsafetyproblems; Relativelybalancedtrafficvolumes; Highpercentageofturningmovements; Highvolumesatpeakhoursbutrelativelylowvolumesatnon‐peakhours; Existingtwo‐waystop‐controlledthathavehighside‐streetdelay; TherequirementstoaccommodateU‐turn; Aroleasgatewayorentrypointtocampus,neighborhood,commercialdevelopment,orurbanarea; Intersectionswhereacommunityenhancementmaybedesirable; Intersectionswheretrafficcalmingisadesiredoutcomeoftheproject; Intersectionswheregrowthisexpectedtobehighandfuturetrafficpatternsareuncertain; Locationswherethespeedenvironmentoftheroadchanges; Locationswithaneedtoprovideatransitionbetweenlanduseenvironments;and Roadswithahistoricalproblemofexcessivespeeds.

However,thelocationsofroundaboutthathavethefollowingconditionsshouldreceiveextraattention:

Intersectionincloseproximitytoasignalizedintersectionwherequeuesmayspillbackintotheroundabout;

Intersectionslocatedwithinacoordinatedarterialsignalsystem; Intersectionswithaheavyflowofthroughtrafficonthemajorroadopposedbyrelativelylight

trafficontheminorstreet; Intersectionswithphysicalorgeometriccomplications; Locationswithsteepgradesandunfavorabletopographythatmaylimitvisibilityandcomplicate

construction; Intersectionswithheavybicyclevolumes;and Intersectionswithheavypedestrianvolumes.

CloselySpacedRoundabout.WisconsinDOTconsidersroundaboutstobecloselyspacedwhenthedistanceislessthan1,000ft.fromthecenterofeachroundabout.4.6GeometryDesignGuidelinesThisreviewhighlightsgeometricaspectsthatdifferamongstates’guidanceandNCHRPReport672,Roundabouts,AnInformationalGuideorotherlistednationaldocuments.WisDOTmentionedtheeffectsofdesignelementsonSafetyandOperationsandoutlinestrade‐offeffectsontherelationshipbetweensafetyandcapacityasshowninFigure32.

Figure32.TheEffectofDesignElements(WisDOT,2011,p.38)



Speed.KansasDOTprovidestheroundaboutdesignspeedbasedonsitecategories:mini‐roundabout,urbancompact,urbansingle‐lane,ruralsingle‐lane,urbandouble‐lane,andruraldouble‐laneroundabout.Table13showstheroundaboutdesignspeedthatKansasDOTapplied.Table13.RoundaboutDesignSpeed

SiteCategory MaximumEntry(R1)DesignSpeedMiniRoundabout 20mi/h(32km/h)UrbanCompactRoundabout 20mi/h(32km/h)UrbanSingle‐LaneRoundabout 25mi/h(40km/h)RuralSingle‐LaneRoundabout 25mi/h(40km/h)UrbanDouble‐laneRoundabout 25mi/h(40km/h)RuralDouble‐LaneRoundabout 30mi/h(48km/h)

Source:KansasDOT,p.67Lanenumbersandarrangements.Indeterminingthese,Caltransusedcapacitymodelstakingcriticalheadwayandfollow‐upheadwayspecificallyasfollows:single‐laneroundabout(4.8sand2.5s,respectively);multilaneroundabouts,leftlane(4.7sand2.2s,respectively);andmultilaneroundabouts,rightlane(4.4sand2.2s,respectively).HeadwayvaluesforWisDOTarepresentedinTable14.Table14.RecommendedHeadwayValues(WisDOT,2011,p31)

Spacing.Caltransdevelopedastandardforspacingentriesandexitstominimizeexit‐circulatingconflicts.Thespacingisconsideredimportantformultilane,morethanforfour‐legandskewed‐legroundabouts.Asaresponsetothecirculating‐exitingpathconflict(Figure33),Caltransofferedtwosolutions,asseeninFigure34.



Figure33.ExampleSolutionDesignwithCirculating‐ExitingPathConflict(Caltrans,2007,p.62)

Figure34.SolutionOptionsforCirculating‐ExitingPathConflict:(i)ModifyLaneConfiguration,and(ii)

RealignApproaches(Caltrans,2007,p.63‐64)SightDistance.AzDOTrequiresthataroundaboutdesignmeettwosightdistancestandards:SSDandISD.TheISDincludestheapproachanddeparturesighttriangles.Caltransfocusesonensuringpropersighttotheleft.Forsightdistancecalculations,“thecriticalheadwayof5.9sec.isrecommendedinsteadofthe6.5sec.presentedinRoundabouts:AnInformationalGuide(Robinsonetal.,2000).ThismethodologyshouldbeconsideredinterimuntilastudyonroundaboutISDiscompleted”(p.viii).Fortheangleofvisibility,CaltranscomparedAASHTO,TheCaltransHighwayDesignManual,andFHWAHighwayDesignHandbookforOlderDriversandPedestrians,whichhadminimumanglesof60degrees,75degreesatgrade,and75degrees,respectively.Figure35showsanexampleofanintersectionthathasaproblemwiththeangleofvisibility.KansasDOTreferstotheFHWAPublication(Robinsonetal.,2000)fortheISDandAASHTOfourthedition.Thecalculationassumedacriticalgapof6.5s.andof4.6s.ifconstraintsfromtopographicfeaturesorbuildingexist(similartothelowerboundoftheHCM2000(TRB,2000)).

( (

( (



Figure35.AngleofVisibility:(i)theAngleisTooSevere(ii)RealignedRampTerminalApproachtoHaveBetterAngleofVisibility(Caltrans,2007,p.65)

KansasDOTdecidesthedesignspeedfromthecalculationofSSDandISD.First,SSDincludestherequirementsofapproachsightdistance,sightdistanceonthecirculatoryroadway,andsightdistancetocrosswalkontheimmediatedownstreamexit.Also,KansasDOTmentionsthatsightdistanceforlandscapingmaterialshavelimitationof2ft.or600mm.height.WisDOTspecifiestheguidanceforclosely‐spacedmultipleroundabouts.Inthedocument,WisDOTusestheminimumvisibilitydistanceshowninTable15.Table15.WisconsinDOTMinimumVisibilityDistance

*MinimumVisibilityDistancesarefromSection2C.36oftheWisconsinSupplementtothe2009MUTCDInscribedCircleDiameter(ICD).TheCaltranscomparedICDforFHWAstandard,Kansas,Arizona,andWisconsindependingontheroundaboutcategories.Table16displaystheICDforthesestates.Table16.TypicalInscribedCircleDiameterRanges(Caltrans,2007,p.67)

Toupdatethosestandards,Caltransincorporateslanenumbersandarrangements,designvehicles,numberoflegs,andapproachalignmentontheirstandards.Table17givesthecommonrangesofinscribedcirclediametersbasedontheaforementionedfactors.



Table17.CommonRangesofInscribedCircleDiameters(Caltrans,2007,p.68)

Geometricdesignforusers.Theneedsofvarioususersareconsideredinthestateguidelines.Forexample:designvehicle,pedestrians,bicyclists,andolderdrivers.First,AzDOTappliedspecialconsiderationstoroundaboutsbyaddingatruckapron.Caltransusesthedesignvehicleasoneofgeometricdesignconsideration,coveringcarsweptpathfordifferenttypesofdesignvehicles.TheguidelinescomparisonfordesignvehiclesformultilaneroundaboutsispresentedinTable18.Inaddition,Caltransprovidesdesignrecommendationsforpedestrians,includingcrossingtreatmentsandmethodologiesasinTCRPReport112andNCHRPReport562.



Table18.TheGuidelinesComparisonforDesignVehiclesonMulti‐laneRoundabouts(Caltrans,2007).

WisDOThascompleteguidancefordesignvehiclesontwo‐laneroundabouts.Theguidebookexploresthreedesigncategoriesforlegaltruckaccess(WisDOT,2013,p.47).Thefirstcaseiswhenroundaboutsallowtruckstoencroachintoadjacentlanesastheyapproach,enter,circulate,andexittheintersection.Thesecondcaseiswhenroundaboutsallowtrucksin‐laneastheyapproachandentertheroundabout,butmayrequiretruckstoencroachintoadjacentlanesastheycirculateandexittheintersection.Thethirdcaseiswhenroundaboutsaccommodatetrucksin‐laneastheyapproachandtraversetheentireintersection.Besidesdesignvehicles,thestates’roundaboutguidesaddressconcernsaboutpedestriansandbicycleaccommodations.KansasDOTfocusesongeometricelementsforpedestriancrossings,suchaslocation,



crossingalignmentandsplitterislands.Inaddition,thedocumentoftheKansasDOTpaysattentiontovisuallyimpairedpedestrians,waystoavoidhavingthepedestriancrossthecentralisland,andtoprovidingmulti‐modalsidewalks.Detaileddesignsforpedestriansincludethefollowingaspects:(1)thepedestriancrossingisexpectedtomaintainonevehiclelengthorabout25ft.awayfromtheroundaboutentrance;(2)curbrampsandpedestriancrossingsshouldbeavailableandbestraightandcontinuouslyalignedontheroundabout;(3)way‐findingandgapdetectionmayneedtobeconsideredforvisuallyimpairedpedestrians;and(4)thedistanceofsidewalksfromthecirculatoryroadwayshouldbeatleast2ft.,althoughtherecommendeddistanceis5ft.Furthermore,itisrecommendedthatthebikelanemergewithsidewalksatleast100ft.(30m)upstreamoftheentranceline.Toaccommodatepedestriansandbicyclists,WisDOTdescribesdesignguidanceforpedestrianfacilities,bicyclemarkings,andbikerampentrancesandexits(WisDOT,2013,p.18).Thepedestrianfacilitiesincludethesidewalks,shared‐usepaths,androundaboutsidepaths.WisDOTfoundthatroundabouts,whencomparedtoothertypeofintersections,dohaveanadvantagewhenpedestrianandbicyclistsafetyisconcerned(WisDOT,2013,p.18).Thisisbecausethelowoperatingspeedsthroughroundaboutsandtherearelessconflictpointbetweenpedestriansandvehicles.Forpedestriancrossingatroundabout,itisimportanttochooseacrosswalklocationthatcanbalancepedestriansafety,theirconvenienceandtheoperationofroundabouts.Forbicyclists,thebiggestchallengeisaccommodateturningmovementatroundabouts.WisDOTrecommendedusingpedestrian‐bicyclepathseparatefromthecirculatoryroadwaytoaccommodatebicyclistatroundabouts(WisDOT,2013,p.19).

Chapter5SafetyAnalysis


ChapterFive:SafetyAnalysis

ThischapterincludesasafetyanalysisthatinvestigatespotentialsafetyconcernsassociatedwithroundaboutsincommercialareasinFlorida.AsidentifiedinChapterThree,thepotentialsafetyconcernsinclude:(1)impactofdrivewaycornerclearancesonroundaboutsafety;(2)safetyimpactofmedianopeningsinthevicinityofroundabouts;(3)safetyatroundaboutsthatprovidedirectaccesstoactivitycenters;and(4)safetyofvulnerableroadusersincludingpedestriansandbicyclists. GeneralstatisticsthatgiveanoverviewofthecrashesthatoccurredinthevicinityofallidentifiedroundaboutsinFloridaareprovidedfirst.Ananalysisbasedoncrashdataanddetailedreviewofpolicereportsisthenconductedtoaddresseachofthepreviouslylistedsafetyconcerns.Thechapterconcludeswithasummaryoffindingsandalistofspecificrecommendations.5.1OverallCrashStatisticsAsindicatedinChapterThree,atotalof1,882crasheswerefoundtooccurduring2007‐2011within500ft.of283roundabouts.Thissectionprovidesanoverallsummaryofthesecrashesinthefollowingorder:(1)areatype;(2)crashtype;(3)crashseverity,and(4)numberofvehiclesinvolvedinacrash.5.1.1AreaTypeThe283roundaboutswerecategorizedintotwodifferentareatypes:commercialandresidential.Commercialroundaboutsarethosethatarelocatedincommercialareasthatservemostlycommercialtraffic.Similarly,residentialroundaboutsarethosethatarelocatedinmainlyresidentialareas.Mixed‐useareas,whichincludebothcommercialandresidential,areincludedwithcommercialroundaboutsbecauseofthetrafficassociatedwiththecommerciallanduse.Table19givesthetotalnumberofroundaboutsandcrashesineachareatype.Table19alsoprovidesthecrashstatisticsbyareatype.Overall,eachroundaboutexperiencedanaverageof6.65crashesduringthefive‐yearanalysisperiod;withcommercialroundaboutsexperiencing8.10crashesperroundaboutwhileresidentialroundaboutsexperienced5.40crashesperroundabout.Thetablealsoshowsahigherstandarddeviationforthenumbersofcrashesforroundaboutsincommercialareas,indicatingthatthecrashfrequenciesvarymoreamongthecommercialroundaboutsthantheresidentialroundabouts. Table19.StatisticsbyAreaType

AreaTypeTotalCrashesinFiveYears

(a)

NumberofRoundabouts

(b)

CrashesperRoundaboutin

FiveYears(a/b)

StandardDeviation

Commercial 1,061 131 8.10 13.65Residential 821 152 5.40 9.20Total 1,882 283 6.65 11.535.1.2CrashTypeTable20givesthesummaryofcrashstatisticsbycrashtypeandareatype.Italsoprovidesthepercentofnighttimecrashesbycrashtype.Figure36providesthepercentageoftotalcrashesandnighttimecrashesbycrashtype.Collisionwithafixedobjectwasthemostfrequentcrashtype.Aboutaquarter(24.7%)ofallcrashesthatoccurredinthevicinityofroundaboutsresultedfromvehicleshittingafixedobject,mostly,



theroundaboutcenterisland.Also,abouttwo‐thirds(62.9%)ofthesecrashes(i.e.,collisionwithafixedobject)occurredatnight.Nexttothecollisionwithafixedobject,angleandrear‐endcrashesweremostcommon,accountingfor21%and18.5%oftotalcrashes,respectively.Additionally,thedistributionofcrashtypeswasfoundtobesimilarincommercialandresidentialareas.

Figure36.StatisticsbyAreaType

Table20.StatisticsbyCrashType

CrashType1

CommercialArea ResidentialArea TotalCrashes

No.(a)

PercentofTotalCrashes(a/1,061)

PercentofNighttimeCrashes

No.(c)

PercentofTotalCrashes(c/821)


No.(d)

PercentofTotalCrashes(d/1,882)


Rear‐end 188 17.7% 19.1% 161 19.6% 20.0% 349 18.5% 19.5%Head‐on 20 1.9% 40.0% 15 1.8% 53.3% 35 1.9% 45.7%Angle 217 20.5% 18.9% 179 21.8% 26.3% 396 21.0% 22.2%Left‐turn 29 2.7% 13.8% 12 1.5% 33.3% 41 2.2% 19.5%Right‐turn 37 3.5% 24.3% 14 1.7% 21.4% 51 2.7% 23.5%Side‐swipe 55 5.2% 23.6% 41 5.0% 19.5% 96 5.1% 21.9%BackedInto 16 1.5% 31.3% 15 1.8% 26.7% 31 1.6% 29.0%CollisionwithParkedCar 27 2.5% 29.6% 18 2.2% 50.0% 45 2.4% 37.8%

CollisionwithMotorVehicle 48 4.5% 20.8% 32 3.9% 34.4% 80 4.3% 26.3%

CollisionwithPedestrian

14 1.3% 40.0% 4 0.5% 25.0% 18 1.0% 36.8%

CollisionwithBicycle

35 3.3% 8.6% 16 1.9% 18.8% 51 2.7% 11.8%

CollisionwithFixedObject 250 23.6% 63.6% 215 26.2% 62.1% 465 24.7% 62.9%

AllOther 125 11.8% 47.2% 99 12.1% 43.6% 224 11.9% 45.7%Total 1,061 100.0% 34.0% 821 100.0% 37.4% 1,882 100.0% 35.5%

1 Thesestatisticsarebasedonthefirstharmfulevent(FHE)codedinthepolicereports.Notethatthesenumbersaredifferentfromthoseprovidedlaterinthechaptersincedetailedanalyseswerebasedonthereviewofpolicereports.

8.10

5.40

6.65

0123456789

Commercial Residential Total

CrashesperRoundabout

in5years



Figure37.TotalandNighttimeCrashStatisticsbyCrashType

5.1.3CrashSeverityFigure38providesthenumberandpercentageofcrashesbycrashseverity.Table21summarizesthecrashesbycrashseverityandareatype.Amajorityofcrashes(i.e.,over60%)thatoccurredatroundaboutsresultedinpropertydamageonly(PDO).Severeinjurycrashes(i.e.,fatalandincapacitatinginjurycrashes)accountedforlessthan5%ofthetotalcrashes.Severeinjurycrashfrequencyperroundaboutwasslightlyhigheratcommercialroundabouts(5.4%)comparedtoresidentialroundabouts(4.4%).However,theoveralldistributionsweresimilar.Also,crashseverityofseveralcrasheswasunknown;mostofwhichwerearesultofhit‐and‐run(i.e.,thedriverfledthecrashsitepriortothearrivalofthelawenforcementofficials).

Figure38.StatisticsbyCrashSeverity

19

2

21

2 35

2 1 3

25

100

20

46

22 2024 22

29

37

12

63

36

0%

10%

20%

30%

40%

50%

60%

70%

80%

90%

100%

PercentageofCrashes

CrashType

PercentofTotalCrashes


Fatal10(0.5%)

Injury634

(33.7%)

PDO1,150(61.1%)



Table21.StatisticsbyCrashSeverityandAreaType

CrashSeverityCommercialArea ResidentialArea TotalCrashesNo.(a)

Percent(a/1,061)

No.(c)

Percent(c/821)

No.(d)

Percent(d/1,882)

FatalInjury 4 0.4% 6 0.7% 10 0.5%IncapacitatingInjury 53 5.0% 30 3.7% 83 4.4%Non‐IncapacitatingInjury 152 14.3% 105 12.8% 257 13.7%PossibleInjury 164 15.5% 130 15.8% 294 15.6%PropertyDamageOnly 642 60.5% 508 61.9% 1,150 61.1%Unknown1 46 4.3% 42 5.1% 88 4.7%Total 1,061 100.0% 821 100.0% 1,882 100.0%

1 Theseverityofacrashisunknownwhenthedriverfledthecrashsitepriortothearrivaloflawenforcementofficials.

5.1.4NumberofVehiclesInvolvedTable22providessummarystatisticsofsingle‐vehicleandmulti‐vehiclecrashesbyareatype.Overall,aboutone‐thirdofthetotalcrashesweresingle‐vehiclecrashes,whiletherestinvolvedmultiplevehicles.Thetableshowsthattheproportionofsingle‐andmulti‐vehiclecrasheswasfoundtobeconsistentacrossareatypes.Table22.StatisticsofSingle‐vehicleandMulti‐vehicleCrashesbyAreaType

CrashTypeCommercialArea ResidentialArea TotalCrashes

No.(a)

Percent(a/1,061)

No.(c)

Percent(c/821)

No.(d)

Percent(d/1,882)

Single‐vehicle 342 32.2% 292 35.6% 634 33.7%Multi‐vehicle 719 67.8% 529 64.4% 1,248 66.3%Total 1,061 100.0% 821 100.0% 1,882 100.0%Table23givesthesummaryofsingle‐vehicleandmulti‐vehiclecrashstatisticsbycrashseverity.Single‐vehiclecrashes(8.9%)hadahigherproportionofsevereinjuriesthanmulti‐vehiclecrashes(2.9%).Also,agreaterpercentageofsingle‐vehiclecrashesresultedininjuriescomparedtomulti‐vehiclecrashes;68.8%ofmulti‐vehiclecrashesresultedinPDOcrashes,whileonly45.9%ofsingle‐vehiclecrasheswerePDOs.Ofthesixfatalsingle‐vehiclecrashes,fourinvolvedmotorcycles,andinallthesefourcrashes,themotorcyclistwasfoundtobeatfault.Anotherfatalcrashinvolvedavehicleandanintoxicatedpedestrianwhoranintothepathofthevehicle.Twoofthefourfatalmulti‐vehiclecrashesinvolvedagolfcart.

5.2ImpactofDrivewayCornerClearancesonRoundaboutSafetyDrivewaycornerclearanceisdefinedinthecontextofthisstudyastheminimumdistancebetweenaroundaboutandanadjacentdrivewayalongeachapproachordepartureleg.AsshowninFigure39,theupstreamdrivewaycornerclearanceismeasuredfromthefirstdrivewayupstreamoftheroundabouttotheroundabout.Likewise,thedownstreamdrivewaycornerclearanceismeasuredfromtheroundabouttothefirstdrivewaydownstreamoftheroundabout.



Table23.StatisticsofSingle‐vehicleandMulti‐vehicleCrashesbyCrashSeverity

CrashSeverity

Single‐vehicleCrashes Multi‐vehicleCrashes TotalCrashes

No.(a)

Percent(a/634)

No.(b)

Percent(b/1,248)

No.(c)

Percent(c/1,882)

FatalInjury 6 0.9% 4 0.3% 10 0.5%IncapacitatingInjury 51 8.0% 32 2.6% 83 4.4%Non‐IncapacitatingInjury 128 20.2% 129 10.3% 257 13.7%PossibleInjury 91 14.4% 203 16.3% 294 15.6%PropertyDamageOnly 291 45.9% 859 68.8% 1,150 61.1%UnknownInjury1 67 10.6% 21 1.7% 88 4.7%Total 634 100% 1,248 100.0% 1,882 100.0%

1 Theseverityofacrashisunknownwhenthedriverfledthecrashsitepriortothearrivaloflawenforcementofficials.

Thefocusofthissectionistoanalyzedriveway‐relatedcrashestoidentifytheimpactsofupstreamanddownstreamcornerclearancesonroundaboutsafety.Inthisanalysis,acrashisconsideredtobedriveway‐relatedifoneofthevehiclesinvolvedinthecrashwasenteringorexitingadriveway.Particularly,crashesinvolvingvehiclesturningfromadrivewayontoamainstreet,turningfromthemainstreetontoadriveway,andbackingoutofadrivewayontoanapproachlegwereidentifiedasdriveway‐relatedcrashes.

Figure39.UpstreamandDownstreamDrivewayCornerClearances

Policereportsofallthe1,882crashesthatoccurredwithin500ft.oftheroundaboutswerereviewedtoidentifydriveway‐relatedcrashes.Ofthe1,882crashesthatoccurredatroundaboutlegs,only74crasheswereidentifiedtobedriveway‐related.Ofthese74driveway‐relatedcrashes,37crashes(50%)occurredatthefirstdriveways(i.e.,thedrivewaythatdefinesthecornerclearance)whileanequalnumberoccurredonallotherdriveways.



Howissafetyatroundaboutsaffectedbycornerclearances?Table24givesthesummarycrashstatisticsofthe37driveway‐relatedcrashesthatoccurredatthefirstdriveway.Ascanbeinferredfromthetable,severalapproacheshaveshorterupstreamanddownstreamcornerclearances.Ofthe37crashes,18occurredatthefirstupstreamdriveway,andtheremaining19occurredatthefirstdownstreamdriveway.Sixof18crashes(33.3%)occurredwhentheupstreamcornerclearancewaslessthan250ft.Ontheotherhand,15of19crashes(78.9%)occurredwhenthedownstreamdrivewaycornerclearancewaslessthan250ft.Intermsofcrashseverity,ofthe37crashes,nonewerefatal,tworesultedinincapacitatinginjuries,eightwerenon‐incapacitatinginjurycrashes,andtheremaining27werePDOs.Table24.Driveway‐relatedCrashesThatOccurredwithinUpstreamandDownstreamDrivewayCornerClearances

CornerClearance(feet)

UpstreamofRoundabout DownstreamofRoundaboutNo.ofCrashes

No.ofLegs

Crashes/100Legs

No.ofCrashes

No.ofLegs

Crashes/100Legs

0‐49 1 29 3.4 0 36 0.050‐99 1 70 1.4 3 64 4.7100‐149 1 55 1.8 4 61 6.6150‐199 1 53 1.9 1 44 2.3200‐249 2 41 4.9 7 28 25.0250‐299 4 35 11.4 1 40 2.5300‐349 0 18 0.0 2 18 11.1350‐399 4 18 22.2 0 22 0.0400‐449 2 13 15.4 0 16 0.0450‐500 2 17 11.8 1 12 8.3Nodrivewaywithin500ft. 0 141 0.0 0 149 0.0Total 18 490a 5.2b 19 490a 5.6c

a The131roundaboutshave490legs.b Thevaluedoesnotincludeapproacheswithnodrivewayswithin500ft.Itiscalculatedas(18×100)/(490‐141).c Thevaluedoesnotincludeapproacheswithnodrivewayswithin500ft.Itiscalculatedas(19×100)/(490‐149).

Theseabovestatisticsindicatethatthedownstreamdrivewaycornerclearancehasagreatersafetyimpactthantheupstreamdrivewaycornerclearance.Thisresultisconsistentwiththefactthatvehiclesexitingadownstreamdrivewayexperiencereducedgapsduetodispersedplatoonsfromtheupstreamroundabout.Thisisfurtheraggravatedbythefactthatroundaboutsalsoprovidelargercornerturningradii,allowingvehiclestoturnrightatahigherspeed.Atcornerswithreducedsightdistance,itfurtherreducesthetimeavailablefordrivewayvehiclestocompletetheirmaneuvers.Figure40showsanexamplelocationthathasadownstreamdrivewaycornerclearanceoflessthan150ft.andwithareducedsightdistanceduetosightobstructions.



Figure40.RoundaboutonSRA1A,NassauCounty,FloridawithReducedSightDistanceatDownstreamCornerClearance

5.3SafetyImpactofMedianOpeningsintheVicinityofRoundaboutsOncorridorswithraisedmedians,accesstoabuttinglanduseisoftenprovidedthroughmedianopenings.Sinceroundaboutsdisperseplatoons,turningtrafficatmedianopeningsinthevicinityofroundaboutsexperiencereducedvehiclegaps,whichcouldresultinmorecrashes.ThissectionexaminesifmedianopeningsinthevicinityofroundaboutsposeasafetyconcernandwhetherclosingthemedianopeningsandhavingvehiclesmakeU‐turnsatthedownstreamroundaboutwouldbebeneficial.Figure41(a)showsacaseinwhichvehiclesfromthemainstreetturnleftatamedianopeningontoadrivewaythatislocateddownstreamofaroundabout.Figure41(b)showsanalternativewithoutthemedianopeningandrequirethevehiclestomakeU‐turnsattheroundaboutdownstreamandthenmakearightturnontothedriveway.Similarly,Figure42(a)showsasecondcaseinwhichvehiclesexitingfromadrivewaylocatedupstreamofaroundaboutturnleftatamedianopeningontothemainstreet.Figure42(b)showsanalternativewithoutthemedianopeningandrequirethevehiclestofirstturnrightandthenmakeaU‐turnattheroundabouttocompletetheleftturn.Inbothoftheabovecases,thefirstquestioniswhethercrashstatisticsshowsignificantsafetyproblemsassociatedwiththeleft‐turningvehicles,eitherontooroutofadriveway.Toanswerthisquestion,crashesinvolvingvehiclesturningleftatmedianopenings(i.e.,vehiclesturningleftfromthemainstreetontoadrivewayandvehiclesturningleftfromadrivewayontothemainstreet)wereidentifiedbyreviewingthepolicereports.The131roundaboutswerefoundtohaveatotalof157medianopeningswithin500ft.Thecrashdatashowthat,during2007‐2011,arelativelylowtotalof15crashesoccurredatthese157medianopenings.Ofthese15crashes,eightinvolvedvehiclesturningleftfromthemainstreetontoadrivewayandseveninvolvedvehiclesturningleftfromadrivewayontothemainstreet.Figure43andFigure44giveexamplesofthesetwoscenarios,respectively.Amongthecrashesinvolvingvehiclesturningleftfromadriveway,onlyonecrashresultedinanon‐incapacitatinginjuryandtherestwerePDOs.AsshowninFigure45,theonlycrashinvolvinganinjuryoccurredwhenavehicleturningleftfromadrivewayontothemainstreetcollidedwithabicyclist.Oftheeightcrashesthatinvolvedvehiclesturningleftfromthemainstreetontoadriveway,threeresultedininjuries,onewasapossibleinjury,andtheremainingfourwerePDOs.



(a)PotentialSafetyProblem:VehiclesTurnLeftfromMainStreetontoaDrivewayatMedianOpeningwith

ReducedGaps

(b)Alternative:VehiclesfromMainStreetTurnontoaDrivewaybyMakingaU‐turn

atDownstreamRoundaboutFigure41.Case1‐VehiclesTurningontoaDrivewayDownstreamoftheRoundabout



(a)PotentialSafetyProblem:VehiclesfromDrivewayTurnLeftatMedianOpeningwithReducedGaps

(b)Alternative:VehiclesfromDrivewayTurnLeftbyMakingU‐turnsatDownstreamRoundabout

Figure42.Case2‐VehiclesTurningLeftfromaDrivewayUpstreamofaRoundabout



Figure43.AnExampleofaCrashataMedianOpeningInvolvingaVehicleTurningLeftFromtheMainStreetOntoaDriveway(CrashID:820970050)

Figure44.AnExampleofaCrashataMedianOpeningInvolvingaVehicleTurningLeftFromaDrivewayOntotheMainStreet(CrashID:801477040)

Figure45.ANon‐incapacitatingInjuryInvolvingaVehicleTurningLeftfromDrivewayandaBicyclist(CrashID:801468970)



Giventhatexistingmedianopeningsdidnotposesignificantsafetyproblemintermsofbothcrashnumberandcrashseverity,thesecondquestioniswhetherclosingthemedianopeningcouldbebeneficial.Whileitisuncertainhowmanyofthe15crashesrelatedtothemedianopeningscouldhavebeenpreventedbyrequiringvehiclestomakeaU‐turnatroundabouts,theU‐turnalternativeisknowntoposetwopotentialtrafficoperationalproblems.First,theU‐turnalternativemayincreasethenumberofsideswipecrashesatroundaboutsespeciallyforlargevehicles.Largetrucksandbusesoftenfinditdifficulttonegotiateasmallerroundabout.Particularly,lackofadequatelateralclearancecouldresultinheavyvehiclessideswipingothervehiclesorbecominginvolvedinacollisionwithafixedobject,usuallywiththeroundaboutcenterisland.During2007‐2011,atotalof18crashesinvolvedheavyvehiclesatthe131commercialroundabouts.Figure46showsexamplesofthesecrashes.Vehiclehittingafixedobject,followedbyangleandsideswipecrasheswerepredominantlyobserved.AllofthesecrasheswerefoundtobePDOs.

(a)Fixed‐ObjectCrash

(b)SideswipeCrash

Figure46.ExamplesofCrashesInvolvingHeavyVehiclesatRoundabouts



Second,theU‐turnalternativepreventscertainturningmovements,whichmayresultincrasheselsewhere.Closingthemedianopeningpreventsthefollowingtwoturningmovements:(1)itpreventsvehiclesfromturningleftfromadrivewayontothemainstreet;and(2)itpreventsvehiclesfromturningleftfromthemainstreetontoadriveway.Figure47andFigure48illustratethesetwoscenarios.AsshownFigure47,thevehiclefromthedrivewaycannotturnleftontothemainstreetandthevehiclehastoturnrightandmakeaU‐turndownstream.Similarly,asshowninFigure48,thevehiclefromthemainstreetcannotturnleftontothedrivewaywhenthemedianopeningisclosed.Thevehicleshastogostraight,makeaU‐turndownstream,andthenturnrightatthedriveway.

Figure47.ClosingMedianOpeningsPreventVehiclesFromTurningLeftFromtheDriveway

OntotheMainStreet

Figure48.ClosingMedianOpeningsPreventVehiclesFromTurningLeftFromtheMainStreet

OntotheDriveway

Thislimitation,however,suggeststhatifthereisasecondadjacentroundaboutdownstream(i.e.,roundaboutsinseries)tofacilitatetheU‐turns,closingthemedianopeningcouldbecomebeneficial,asit



couldpotentiallypreventsomeofthemedianopeningrelatedcrasheswithoutmakingsometurningmovementsdifficult.Figure49providesanexampleofacandidatelocationforconstructingraisedmedianstoeliminateleftturningmovementsinvolvingvehiclesenteringandexitingthedriveways.Again,thisalternativeisviableonlywithlowvolumeofheavyvehiclesorwithlargerroundaboutsthatcouldbetteraccommodatelargevehicles.

(a) Withoutraisedmedians(b)Withraisedmedians

Figure49.ACorridorwithTwoRoundaboutsonSegoviaStreet,MiamiDadeCounty,Florida

5.4SafetyatRoundaboutsThatProvideDirectAccesstoActivityCentersAccesstomajoractivitycenters,suchasbigboxretailstores,shoppingcenters,andmalls,isoftenprovidedatmid‐blocklocationsonacorridor.Figure50givesanexampleofthisscenario.Suchaccesscreatesanintersectionoramajordrivewaytothedetrimentoftrafficflowonthecorridor.Onealternative,asshowninFigure51,istohavetheaccesspointconnecteddirectlytotheroundabout,sendingallaccesstrafficthroughtheroundaboutcirculationlane(s).Doroundaboutsthatprovidedirectaccesstoactivitycentersthroughadedicatedlegperformlessfavorablyinsafetythanotherroundabouts?



Figure50.AnActivityCenterwithAccessThroughaMajorDriveway

Figure51.AnActivityCenterwithDirectAccessfromaRoundabout

Ofthe131commercialroundaboutsinFlorida,19roundaboutswerefoundtoprovidedirectaccesstotheactivitycenters.Ofthese19roundabouts,15haveeitherthreeorfourlegs.Thecrashexperienceofthree‐andfour‐leggedroundaboutswithandwithoutdirectaccesstotheactivitycenterswascomparedusinganindependentt‐testwiththefollowinghypothesis:

H0:thereisnodifferenceinmeans(i.e.,averagecrashesperroundabout)betweentheroundabouts

withandwithoutdirectaccesstoactivitycenters(µ1=µ2), H1:therearedifferencesinmeans(i.e.,averagecrashesperroundabout)betweentheroundabouts

withandwithoutdirectaccesstoactivitycenters(µ1≠µ2).Table25summarizestheseresults.Ata5%significancelevel,theperformanceofthethree‐leggedroundaboutswithandwithoutdirectaccesswasstatisticallyinsignificant,whiletheperformanceofthefour‐leggedroundaboutswithandwithoutdirectaccesswasstatisticallysignificant.Overall,therewassufficientevidencetosupporttheconclusionthatata5%significancelevel,therewasnosignificantdifferenceintheperformanceofthree‐andfour‐leggedroundaboutswithdirectaccesstoactivitycentersandthosewithoutdirectaccess.



Table25.StatisticsofRoundaboutswithThreeandFourLegs

No.ofLegs

RoundaboutsWITHDirectAccesstoActivityCentersThrough

aDedicatedLeg

RoundaboutsWITHOUTDirectAccesstoActivityCentersThrough

aDedicatedLeg

Ata5%SignificanceLevel,Isthe

PerformanceofRoundaboutsWithand

WithoutDirectAccess

SignificantlyDifferent?1

TotalCrashesinFiveYears(a)

NumberofRoundabouts

(b)


(a/b)

TotalCrashesinFiveYears(c)

NumberofRoundabouts

(d)


(c/d)

3 23 5 4.6 163 39 4.2 No(p‐value:0.925)

4 33 10 3.3 473 60 7.9Yes

(p‐value:0.021)

3and4 56 15 3.7 636 99 6.4No

(p‐value:0.145)1 Ata5%significancelevel,ifP‐value<0.05,itisconcludedthatthereisasignificantdifferenceintheperformance

ofroundaboutswithdirectaccesstoactivitycentersandthosewithoutdirectaccess.Similarly,ifP‐value>0.05,itisconcludedthatthereisnosignificantdifferenceintheperformanceofroundaboutswithdirectaccesstoactivitycentersandthosewithoutdirectaccess.

Thenextquestioniswhetheritwouldbebeneficialifprovidingdirectaccesstoactivitycentersresultsinfiveormorelegsataroundabout,i.e.,morethanthetypicalroundaboutswiththreeorfourlegs.Table26showsthecrashstatisticsofcommercialroundaboutswithfiveandsixlegs.Itcanbeseenthattheseroundaboutsexperiencedasignificantlyhighernumberofcrashes,especiallyinthesix‐leggedcase,whencomparedwiththoseofthree‐andfour‐legged.Thesignificantincreaseintheaveragecrashesisexpectedastheadditionallegsquicklyincreasethenumberofconflictpointsinthecirculationlanesandbecomeconfusingtothedrivers.Figure52givesexamplesoftwosix‐leggedroundaboutswhichcollectivelyexperienced154crashesduringthefive‐yearanalysisperiod.Theabovecrashstatisticssuggestthatprovidingdirectaccesstoactivitycentersatroundaboutsisdesirable,butonlyifitdoesnotincreasethenumberofroundaboutlegsbeyondthestandardfourlegs.Table26.StatisticsofRoundaboutswithFiveandSixLegs

No.ofLegsTotalCrashesinFiveYears

(a)Numberof Roundabouts

(b)CrashesperRoundabout

(a/b)5 157 10 15.76 213 4 53.3

5and6 370 14 26.4



(a)PonceDeLeon,MiamiDadeCounty,Florida

(b)MemorialCausewayBoulevard,PinellasCounty,Florida

Figure52.ExamplesofSix‐leggedRoundaboutsthatExperiencedHighCrashes



5.5SafetyofVulnerableRoadUsersThissectionfocusesonevaluatingthesafetyofvulnerableroadusers(i.e.,pedestriansandbicyclists)inthevicinityofroundabouts.5.5.1PedestriansDuring2007‐2011,the131roundaboutsincommercialareasexperiencedatotalof20pedestriancrashes,constituting1.06%ofthetotalcrashes.Ofthe20pedestriancrashes,onewasfatalandtworesultedinsevereinjuries.Thefatalcrashinvolvedapedestrianwhowasintoxicated.Figure53givestheillustrativesketchofthecrash.Besidesthisfatalcrash,apedestrianwasfoundtobeintoxicatedinoneothercrash,whichresultedinanon‐incapacitatinginjury.Illustrativesketchesanddescriptionsofthe20pedestriancrasheswerereviewedindetailtodeterminetheat‐faultroaduser.Ofthe20pedestriancrashes,10(i.e.,50%)occurredduetodriverfault,andthepedestrianwasfoundtobeatfaultinsevencrashes(i.e.,35%).Forthreecrashes,identifyingtheat‐faultroaduserwasnotpossibleduetoinconclusiveinformationinthepolicereports.Whenthepedestrianwasfoundtobeatfault,thefollowingwerethemostfrequentcontributingcauses(numberinparenthesesindicatesthenumberofrelatedcrashes):

pedestrianobstructedthepathofvehicles(3), pedestrianfailedtoyieldright‐of‐waytothevehicle(2),and pedestrianwasundertheinfluenceofalcoholand/ordrugs(2).

Figure53.FatalCrashInvolvingaPedestrian(CrashID:772427040)

Whenthedriverwasfoundtobeatfault,themostfrequentcontributingcauseswere:

carelessdriving(5), driverfailedtoyieldright‐of‐waytothepedestrian(4),and driverdisregardedtrafficsignalorothertrafficcontrol(1).

Ofthe20pedestriancrashes,onlytwocrashesoccurredatroundabouts,andtheremaining18crashesoccurredontheapproachlegs.Crashesthatoccurredontheroundaboutlegswerereviewedindetailto



identifyanyspecificcontributingfactors.Table27providespedestriancrashstatisticsbymediantype.Ofthe18pedestriancrashesthatoccurredontheroundaboutlegs,11crashes(61.1%)occurredatraisedmedians,oneoccurredonalegwithTWLTL,whiletheremainingsixoccurredonundividedsections.Fromthetable,itisalsoclearthatthenumberofpedestriancrashesper100legswashighestforraisedmediansat6.40pedestriancrashesper100legs.Further,itwasfoundthatallthreesevereinjurypedestriancrashesoccurredonapproacheswithapostedspeedgreaterthan30mph,andlow‐speedcorridors(i.e.,postedspeedlimit≤30mph)didnotexperienceseriousinjuries.Table27.PedestrianCrashStatisticsbyMedianType

MedianTypeNumberof

PedestrianCrashes(a)

NumberofApproachLegs

(b)

NumberofPedestrianCrashesper100ApproachLegs

(a)/(b)RaisedMedian 11 172 6.40

TWLTL 1 18 5.56

UndividedSections 6 281 2.14

Other 0 19 0.00

Total 18 490 3.67

5.5.2BicyclistsDuring2007‐2011,atotalof47bicycle‐vehiclecrashesoccurredinthevicinityofthe131roundabouts.Althoughnoneofthecrasheswerefatal,amajorityofthecrashesresultedinaninjury.Asitcanbeinferredfromthetable,48.9%ofbicyclecrasheswerearesultofdrivererrorwhile40.4%ofthecrasheswereduetobicyclisterror.Whenthebicyclistwasfoundtobeatfault,thefollowingwerethemostfrequentcontributingcauses(numberinparenthesesindicatesthenumberofrelatedcrashes):

1. bicyclistfailedtoyieldright‐of‐waytothedriver(7),2. bicyclistobstructedvehicles’pathbyeitherfallingoffthebikeorlosingcontrolofthebikeintothe

pathofthevehicle(6),and3. bicyclistrodeintoastoppedvehicle(3).

Whenadriverwasfoundtobeatfault,themostfrequentcontributingcauseswere:

driverfailedtoyieldright‐of‐waytothebicyclist(13)and carelessdriving(9).

ThecorridoronSWSecondAvenueinGainesvillehasthreeroundaboutsandhad12bicyclecrashes(i.e.,25.5%oftotalbicyclecrashes)duringthefive‐yearanalysisperiod.Figure54showstheaerialviewofthiscorridor,whichisneartheUniversityofFlorida(UF)mainentrance.Thiscorridorwasfoundtohaveasignificantamountofbicycletraffic;thisdisproportionatelyhighexposureresultedinahighnumberofbicyclecrashes.

Table28givesbicyclecrashstatisticsbasedonwherethecrashhadoccurred(i.e.,eitherattheroundaboutoronanapproachleg).The131roundaboutshave490legs;86ofthesehavedesignatedbikelanes.During2007‐2011,these86legsexperiencedeightbicyclecrashes,whiletheremaining404legswithoutdesignatedbikelanesexperienced20bicyclecrashes.However,thesestatisticsdonottakeintoaccountbicycleexposuredata.Inotherwords,locationswithdesignatedbikelanesmightexperiencemorebicyclecrashessimplybecausemorebicyclistsusethefacility.



ThecorridoronSWSecondAvenueinGainesvillehasthreeroundaboutsandhad12bicyclecrashes(i.e.,25.5%oftotalbicyclecrashes)duringthefive‐yearanalysisperiod.Figure54showstheaerialviewofthiscorridor,whichisneartheUniversityofFlorida(UF)mainentrance.Thiscorridorwasfoundtohaveasignificantamountofbicycletraffic;thisdisproportionatelyhighexposureresultedinahighnumberofbicyclecrashes.

Table28.BicycleCrashStatisticsbyLocationandCrashSeverity

CrashSeverity CrashesatRoundabout CrashesonApproachLeg TotalBicycleCrashes

FatalInjury 0 0 0IncapacitatingInjury 1 4 5Non‐IncapacitatingInjury 12 11 23PossibleInjury 3 9 12PropertyDamageOnly 3 4 7TotalCrashes 19 28 47

Figure54.CorridoronSW2ndAvenue,Gainesville,AlachuaCounty,Florida

5.6SummaryofFindingsAtotalof283roundaboutsinFloridawereincludedintheanalysis.During2007‐2011,1,882crashesoccurredwithin500ft.oftheseroundabouts.Policereportsofthesecrasheswerereviewedindetailtoinvestigatethefollowingpotentialsafetyconcernsassociatedwithroundaboutsincommercialareas:

Impactofdrivewaycornerclearancesonroundaboutsafety. Safetyimpactofmedianopeningsinthevicinityofroundabouts. Safetyatroundaboutsthatprovidedirectaccesstoactivitycenters. Safetyofvulnerableroadusersincludingpedestriansandbicyclists.

Onaverage,eachroundaboutexperienced6.65crashesduringthefive‐yearanalysisperiod.AmajorityofcrasheswerefoundtobePDOs.Lessthan5%ofcrashesresultedinsevereinjuries(i.e.,fatalinjuryandincapacitatinginjury).Intermsofcrashtype,collisionwithafixedobject,anglecrashesandrear‐endcrasheswerepredominant,constitutingover60%oftotalcrashes.



During2007‐2011,the131commercialroundaboutsexperiencedatotalof74driveway‐relatedcrashes.Ofthesecrashes,37(50%)occurredatthefirstdriveway(i.e.,thedrivewaythatdefinesthecornerclearance),including18thatoccurredattheupstreamdrivewaycornerclearance,and19thatoccurredatthedownstreamdrivewaycornerclearance.Morecrasheswerefoundtooccuratthefirstdrivewaydownstreamratherthanupstreamofroundabouts,indicatingthatdownstreamdrivewaycornerclearanceshaveagreatersafetyimpactthanupstreamdrivewaycornerclearances.Thisresultisconsistentwiththefactthatvehiclesexitingadrivewaydownstreamofaroundaboutexperiencereducedgapsduetodispersedplatoonsfromtheupstreamroundabout.Further,largercornerturningradiitypicalofroundaboutsincreasesvehicle‐turningspeed.Whencombinedwithreducedsightdistanceduetosightobstructions,thetimeavailablefordrivewayvehiclestocompletetheirmaneuverscouldbesignificantlyreduced.Athigh‐volumelocations,turningtrafficatmedianopeningsinthevicinityofroundaboutsexperiencereducedvehiclegaps,whichcouldresultinmorecrashes.Toaddressthispotentialsafetyconcern,crashesatmedianopeningsinvolvingleft‐turningvehicleswereidentified.Arelativelylowtotalof15crasheswerefoundtoinvolveturningvehiclesatthemedianopenings,andamajorityofthesewerenotsevere.Crashdatadidnotindicateanyserioussafetyissueswithmedianopeningsinthevicinityofroundabouts.Nonetheless,closingthemedianopeningsandhavingvehiclesmakeU‐turnsatthedownstreamroundaboutcouldpotentiallypreventsomeofthesecrashes.However,thisalternativewasfoundtoposetwotrafficoperationalproblems.First,thisalternativemayincreasethenumberofsideswipecrashesatroundaboutsespeciallyforlargevehicles.Thesecondproblemwithclosingmedianopeningsisthatitpreventscertainturningmovements,whichmayresultinmigrationofcrashes.ThisproblemexistsbecausethereisnotanotherroundaboutavailabletofacilitatealltheU‐turnsneededwhenmedianopeningsareclosed.Atlocationswithbothupstreamanddownstreamroundabouts(i.e.,roundaboutsinseries),closingthemedianopeningcouldbecomebeneficial,asitcouldpotentiallypreventsomeofthemedianopeningrelatedcrasheswithoutmakingsometurningmovementsdifficult.Accesstomajoractivitycentersisoftenprovidedatmid‐blocklocationsonacorridor.Onealternativeistohavetheaccesspointconnecteddirectlytotheroundabout(i.e.,throughadedicatedleg).Ofthe131commercialroundaboutsinFlorida,19roundaboutswerefoundtoprovidedirectaccesstotheactivitycenters.Averagecrashesperroundaboutatthree‐andfour‐leggedroundaboutswithandwithoutdirectaccesstotheactivitycenterswerecomparedusinganindependentt‐test.Ata5%significancelevel,therewasnosignificantdifferenceintheperformanceofthree‐andfour‐leggedroundaboutswithdirectaccesstoactivitycentersandthosewithoutdirectaccess.Itwasalsofoundthatroundaboutswithmorethanfourlegsexperiencedasignificantlyhighernumberofcrashes.Thiswasexpectedastheadditionallegsincreasethenumberofconflictpointswithinthecirculationlanesandbecomeconfusingtothedrivers.Overall,thecrashstatisticssuggestthatprovidingdirectaccesstoactivitycentersatroundaboutsisdesirable,butonlyifitdoesnotincreasethenumberofroundaboutlegstobeyondthestandardfourlegs.Safetyofvulnerableroadusers(i.e.,pedestriansandbicyclists)inthevicinityofroundaboutswasevaluated.Duringthefive‐yearanalysisperiod,the131commercialroundaboutsexperienced20pedestriancrashes.Ofthese20crashes,onlytwooccurredatroundabouts,whiletheremaining18occurredontheroundaboutlegs.Comparedtopedestriancrashes,bicyclecrashesweremorefrequent;during2007‐2011,47bicyclecrasheswerereported.Ofthese47,19occurredatroundaboutsandtherestwereontheroundaboutlegs.Roundaboutlegswithdesignatedbikelanesresultedinaslightlygreaterproportionofbicyclecrashescomparedtothosewithoutbikelanes.However,thisobservationdidnottakeintoaccountbicycleexposuredata,whicharenotavailableforthisstudy.Basedontheresultsfromthesafetyanalysis,thefollowinggeneralrecommendationsrelatedtotheaccessfeaturesinthevicinityofroundaboutsaremade:



Crashdatashowthatdownstreamdrivewaycornerclearanceshaveagreatersafetyimpactthanupstreamdrivewaycornerclearances.Longerdownstreamcornerclearancesaredesirabletoprovideadditionaltimefordrivewayvehiclesthatexperiencereducedvehiclegapsandhigherapproachvehiclespeedfromupstreamroundabouts.

Crashdatadidnotindicateserioussafetyissueswithmedianopeningsinthevicinityofroundabouts.However,closingmedianopeningslocatedbetweentwoadjacentroundaboutscouldpreventsomeofthemedianopeningrelatedcrashesandisdesirableifthecorridorisdesignedtoservelowheavyvehiclevolumesoriftheroundaboutsaresufficientlylargetosafelyaccommodateU‐turnsbyheavyvehicles.

Crashdatadidnotshowanincreasedsafetyhazardatroundaboutsthatprovidedirectaccesstoactivitycenters.Providingdirectaccesstoactivitycentersthroughadedicatedlegisdesirabletoimprovetrafficoperationsonthecorridoriftheprovisiondoesnotincreasethenumberofroundaboutlegstobeyondthestandardfour.

Chapter6OperationalAnalysis


ChapterSix:OperationalAnalysisThissectionpresentsthefindingsoftheoperationalanalysisofroundaboutsandaccessmanagement.Analysisofconflicts(involvingvehicles,pedestriansandbicyclists,etc.),accesstodrivewaysandviolationoftrafficrulesatroundaboutsareconductedtosummarizetheissuesrelatedtoaccessmanagement.6.1OverviewofDataCollectionSitesSiteobservationsanddatacollectionwereconductedatthe13selectedsitesinFlorida.DetailsoftheroundaboutsareincludedinAppendixD.Ingeneral,thesiteswereselectedbasedupontrafficvolume,proximityorlocationonstatehighways,orincloseproximitytodriveways.Adiversityofconditionswereselectedforthefollowingcharacteristics:singleandmultiplelanes,commercialandmixed‐laneusesadjacenttotheroundabout,proximitytoparking,asingleisolatedroundaboutandaroundaboutcorridor.All13roundaboutsselectedforobservationareconsideredtohaveatleastamoderatetrafficvolumelevelduringpeakperiods.Sevensitesaresingle‐laneroundaboutsandfivearemulti‐laneroundabouts.Theotherisconsideredacomplexroundabout,whichincludespiralroundabouts,turboroundaboutsoraroundaboutthathasmultiplesliplanes.Ninesitesarelocatedincommercialareas;theremainingfourarelocatedinamixed‐usearea.Allofthesiteshaveadrivewaynearby.Allofthemhavedrivewaysnearbothaccessandegresslegsoftheroundabout.Sevenofthesiteshavedrivewaysnearboththeaccessandegressapproachesoftheroundabout.Twositeshavedrivewaysinthemiddleoftheroundabout.Allofthesitesarelocatednearstatehighways,andoneisonastatehighway.Twositesarelocatedonstreetswithon‐streetparking,whereintheparkingmaneuveronthestreetcouldaffecttheoperatingspeed,safety,andperhapsaccessoftheroundabout.Foursitesarelocatedinaseriesofseveralroundabouts.Theliteraturesuggeststhataseriesofroundaboutsinacorridor,particularlyacommercialcorridor,canprovideamoreaestheticallypleasingarea,slowtraffic,andimproveaccessandsafety.Buildingaseriesofroundaboutscancreateavibrantbusinessarea.Therefore,itisdesirabletolookattheperformanceandaccessissuesofaseriesofroundabouts.6.2AnalysisofAccessManagementIssuesAffectingOperationsDuringthefieldobservations,severalaccessmanagementissueswereidentifiedattheroundabouts.Theseinclude:(1)conflictataccesspointswithinthefunctionalarea,whichincludesintersectionsofadrivewayandapproachinglaneofaroundabout,andtheimpactofqueuingontheoperationofanearbystop‐controlleddriveway;(2)conflictsatroundaboutsinvolvingpedestriansandbicyclists;and(3)violationoftrafficrulesanditsimpactontheroundaboutoperations.Eachoftheseissuesisaddressedseparatelybelow.6.2.1ConflictsatAccessPointwithinRoundabout’sFunctionalAreaIfanaccesspoint,suchasadrivewayoranotherintersection,islocatedwithintheroundaboutfunctionalarea,vehicleconflictsmayoccurandcompromisetheoperationoftheroundabout.Theconflictbetweenavehiclemakingaleftturnintoadrivewayandtheopposingtrafficflowenteringtheroundaboutwasacommonfieldobservation.Figure55showsanexamplethatwasobservedatSW2ndAvenueandSW6thStreetintheCityofGainesvilleinAlachuaCounty.Thedrivewayislocatedneararoundabout(60ft.).Whenthequeuespillsbackattheleft‐turninglane,leftturningvehiclesfromtheexitinglaneintothedrivewaycanbeblockedattheturningbay,causingaspillbackintotheroundabout,whichtheninterfereswiththeoperationoftheentireintersection.



Figure55.ConflictofLeft‐turnVehicleatRoundabout(SW2ndAvenueandSW6thinAlachuaCounty)

AnothercaseiswhenanAWSCdrivewayisclosetoaroundabout.Inthissituation,thetrafficenteringtheroundaboutcanspillbackintothedriveway.Figure56showsaroundaboutinMiami‐DadeinwhichthetrafficspillsbackfromtheapproachinglaneandblockstheoperationoftheAWSCdriveway.Acertaindistanceisclearlyneededbetweentheroundaboutandthenearbyintersection.

Figure56.RoundaboutObservationonSpillBackofEnteringTrafficintoanAdjacentAWSCIntersection(NE10thCt.andSW152ndAve.,Miami)

6.2.2ConflictswithPedestriansFigure57showstheinteractionbetweenpedestriansandvehiclesatIndependentDriveandSouthLauraStreet,inJacksonville.Sincethisroundaboutislocatedinabusinessandcommercialarea,wecanobservearelativelyhighflowofpedestriantraffic.Whenacarstopsforapedestrianatacrosswalk,thequeuebehindthecarspillsbackintothecirculatinglane,andaffectstheoperationoftheroundabout.



Figure57.RoundaboutObservationwithPedestrianConflict(IndependentDr.andS.LauraSt.,Duval

County)6.2.3ViolationofTrafficRulesThereareseveralcaseswheredriversviolatetrafficrulesandstopinthemiddleofroundabouts.Figure58showsacaseatIndependentDriveandSouthLauraStreet,inJacksonville,wheretheroundaboutisplacednearabusinessandshoppingcenterdowntown.Peopletendtopickuppeopleattheroundaboutandcauseaqueueback‐upinthecirculatinglane.

Figure58.RoundaboutObservationwithDriverViolationofTrafficRules(IndependentDr.andS.Laura

St.,DuvalCounty)Anotherexampleofviolationoftrafficrulesiswhenvehiclesstopatthedrivewayandpickuppeople.Thequeuespillsbackintothecirculatinglaneandcausesonelanetojam.Carsinthislanetrytochangetotheothercirculatinglaneanddisrupttheoperationoftheroundabout.



Figure59.RoundaboutObservationwithSpillBackfromDrivewayintoCirculatingLanes(CausewayBlvd.

andMandalayAve.,PinellasCounty)6.2.4SummaryofOperationalAnalysisInmostcases,roundaboutsoperateinamannersimilartoothertypesofintersections,suchasnon‐signalizedintersections.Thus,fromanoperationalperspective,accessmanagement,shouldbemanagedinawaythatissimilartootherintersections.Howeverthecombinationofroundaboutandaccessmanagementdoeshavesomeuniquefeaturesforoperations.Insummary,thefollowingsuggestionsaremadetocountertheproblemsfoundinthesiteobservations.Beforethedesignandconstructionoftheintersection,thedistancebetweentheroundaboutandnearbydrivewaysshouldbecarefullyconsideredinordertokeepthedrivewayandroundaboutsinoperation.Thedistancebetweentheroundaboutandthenearbyintersectionshouldalsobecarefullyconsideredandenoughstoragecapacityshouldbeprovidedtokeeptheroundaboutandanyadjacentintersectionsfunctioningproperly.Ifthetrafficvolumeismoderateandthepercentageofheavyvehiclesislow,whenadrivewayhastobelocatedclosetoaroundabout,amedianclosingshouldbeusedandanotherroundaboutatthenextintersectionisrecommendedtoallowU‐turnsforaccessingdriveways.Ifaroundabouthaslessthan4legs,accesstonearbyactivitycentersshouldbeprovidedbyusingaseparatedriveway,insteadoflinkingtheroundabouttotheactivitycenteritself(asshowninFigure58wherevehiclesstoppedintheroundabouttopickupapassenger);ifmorethan4legsareincluded,trafficdesignersshouldavoidaddingonemorelegtotheroundaboutbasedonthefindingsinChapter5.Additionally,drivereducationisnecessarytomaintainroundaboutoperations.6.3AssessmentofSoftwareAnumberofsoftwarepackagescanbeusedtoanalyzetheoperationaleffectofroundabouts.Basedontheirmethodology,wecandividethemintotwodifferentgroups:deterministicsoftwaretoolsandsimulationtools.Deterministicmethodsmodelvehicleflowsasflowratesandaresensitivetochangesinflowrateandthegeometricdesignofroundabouts(Rodegerdtsetal.,2010,p.4‐18).Macroscopicanalysistoolsalsofallintothiscategory(Trueblood,2013).ExamplesofsoftwarepackagesthatimplementdeterministicanalysismethodsareHighwayCapacitySoftware(HCS),ARCADY,RoundaboutDelay(RODEL),SIDRA,andSynchro.Microscopicsimulationisanotherwaytomodelroundabouts.Suchtoolscanmodelanddisplayindividualvehiclesandthusaresensitivetofactorsatthatlevel:car‐followingbehavior,lane‐changingbehavior,anddecision‐makingatintersectionssuchasgapacceptance(Rodegerdtsetal.,2010,p.4‐19).ExamplesofsoftwarepackagesthatperformmicroscopicsimulationareCORSIMandVISSIM.



6.3.1HCSHCSstandsforHighwayCapacitySoftware,whichisasoftwarepackagethatimplementsthedeterministic,macroscopicanalysismethodsoftheHighwayCapacityManual.TheprocessitemploysistheHighwayCapacityManualprocedure,whichusescriticalgapandfollow‐uptimealongwithturningmovementtocomputethecapacityofeachapproach.ThenewestversionofHCS2010,basedontheHCM2010,providedanewanalyticalmethodinassessingroundaboutoperations.Approachcontroldelay,approachLOS,intersectiondelayandintersectionLOScanbecalculatedbythesoftware(TRB,2010a).ThemethodologyinHCM2010focusedontheoperationofroundaboutswithintheboundariesoftheroundabout.ThismethodologyprovidesacombinationofanempiricalapproachandananalyticalapproachforevaluatingroundaboutoperationsbasedonrecentU.S.fielddata(Rodegerdtsetal.2010).Evaluationforbothsingle‐laneanddouble‐laneroundaboutsareprovidedinHCM2010.ThereforeinHCS,wecanonlymodelroundaboutswithtwoorlesscirculatinglanes.

Figure60.InterfaceofHCS2010

InTable29.InputandOutputforRoundaboutComponentsinHCS2010,theinputforcalculationroundaboutsinHCS2010isshown.SinceHCS2010adoptedthemethodologyinHCM2010,morefeatureshavebeenavailableinassessingroundaboutperformance.



Table29.InputandOutputforRoundaboutComponentsinHCS2010

InputParameters HCS2010

TurningFlows InputPeakHourFactor Input

CriticalGap InputFollow‐upHeadway Input

OutputPerformanceMeasures HCS2010

Capacity YesApproachDelay YesApproachLOS Yes

Queue YesIntersectionDelay YesIntersectionLOS Yes

ThetwomostimportantparametersintheHCM2010roundaboutmodelarecriticalgapandfollow‐upheadways.Thesetwovaluesplayanimportantroleintheoperationalanalysisofbothsingle‐laneanddouble‐laneroundabouts(TRB,2010a).OneofthedisadvantagesoftheHCM2010modelforassessingroundaboutandaccessmanagementisthatitdoesn’taccountforeffectsrelatedtogeometrysuchaslanewidth,ortrafficflowfromadjacentintersections(Trueblood,2013).HCS2010hastheabilitytocalculateroundaboutapproachqueuelengths.Thisfeatureisessentialtounderstandingaccessmanagementissuerelatedtoroundabouts.6.3.2SynchroSynchroisananalysistoolforstudyingintersectionsatamacroscopicscale.SimilartoHCS,SynchrocanalsobeusedtoassessroundaboutperformancebasedontheHCM2010methodology.CodingaroundaboutisverystraightforwardwithinSynchro.Theuseronlyneedstospecifytheintersectioncontroltypeasaroundaboutaftersettingupanintersectionwiththespecificgeometryandvolumedata.IftheHCM2010methodwasselectedinSynchro,theoutputresultsshouldbepresentedinthemannershowninFigure61.

Figure61.UserInterfaceofSynchro(Trueblood,2013)



Synchroalsocomeswithamicro‐simulationtoolcalledSimTraffic.ThistoolallowstheusertodesignandevaluateadvancedroundaboutsdesignsthatexceedtheHCM2010methodologylimitations.Forinstance,HCScannotmodelroundaboutswithmorethantwocirculatinglanes(Trueblood,2013).Synchrocanalsoassesstheperformanceofaseriesofroundaboutsinacorridor.6.3.3SIDRASIDRAwasoriginallydevelopedbyARRBTransportResearchLtd.andlaterbyAkcelik&Associates(Akcelik&Associates,2014).ItisoneofthemostwidelyusedroundaboutanalysissoftwareprogramsintheUnitedStates(Jacquemart,1998).Themodelisbasedonananalyticalmethod,whichusesgap‐acceptancetechniquestodetermineroundaboutcapacity,delay,queuelength,andotherperformancemeasures.SimilartotheHCM2010,SIDRAincludestwoimportantgapparameters:criticalgapandfollow‐upheadway.Thecriticalgapandfollow‐upheadwayvaluescanbeeitherspecifiedbytheuserorautomaticallyestimatedbySIDRAaccordingtothegeometryandflowconditionsateachentry(Yinetal.,2011).AlthoughSIDRAwasdevelopedinAustralia,itdoesincludeseveralmodeloptionstoaccountforroundaboutcapacitydifferencesinotherpartsoftheworld.Anenvironmentfactorof1.2wasadoptedasaglobalcalibrationfactorfortheSIDRAversionissuedintheUnitedStates(Yinetal.,2011).Thisfactoradjuststhecriticalgapandfollow‐upheadwayvalues;thereforethecapacityvalueisadjusteddownwardandtheresultingroundaboutperformancemeasureswillbeworsethanthoseforaroundaboutinAustralia,allelsebeingequal.ThenewestversionofSIDRAcanaccommodatebothHCMmodelandSIDRAmode. 6.3.4RODELandARCADYThesoftwareARCADYwasdevelopedbyTransportationResearchLab(TRL)intheUnitedKingdom.Itusesalinearregressionformulatopredictcapacity,queuelength,delays,andcrashfrequenciesasafunctionofgeometry(Elias,2009).Queuesanddelayswerebasedontime‐dependentqueuingtheory.ARCARDYcanmodelroundaboutwiththeinclusionofcrashprediction,geometricdelay,andpedestriancrossing(Waddell,1997).RODELstandsforRoundaboutDelay,whichwasfirstdevelopedin1987.Itisusedtoexperimentwithdifferentgeometricdesignsofroundabouts.RODELcanprovidecapacityestimates,averageandmaximumdelay,queuesforeachapproach,andanestimateofoveralldelay(Elias,2009).RODELcanuseobservedvariationincapacitytoallowtheuserstosettheirdesiredconfidencelevel.TheinclusionofstatisticalvariabilityinRODELgavedesignersapreciselevelofconfidencethattheirdesignswouldmeettherequirementofcapacityanddelaywithsignificantflexibility(Waddell,1997).RODELcanalsoprovidethemaximumprobablequeueover40daysratherthantheaveragequeueasinotherroundaboutmodels.(Waddell,1997).6.3.5VISSIMVISSIMisamicro‐simulationprogramdevelopedbyPTVinGermany(PTVGroup,2013).CriticalfeaturesinVISSIM,suchaslinkandconnectors,routingdecisions,priorityrules,andreducedspeedzones,providesarealisticrepresentationofroundabouttrafficoperations(TruebloodandDale,2003).VISSIMusesalinkandconnectorsystemratherthanthelinkandnodesystemthatCORSIMuses.ThissystemallowsVISSIMtoemphasizethelinkbyusingconnectorstojoindifferentlinkswithoutconsidering



thenode.Forroundaboutsimulation,thissystemissuitablesincealinkinVISSIMallowsmultipleinternalinflectionpointswithoutaffectingthesimulationoftrafficflow(TruebloodandDale,2003).

Figure62.ExampleofRoundaboutSimulationinVISSIM(FHWA,2011)

ManyotherfeaturesinVISSUMfacilitateitsusagetosimulatetrafficmovementthrougharoundabout.TheavailabilityofsettingroutechoicedecisionsinVISSIMallowstheusertodetermineaspecificpaththrougharoundaboutandthespecificvolumepercentage.Thereforeitalsoallowsausertospecifywhichlaneavehicleusestocompleteitsroutingdecisionthroughmulti‐laneroundabouts(TruebloodandDale,2003).ThepriorityrulesinVISSIMallowsuserstospecifytheyieldprocessattheconflictpoint.Adjustmentofgap‐acceptancetimes,dependingondifferentvehicletypes,canalsobedeterminedusingthesettingofpriorityrules(2003).ReducedspeedzonesinVISSIMarealsogreatfeaturestouseinmodelingroundabouts,sincevehiclesusuallyslowdownto15‐25mi/htocirculatetheroundabout(2003).VISSIMprovidesaflexibletoolforuserstoaccuratelysimulatetheoperationofroundabouts.ResearchalsopointedoutthatVISSIMallowsuserstofine‐tunethegapacceptanceparametersrequiredforthesimulation(StanekandMilam,2005).Withgreatflexibilityandaccuratefeatures,itisbelievedthatVISSIMisthebestmicro‐simulatorforroundaboutmodeling(Elias,2009).6.3.6CORRIDORSIMULATION(CORSIM)CORSIMincludestwomicroscopicsimulationsubprograms,NETSIMandFRESIMthatarespecializedforurbanstreetsandfreeways,respectively.AlthoughitiswidelyusedintheUnitedStates,CORSIMhaslimitedcapabilitiesforsimulatingroundabouts(Elias,2009).SinceCORSIMusesalinkandnodestructuretomodelatransportationnetwork,withnodesbeingintersectionsandlinksrepresentingtheconnectingroadways,itdoesnotprovideadirectrepresentationofroundabouts.TomodelaroundaboutinCORSIM,theuserneedstocreateaseparatenodeforeachapproachandconnectthesenodestogetherwithaone‐waylinksegmentinacounterclockwisedirectionasshowninFigure63.(Elias,2009).TheinputsforroundaboutsimulationinCORSIMincludethefollowing:approachvolumesforeachleg,origin‐destinationofalltraffic,geometriccharacteristics,andspeeddistribution.TheoutputsfromCORSIMincludecontroldelay,averagequeue,andmaximumqueue,andotherstandardperformancemeasures. Whenstartingthesimulation,thevehicleentryheadwaydistributionshouldbemodifiedbasedonfielddatatocloselymatchthearrivalsateachapproach.Thentheuserneedstoconnecteachapproachusingacounterclockwiseone‐waylinkasinFigure63.Itisimportanttoverifythatthelengthoftheone‐waylinkmatchesthesizeoftheactualroundabout.Inordertoreplicatethetrafficruleatroundabout,itisimportanttoimplementyieldcontrolateachapproachlane.Thefinalstepistoadjustthegapacceptance



Figure63.ExampleofModelingRoundaboutinCORSIM(Elias,2009)

modeltomatchthefindingonroundaboutdriverbehavior.Additionally,itispossibletomodelorigin‐destinationsinCORSIMusingconditionalturnmovements(Elias,2009).OnemajordifficultyinmodelingroundaboutsinCORSIMistheinputofturnmovements.Sinceroundaboutsarereplicatedusingdifferentsegmentsoflinksconnectedwitheachotherbyjoiningnodeswithapproaches,theturnmovementsneedtobesetbasedonconditionallogicasshowninFigure64.

Figure64.ConditionalTurnMovementinCORSIM(Elias,2009)

ResearchshowedthattheoutputofCORSIMwhensimulatingroundaboutsisinaccurateandquitedifferentfromsiteobservation.Averagequeuewasleastwellpredictedforthethreeperformancemeasures(Elias,2009).ThiscouldpotentiallybringsomedifficultieswhenusingCORSIMformodelingroundabouts,especiallyforaccessissues.6.3.6.1ImprovementofCORSIMforRoundaboutModeling.SinceCORSIMdoesnotprovideadirectmethodforroundaboutsimulation,severalrevisionstoCORSIM’sdefaultparametersshouldbeconductedbeforesimulatingroundabouts(Elias,2009).BasedontheresearchofElias,thecurrentversionofCORSIMdoesnotreplicateroundaboutoperationsaccurately(Elias,2009).AlthoughCORSIMhasallthenecessaryfeaturesforroundaboutsimulation,improvementsshouldbeconsideredinordertogiveCORSIMthe



abilitytomodelroundaboutswell.(Elias,2009).Elias(2009)hasmadethefollowingrecommendationsforimprovementstoCORSIM:

Multiplenodesshouldbeabletobegroupedtogetherasaroundabout.Oncegrouped,thesoftwareseeksinputsforinscribeddiameterandsuper‐elevation.Theprogramthenusesthisinformationtocalculatethelimitingspeedforcirculatingvehicles.

Addinputsforturnmovementsandconditionalturnmovementsateachapproachnode. Revisecriticalgapandfollow‐uptimeparameterstobeapproachspecific.Defaultvaluesshouldbe

basedonNCHRP3‐65,withtheabilitytooverwritebasedonavailablefielddata.AdjustthelinklengthsandcurvatureforrealisticanimationinTrafVu.

6.3.7SummaryThesoftwarepackagesincludedintheassessmentsectionarethosewhichareoftenusedtoanalyzeroundaboutoperation.Insum,deterministicsoftware,suchasHCS,Synchro,SIDRA,RODELandARCADY,canperformqueuinganalysisandprovideusefulinformationrelatedtoaccessmanagement,especiallyforplacingdriveways.Simulationsoftware,suchasVISSIM,canbeusedtoevaluatetheoperationofroundaboutsandtheinteractionbetweentrafficflowsatroundaboutandadjacentdrivewaysbyconductingmicroscopicanalysis.Itisclearfromthisanalysisthatdeterministicsoftwarecanprovideguidanceonwherethedrivewayshouldbeplacedbeforeconstructionofintersections,whilesimulationcanbeusedtoevaluatetheimpactofdrivewayandotheraccessmanagementissuesonroundaboutoperation.HCScandoqueuinganalysis,whichcandeterminetherecommendeddistancebetweentheroundaboutandadjacentdriveways.Table30showstherecommendationforselectionofanalysistoolfordifferentdesignandevaluationapplicationsregardingroundaboutsandaccessmanagement.Table30.RecommendedSelectionofAnalysisToolforDifferentApplicationsRegardingRoundaboutsandAccessManagement

Application ExpectedOutcome RequiredInput PotentialAnalysisToolPlanningdrivewaylocation

Distanceofdrivewaytoroundabout(vehiclequeuing)

Trafficvolume,roundaboutgeometriccharacteristics

HCM,deterministicsoftware

Pedestrianaccessatroundabout

Vehicledelay, vehiclequeuing,pedestriandelay

Trafficvolume(vehicleandpedestrian),crosswalkdesign

HCM,deterministicsoftware,simulation

Accesstoactivitycenter,parking

Vehicledelay,vehiclequeuing

Trafficvolume, Simulation

Evaluationofinteractionbetweendrivewayandroundabout

Delayandqueuesbetweenintersections,traveltime

Trafficvolume,roundaboutgeometriccharacteristics

Simulation

OthermajorsoftwarepackagethattheFDOTusesforperformingLOSanalysisisLOSPLAN.However,atthistime,theabilitytoanalyzeroundaboutsisnotincludedinanyoftheLOSPLANcomponentsoftwareprograms:ARTPLAN,FREEPLAN,andHIGHPLAN.Thereforediscussionsofthesesoftwarepackagesarenotincludedinthisstudy.Furtherdevelopmentofsuchsoftwarepackagesmaytakeroundaboutsintoconsideration.Somesoftwarepackages,suchasCAP‐X(developedbyFHWA),GIRABASE(French)andKreisel(German),canalsoanalyzeroundabouts,butarenotcurrentlyusedbyFDOT.

Chapter7Discussion


ChapterSeven:Discussion7.1OverviewFloridahasrecentlybeguntoencouragetheuseofroundaboutsonthestatehighwaysystemandissystematicallyupdatingitsguidancedocuments(e.g.,PlansPreparationManual,IntersectionDesignManual,andManualonUniformTrafficStudies)butneedsadviceonwhattoincludeintheMedianHandbook,andDrivewayInformationGuide.Thepolicyjustificationforthischangeinpolicyresultsfromincreasingevidencethatroundaboutsmaycostlesstoinstall,havegreatersafetypotentialbyreducingthenumberofconflictpoints,anddependinguponthecontext,haveloweroperationsandmaintenancecosts.Toaccomplishthisgoalitisimportanttounderstandtheconnectionbetweenroundaboutsandaccessmanagementandotherformsoftrafficcontrol.Roundaboutsarebeingimplementedinavarietyofcontexts,butexistingresearchdoesnotprovidedetailedguidancetoevaluatehowtheroundaboutscanbeimplementedasaformofaccessmanagement.AccessmanagementisdefinedbytheTRBAccessManagementCommitteeas“thesystematiccontrolofthelocation,spacing,design,andoperationofdriveways,medianopenings,interchanges,andstreetconnectionstoaroadway”(TRB,2003,pp.3).RoundaboutsfacilitateU‐turnsthatcansubstituteformid‐blockleftturnsand,whenincorporatedintoacorridorofmultipleroundabouts,canaccommodateaseriesofU‐turnsandleft‐turnlanesthatcanreducedelayinthecorridor(Rodegerdtsetal.,2010).Left‐turnlanesandmedianopeningscanbereducedoreveneliminatedasvehiclesthatwanttomakealeftturncanmakeaU‐turnandthenarightturntoadriveway.However,becauseoftheiroperationalcharacteristics,roundabouts“mayalsoreducethenumberofavailablegapsformid‐blocksignalizedintersectionanddriveways”(Rodegerdtsetal.,2010,p.29).Thismayjustreducethecapacityattheseaccesspoints.Attheveryleast,thetrafficalongacorridorchangeswiththeintroductionofroundabouts;thetrafficmaybemoreuniformlydistributedwithalargenumberofsmallergapsratherthanfewerlargerones.Additionally,asingleroundaboutfunctionsdifferentlythanacorridorofroundabouts;acorridorofroundaboutscannotbeactivelymanagedtoprovideprioritytoamajorstreetcorridorinthesamewaythatcoordinatedplatoonsoftrafficcanbemanagedtoimprovetheefficiencyoftrafficsignals.Furthermore,“roundaboutscannotbemanagedwithacentralizedmanagementsystemtofacilitatespecialevents,diverttrafficflows,andsoonunlesssignalsattheroundaboutsorinthevicinityareusedforsuchapurpose”(TRB,2010a,pp.2‐6).Developingguidanceforaccessmanagementnearroundaboutsisfurthercomplicatedbytheneedtounderstandtheirbenefitsandchallengesforthevarietyofusersoftheroadway.While,inmostcontexts,roundaboutsaregenerallyfoundtobesaferthantheprevioustreatmentsinbefore‐and‐afterstudies(Kittelson&Associates,Inc.2013),theactualandperceivedsafetyofroundaboutsvariesamongusers.Yet,roundaboutsarenotalwayssafeforallusers.Inparticular,insomecontexts,pedestrians,especiallythosewithvisualimpairments,bicyclists,andtruckdriversmayfacespecificchallengesinnavigatingthroughroundabouts.Theuseofroundaboutsandotheraccessmanagementtechniquesmayestablishpriorityforspecificmovementsatornearroundaboutsthataffecttheiroperations.Thepurposeofthisstudyistounderstandpreviousresearchonroundaboutsandaccessmanagement,todocumenthowotherstatesareprovidingguidanceonroundaboutsandaccessmanagement,andtoprovideempiricalresearchonthesafetyandoperationsofroundaboutsinFlorida.Thepurposeistopresentinformationaboutincorporatingguidanceonroundaboutsandaccessmanagementintotheaccessmanagementguidelines,ingeneral,and,specifically,intotheMedianHandbook,andDrivewayInformationGuide.Thischapterisorganizedasfollows.First,thecontextforunderstandingtheresearchisprovidedbydescribinggapsintheliterature,andtheresultsofsafetyandoperationalanalysis.Next,thefindingsfrom

Chapter7Discussion


thereviewofnationalandstates’guidanceonroundaboutsandaccessmanagementaresummarized.Basedupontheseresults,specificrecommendationsaremaderegardingtheneedforadditionalresearchonroundaboutsandaccessmanagement,specificguidancefortheroundaboutsandaccessmanagement,andrecommendationsforsoftwaretoanalyzetheoperationsatroundabouts.7.2RoundaboutsandAccessManagementinFloridaThestateofFloridahasarelativelylargenumberofroundaboutsthataresafelyoperatingandprovidingtheoperationalefficienciesofroundabouts,butfewofthemarelocatedonthestatehighwaysystem.Theresearchteamidentifiedatotalof283roundaboutsthroughoutthestatebutonlyfourofthoseroundaboutsarelocatedonthestatehighwaysystem.Theroundaboutsarelocatedinavarietyofregionalcontextswithdiversedesignsandaccessconsiderations.Theregionalcontextvariesfromurbantosuburbantoruralanddifferentdistancesfromthenearestcommunitycenters,highways,interstates,andstatehighways.Thedesignoftheroundaboutsvariesfromthemorecommonthreeorfourlegroundabouttoroundaboutswithuptosixlegs.Thetypeofroundaboutvariesfromasingle‐lanetomulti‐laneandturbo,spiralandothercomplexroundaboutdesigns.Someroundaboutshavemediansononeormorelegs,sliplanesandstub‐outs.Accessconsiderationsinvolvedrivewayplacement,thepresenceorabsenceofmedians,andthetypeofadjacentlanduses,whichincluderesidentialsingle‐family,residentialmulti‐family,commercialandmixed‐use.Althoughonlyfourarelocatedonthestatehighwaysystem,themajorityarelocatednearstatehighwaysandinsomecasesprovideaccessthatallowsdriversalternativestousingthestatehighwaysystem.Inthissection,asummaryofthesafetyandoperationalanalysisispresented.Thesafetyanalysisconsideredfourdifferentaspectsofsafetyrelatedtoaccessmanagementnearroundabouts:(1)impactofdrivewaycornerclearancesonroundaboutsafety;(2)safetyimpactofmedianopeningsinthevicinityofroundabouts;(3)safetyatroundaboutsthatprovidedirectaccesstoactivitycenters;and(4)safetyofvulnerableroadusers,includingpedestriansandbicyclists.Next,theresultsoftheoperationalanalysisweresummarizedbyconsideringthreedifferentaspectsoftheoperationsofroundabouts:(1)conflictswithinthefunctionalareaofroundabouts;(2)conflictsatroundaboutsinvolvingpedestriansandbicyclists;and(3)violationoftrafficrulesandtheirimpactontheoperationofroundabouts.Then,asummaryoftheanalysisofthereviewofthenationalandstateguidanceonroundaboutsandaccessmanagementispresented.Finally,theFloridaguidelinesforroundaboutsandaccessmanagementareexploredandtheresultsareplacedwithinthecontextofFloridapractice.7.2.1SummaryofSafetyAnalysisThefindingsofthesafetyanalysisoneachofthefourdifferentaspectsofsafetyareaddressedafterthesummaryofthecrashdataispresented.7.2.1.1SummaryofOverallCrashData.Atotalof1,882crasheswithin500ft.ofthe283roundaboutslocatedinFloridathatweredirectlyrelatedtotheroundaboutwerefoundtooccurduring2007‐2011.Overall,eachroundaboutexperiencedanaverageof6.65crashesperroundaboutduringthefive‐yearanalysisperiodwithcommercialroundaboutsexperiencing8.10crashesperroundaboutwhileresidentialroundaboutsexperienced5.4crashesperroundaboutduringthefive‐yearanalysisperiod.Consistentwiththepreviousfindingsonthesafetyoftheroundabouts,ananalysisofallofthecrashesrelatedtoroundaboutsshowedarelativelyfewercrashes.Acollisionwithafixedobjectwasthemostfrequentcrashtype,withaboutaquarter(24.7%)ofallcrashesinthevicinityofroundaboutsresultingfromvehicleshittingafixedobject,mostly,theroundaboutcenterisland.Abouttwo‐thirds(62.9%)ofthesecrashes(i.e.,collisionwithafixedobject)occurredatnight.Aftercollisionwithafixedobject,angleandrear‐endcrashesweremostcommon,accountingfor21.0%and18.5%oftotalcrashes,respectively.Thedistributionofcrashtypeswasfoundtobesimilarincommercialandresidentialareas.

Chapter7Discussion


Overall,aboutone‐thirdofthetotalcrashesweresingle‐vehiclecrashes,whiletherestinvolvedmultiplevehicles;thesecrasheswereequallydistributedacrosscommercialandresidentialareas.Onehalfofonepercent(0.5%)ofallcrasheshadafatality,4.5%involvedanincapacitatinginjury,andaboutathird(29.7%)involvedapossibleornon‐incapacitatinginjury;theremaining61.1%involvedonlypropertydamage.Single‐vehiclecrashes(8.9%)hadahigherproportionofsevereinjuriesthanmulti‐vehiclecrashes(2.9%)andagreaterpercentageofsingle‐vehiclecrashesresultedininjuriescomparedtomulti‐vehiclecrashes.Ahigherpercentageofmulti‐vehiclecrashes,at68.8%,resultedinPDOcrashes,whileonly45.9%ofsingle‐vehiclecrasheswerePDOs.Ofthesixfatalsingle‐vehiclecrashes,fiveinvolvedvulnerableroadusers(fourweremotorcyclistswhowerefoundatfaultandoneinvolvedanintoxicatedpedestrian).Twoofthefourfatalmulti‐vehiclecrashesinvolvedagolfcart.7.2.1.2ImpactofDrivewayCornerClearanceonRoundaboutSafety.Ofthe1,882crashesthatoccurredatroundaboutlegs,only74crashes,orabout4%,wereidentifiedtobedriveway‐related.Ofthese74driveway‐relatedcrashes,37crashes(50%ofthedriveway‐relatedcrashes)occurredatthefirstdriveways(i.e.,thedrivewaythatdefinesthecornerclearance),whileanequalnumberoccurredonallotherdriveways.Ofthe37crashes,18occurredatthefirstupstreamdriveway,andtheremaining19occurredatthefirstdownstreamdriveway.Sixof18crashes(33.3%)occurredwhentheupstreamcornerclearancewaslessthan250ft.;thiscanbecomparedto15of19crashes(78.9%)thatoccurredwhenthedownstreamdrivewaycornerclearancewaslessthan250ft.Intermsofcrashseverity,ofthe37crashes,nonewerefatal,tworesultedinincapacitatinginjuries,eightwerenon‐incapacitatinginjurycrashes,andtheremaining27werePDOs.Theabovestatisticsindicatethatthedownstreamdrivewaycornerclearancehasagreatersafetyimpactthantheupstreamdrivewaycornerclearance.Althoughthisresultisbasedonasmallsample,theresultisconsistentwiththefactthatvehiclesexitingadownstreamdrivewayexperiencereducedgapsduetodispersedplatoonsfromtheupstreamroundabout.Thegeometryoftheroundaboutwithalargercornerturningradii,allowsvehiclestoturnrightatahigherspeed.Atcornerswithreducedsightdistance,itfurtherreducesthetimeavailablefordrivewayvehiclestocompletetheirmaneuvers.7.2.1.3SafetyImpactofMedianOpeningsintheVicinityofRoundabouts.Crashesinvolvingvehiclesturningleftatmedianopenings(i.e.,vehiclesturningleftfromthemainstreetontoadrivewayandvehiclesturningleftfromadrivewayontothemainstreet)wererelativelyrare.Ofthe283roundabouts,131roundaboutswerefoundtohaveatotalof157medianopeningswithin500ft.During2007‐2011,arelativelylowtotalof15crashesoccurredatthese157medianopenings.Ofthese15crashes,eightinvolvedvehiclesturningleftfromthemainstreetontoadrivewayandseveninvolvedvehiclesturningleftfromadrivewayontothemainstreet.7.2.1.4SafetyatRoundaboutsthatProvideDirectAccesstoActivityCenters.Accesstomajoractivitycenters,suchasbigboxretailstores,shoppingcenters,andmalls,isoftenprovidedatmid‐blocklocationsonacorridor;assuch,aquestionremainsaboutthesafetyofdirectaccesstoactivitycentersascomparedtoaccessatmid‐blocklocations.Thesafetyanalysisconfirmsthatroundaboutswiththreeorfourlegswithdirectaccesstoactivitycentersareassafeasroundaboutswithoutdirectaccesstoactivitycenters.Oncethenumberoflegsincreasestomorethanfourlegs,theroundaboutswithdirectaccesstotheactivitycenterarelesssafe.7.2.1.5SafetyofVulnerableRoadUsers,IncludingPedestriansandBicyclists.Atotalof20pedestriancrashesand47bicycle‐vehiclecrashesoccurredatornearthe131roundaboutsincommercialareas,constitutinglessthan4%ofallcrashes.Ofthepedestriancrashes,18occurrednearmedians,withaslightlyhigherrate(6.64)per100roundaboutlegsthanTWLTL(5.56)andmuchhigherthanothermediantreatments.Becauseofthesmallsamplesizeandthelackofgoodexposuredataforpedestriansandbicyclists,itisdifficulttogeneralizefromtheresultsofthesafetyanalysis.

Chapter7Discussion


7.2.2SummaryofOperationalAnalysisThethreeoperationalanalysisissuesrelatedtoaccessmanagementarepresentedinthissection.Roundaboutssharetheseissuesincommonwithothertypesofintersections.Withtheexceptionofthelastissue“violationoftrafficrulesanditsimpactontheoperationofroundabouts,”theseconcernsoverlapwiththeissuesinthesafetyanalysis.Oneotherissue,thespillbackintoaroundaboutfromadownstreambottleneck,wasnotfoundattheroundaboutsincludedintheoperationalanalysis.Incaseswherethisoccurs,itwouldresultinalockedroundabout.7.2.2.1ConflictswithintheFunctionalAreaofaRoundabout.Conflictcanoccurinthefunctionalareaofaroundaboutwhendrivewaysorotherintersectionsarelocatedtooclosetoaroundabout.Theseconflictscanoccurwithacoupleoftypesofmovements,suchasleft‐turnsintodrivewaysthatarepreventedordelayedbecauseofatrafficqueueontheopposinglegoftheroundabout(seeFigure65).Inaddition,left‐turningvehiclesturningfromadrivewayontooneofthelegsofaroundaboutarepreventedfromenteringtheroadway,aqueue,ortrafficbacksintoanotherintersectionbecausetheyaretoocloselyspaced.Ineachcase,thefailuretodesignforthetrafficqueuecaninterferewiththeoperationoftheentireintersection,anadjacentintersection,orbothintersections,andcanposeapotentialsafetyrisk,whilereducingthecapacityoftheroundabout.Thesafetyandoperationalconcernsassociatedwithconflictswithinthefunctionalareaofaroundaboutreinforcestheimportanceofensuringthatintersectionsarenottoocloselyspacedandthatthefunctionalareabeprotectedtoensuretheefficientmovementoftraffic.Thechallengeisthatthefunctionalareaofaroundaboutmaybedifferentfromotherintersections,especiallyinareaswherethespeedissignificantlylowerthanmostun‐signalizedintersectionscurrentlyoperate.Figure65‐Figure67showexamplesofdealingwithaccesstodrivewaysatroundabouts.Whenleft‐turnaccesstoaroundaboutisdesigned,spillbackandconflictwithvehiclesfromtheoppositedirectionmayoccur,asFigure65shows.Onesolutiontothissituationistoaddadedicatedleftturnlaneinthemiddlewithenoughstoragecapacity(Figure66).Anothersolutionistodesignthedrivewayattheexitinglaneandallowright‐turnaccesstothedriveway(Figure67).

Figure65.ConflictandSpillbackassociatedwithLeft‐turnAccesstoDriveway

Chapter7Discussion


Figure66.Solution1‐DedicatedLeft‐turnLaneforAccesstoDriveway

Figure67.Solution2–Right‐laneAccess

7.2.2.2ConflictsatRoundaboutsInvolvingPedestriansandBicyclists.Fromanoperationalperspective,locatingroundaboutsinanareawithhighpedestriantrafficcanreducethecapacityofroundabouts.Whenacarstopsforapedestrianatacrosswalk,thequeuebehindthecarspillsbackintothecirculatinglane,andaffectstheoperationoftheroundabout.Thisdelayduetopedestrianmovements

Chapter7Discussion


arenotunliketheconflictsbetweenmovingvehiclesandpedestriansincrosswalksatothertypesofintersections.7.2.2.3ViolationofTrafficRulesanditsImpactontheOperationofRoundabouts.Examplesofdriversviolatingtherulesoftheroadcanbeseenwhentheystopinthemiddleofroundaboutstoeitherpick‐upordrop‐offapassenger.Whenthedriverstopsintheroundabout,theresultcanbeaqueuethatcausesdriverstoqueueinsidetheroundaboutorchangetheirdirectiontogetaroundthestoppedvehicle.Pick‐upsanddrop‐offsaremorelikelytooccurinareaswithhighpedestriantrafficoratcertainactivitycenters.Thisresultconflictswiththesafetyanalysis,whichreinforcedtheadvantagesofusingroundaboutsforaccesstoactivitycentersbecausetheyreducethechallengesofaccessthroughopenmediansortheplacementofanAWSCintersectionincloseproximitytotheroundabout.7.3RoundaboutsandAccessManagementGuidanceInthissection,thenationalandstateguidanceonroundaboutandaccessmanagementissummarized.Thenthenationalandstateguidanceonboth,incombinationwitheachother,areexplored.Finally,Florida’sguidanceonroundaboutsandaccessmanagementaresummarized.Followingthissection,thefindingsoftheresearcharecomparedtoeachothertoestablishabasisformakingrecommendations.7.3.1SummaryofNationalandStateGuidanceonRoundaboutsInthisreportfourNCHRPreportsaresummarizedastheyrelatetoaccessmanagement.Theyinclude:NCHRPReport672:Roundabouts:aninformationalguide.SecondEdition,(Rodegerdtsetal.,2010),NCHRPReport674:CrossingSolutionsatRoundaboutsandChannelizedTurnLanesforPedestrianswithVisionDisabilities,(Schroederetal.,2008),NCHRPReport572:RoundaboutsintheUnitedStates.Report572,(Rodegerdtsetal.,2007),andNCHRPSynthesis264:ModernroundaboutpracticeintheUnitedStates,(Jacquemart,1998).Twoofthesedocuments–NCHRPReport672,Roundabouts,AnInformationalGuide(Rodegerdtsetal.,2010)andNCHRPReport572,RoundaboutsintheUnitedStates(Rodegerdtsetal.,2007)areofgreatestrelevancetothisstudy.NCHRPReport674(Schroederetal.,2008)focusesonroundaboutsforpedestrianswithvisiondisabilities.NCHRPSynthesisReport264(Jacquemart,1998)isanearlyreportontheuseofroundaboutsintheUnitedStates;itincludesdiscussionsofsafety,capacityanddelay,issuesofroundaboutsforvarioususers,locationcriteriaforroundabouts,andexamplesoftheuseofroundaboutsintheUnitedStates.AnadditionalstudythatisbeingcompletedunderNCHRPProject3‐100–EvaluatingthePerformanceofCorridorswithRoundabouts–willalsobeofrelevancetothisreport.Thecontractor’sreportshouldbeavailablewithinthenextmonth.NCHRPReport672,Roundabouts,AnInformationalGuide,(Rodegerdtsetal.,2010),iscomprehensive,coveringplanning,operation,safety,geometricdesign,trafficdesignlandscaping,andsystemconsiderationsofroundabouts.Inonesectiononplanning,thisdocumentcomparesoperationalperformancefromtheroundaboutswithintersectioncontrols,suchasTWSC,AWSC,andsignalcontrol.Theoperationsectionincludesthecapacityandperformanceanalysisoftrafficoperation,e.g.degreeofsaturation,delay,queuelength,andfieldobservation.Specificallyforgeometricdesignasrelatedtoaccessmanagement,thisdocumentexplainshowtodesignroundaboutswith:entrycurvesandexitcurves,splitterislands,SSD,ISD,andparkingandbusstoplocations.Inthesafetysection,thisdocumentreviewsconflictpointsfordifferentusers,andcommoncrashtypesinroundabouts.Signage,pavementmarkings,illumination,workzonetrafficcontrol,andlandscapingareexploredinthesectionontrafficdesignandlandscaping.Thelastsectionsystemfocusesonthefollowingconsiderationsrelatedtoaccessmanagement:trafficsignalsatroundabouts,closelyspacedroundabouts,roundaboutinterchanges,androundaboutsinanarterialnetwork.ThisreportistheonemostfrequentlyadoptedbystateDOTs,includingthestateofFlorida,astheirroundaboutsdesignguidancedocuments.Asisdiscussedbelow,italsoincludesconsiderationsofbothroundaboutsandaccessmanagement.

Chapter7Discussion


NCHRPReport572:RoundaboutsintheUnitedStates(Rodegerdtsetal.,2007)focusesonpriorresearchonroundaboutsintheUnitedStatesanddescribesthemethodsofpredictingsafetyandoperationalaspectsofroundabouts.Thisdocumentincludesfourmainsections:safetyperformance,operationalperformance,geometricdesign,andpedestrianandbicyclistobservation.Thefindingsonoperationalperformanceincludedentrycapacityandcontroldelaymodelforone‐laneandmultilaneroundabouts;theproposedLOScriteriaaresimilartothoseatunsignalizedintersections;andthedraftproceduresthatincorporatethosemodelsintotheHCM2010.Furthermore,aspectsofdesignthatmaybeimportanttoconsiderare:accelerationanddecelerationeffectsonspeeds,ISD,anddesigndetailonmultilaneroundabouts,suchasvehiclepathalignment,lanewidth,anddriverinformationregardinghowtouselanemarkings.Twenty‐sixstateshaveroundaboutwebsiteswithvaryingdegreesofinformation.MostofthesestatesadoptthenationalguidancefromNCHRPReport672:Roundabouts,AnInformationalGuide,2ndEdition,(Rodegerdtsetal.,2010).Theguidanceoffourteenofthesestateswasreviewedingreaterdetailtounderstandhowroundaboutsguidancerelatestoaccessmanagement.ThethreestatesthataddressthecoordinationroundaboutsandaccessmanagementincludeWisconsin,Virginia,andKansas;thesestatesareprofiledingreaterdetailinthesectiononroundaboutsandaccessmanagement,below.Thestateguidanceinseveralofthefourteenstatesprovidesguidanceontheuseoflocallydevelopedparametersforvariousaspectsofdesignandoperationalanalysis(e.g.,Californiaforaccelerationanddecelerationeffects;MichiganforSPFsandCMFs;andWashingtonforcornerclearance,parallelroundabouts,U‐Turns,parkingandtransitstops,andWisconsinforlocationofdrivewaysandsitedistancebetweenusers).Somestates(e.g.,Minnesota,WisconsinandNewHampshire)recommendspecificsoftwarefortheassessmentoftheuseofroundaboutsforanintersectiondesign.7.3.2SummaryofNationalandStateGuidanceonAccessManagementTwelvenationalpublicationsthatdescribetheadvantagesanddisadvantagesofaccessmanagementanddocumenthowtoimplementitwereidentified.Thesedocumentsinclude:APolicyonGeometricDesignofHighwaysandStreets(GreenBook),6thEdition,AASHTO,2011,NCHRPSynthesis404:StateofPracticeinHighwayAccessManagement(GluckandLorenz,2010),NCHRPReport548:AGuidebookforIncludingAccessManagementinTransportationPlanning(Roseetal.,2005),NCHRPSynthesis351:Accessrights:asynthesisofhighwaypractice(HuntingtonandWen,2005),NCHRPReport524:SafetyofU‐turnsatUnsignalizedMedianOpenings(Potts,2004),NCHRPSynthesis337:CooperativeAgreementsforCorridorManagement(Williams,2004),TRBAccessManagementManual(TRB,2003),NCHRPSynthesisofHighwayPractice332:AccessManagementonCrossroadsintheVicinityofInterchanges(ButoracandWen,2002),NCHRPSynthesis304:DrivewayRegulationPractices(Williams,2002),NCHRPReport420:ImpactsofAccessManagementTechniques(Glucketal.,1999), NCHRPReport395:CapacityandOperationalEffectsofMidblockLeft‐TurnLanes(BonnesonandMcCoy,1997),andNCHRPReport348:AccessManagementGuidelinesforActivityCenters(KoepkeandLevinson,1992).Collectively,thesereportsdocumentvariousaspectsofplanningforaccessmanagement,includingsafety,capacity,economicdevelopment,andbroadconceptsrelatedtotheimplementationofaccessmanagement,cooperativeagreementsforcorridormanagement,andtheuseofaccessmanagementasapartoftransportationpractice.Landuseandenvironmenteffectsofaccessmanagementincludeaesthetics,unificationofactivitycenters,maintainingthecapacityofavailableroadways,minimizingtheenvironmentalimpactofindividualaccessroads,andmoreefficientfuelconsumption.Someofthesedocumentsfocusonaccessmanagementinspecificcontexts,suchasactivitycenters,U‐turnsatunsignalizedmedianopenings,crossroadsinthevicinityofinterchanges,drivewayregulations,andcapacityandoperationalaspectsofmidblockleftturns.Becausesomeofthesedocumentswerepreparedinthe1990s,theydonotaddressroundaboutsinmuchdetail.Asisdescribedbelow,noneofthesedocuments,withtheexceptionoftheAASHTOGreenBook(AASHTO,2011),specificallyexplaintheconsiderationsforroundabouts.

Chapter7Discussion


Twoofthesedocuments–NCHRPReport420,ImpactsofAccessManagementTechniques(Glucketal.,1999)andTRBAccessManagementManual(TRB,2003)–areusefulinprovidinggeneralconsiderationsrelatedtoroundaboutsandaccessmanagementthatcouldbeappliedtoroundabouts;bothofthesedocumentsareovertenyearsold,whichmayexplainthelackofcoverageofroundabouts.NCHRPReport420:ImpactsofAccessManagementTechniques(Glucketal.,1999)focusesonthemethodsforevaluatingparticularaccessmanagementtechniquesintermsofsafetyandtrafficoperations.Thisresearchidentifiesavailabletechniques,andcollectsandanalyzesthemethodsanddatafromvarioussources.Theprioritiesforaccessmanagementanalysisare:trafficsignalspacing,unsignalizedaccessspacing,cornerclearance,medianalternatives,left‐turnlanes,U‐turnsasalternativestodirectleftturns,accessseparationatinterchanges,andfrontageroads.Thereportreachesseveralconclusions.Crashratesarehigherwheresignaldensityishigher,orwhereun‐signalizedintersectionsaremorecloselyspaced.Safetyandoperationsaspectsarebetterifthereismorecornerclearance.Safetyisalsoassociatedwithraisedmedians.Left‐turnstoragelanesupgradesafetyandcapacitybyprovidingspacesforturningvehicles.Indirectleft‐turnsorU‐turnsmayimprovesafety,capacityandtraveltime.Frontageroadsalongfreewaysmayneedtobeallocatedproperlytodecreasearterialleftturns,weavingmovements,andenhanceaccess.Theymayalsoneedtobeplacedfarenoughfromtheramptoavoidconflicts.TRB’sAccessManagementManual(TRB,2003)exploresthegeneralbenefitsofmanagingaccesstoroadways,explainshowaccessmanagementcanbeachieved,itsaspectsandprinciples,andtherolesofvariousinstitutionsinaccessmanagement.Threebasicstepsinimplementingaccessmanagementtoaroadwayaredefiningaccesscategories,establishingaccessmanagementstandards,andassigningcategoriestotheroadwaysorroadwaysegments.Initialfactorstobeconsideredarethedegreeofroadwayimportance,roadwaycharacteristics,landuseandgrowthmanagementobjectives;andthecurrentandpredictedflowsofgeneraltransit,aswellaspedestrianandbicycletraffic.Fourgeneralaspectsofdevelopingaccessmanagementstandardsincludemedians,degreeofurbanization,speed,andsafety(TRB,2003,p.71).Finally,theassignmentofcategoriesinroadwaysystemsneedstotakeintoaccountthefollowingfactors:theintendedfunctionoftheroadwayasacomponentofacompletetransportationsystemnetwork;theroadwaysegment’senvironment(ruralandundeveloped,urbanfringe,sub‐urban,urban,anddenselydevelopedorurbancore);theavailabilityofasupportingroadwaysystemtosupplyalternativeaccess;andthedesiredorappropriatebalancebetweensafetyandfrequencyofaccess(TRB,2003,p.77).Forty‐threestates,includingtheDistrictofColumbia,haveincorporatedaccessand/oraccessmanagementintotheirplanninganddesignpolicies.Nineteenstateshaveaccessmanagementmanuals,separatefromgeneraldesignmanuals,andelevenstateDOTsmentionaccessmanagementordesignmanuals,whileanothersixteenDOTshaveotherdocumentswithvariousnames.Onlysevenstatesincorporateroundaboutsintotheiraccessmanagementguidance;thesestatesarediscussedbelow.7.3.3SummaryofNationalandStateGuidanceonRoundaboutandAccessManagementAmongallthenationalguidancedocumentsonroundaboutsandthedocumentsonaccessmanagement,onlyNCHRPReport672:Roundabouts,AnInformationalGuide,SecondEdition(Rodegerdtsetal.,2010)referstoaccessmanagementinthecontextofroundaboutsunderthegeneralcharacteristicsofroundaboutsandaspartofthegeometricprocess(Sections2.2.5p.2.9and6.11,pp.6‐95to6‐98).Thisdocumentreinforcestheideathat“[m]ostoftheprinciplesusedforaccessmanagementatconventionalintersectionscanalsobeappliedatroundabouts.”(Rodegerdtsetal.,2010,p.2‐9)and“[a]ccessmanagementatroundaboutsfollowsmanyoftheprinciplesusedforaccessmanagementatconventionalintersections”(p.6‐95).However,thedifferenceinoperationalcharacteristicsofroundaboutscomparedtoothertypesofintersectionsmayjustifythedifferenceincertaindetailsofaccessmanagement.

Chapter7Discussion


Asapartofanoverallroadwaysystemthatinvolvesaccessmanagement,thetreatmentofdrivewaysandparkingwithinthefunctionalareaoftheroundaboutsintersectioniscritical.Theabilitytoprovidepublicandprivateaccesspointsneararoundaboutisinfluencedbyanumberoffactors,suchasthecapacityoftheminormovementsattheaccesspoints,theneedtoprovideleft‐turnstorageonthemajorstreettoservetheaccesspoint,theavailablespacebetweentheaccesspointandtheroundabout,andsightdistanceneeds.Figure29,above,whichwastakenfromNCHRPReport672,Roundabouts,AnInformationalGuide(Rodegerdtsetal.,2010)showsthetypicaldimensionsforleft‐turnaccessnearroundaboutsshouldbeabout275ft.subjecttolocalconditions.Thefunctionalareaofabout275ft.fromthecenterdiameterincludesthedistancefromthecenterfortheroundabouttotheedgeofthesplitterisland,aminimumof50ft.toclearthemedianandaminimumof75ft.toallowfortheleftturningmovementinadditiontothedistanceformaneuvering,decelerating,andqueuingintotheleftturnlane.Asmallnumberofstatesexplicitlyrefertoaccessmanagementwithinthecontextofroundabouts.ManystatesadopttheguidanceofNCHRP672,Roundabouts,AnInformationalGuideintheirroundaboutplansand,assuch,adopttheunsignalizedintersectionspacingguidance.Someincludesuchinformationintheirroundaboutsmanualsandsomeintheiraccessmanagementmanuals.Fromthesevenstatesthatspecificallyrefertoaccessmanagementinthecontextofroundabouts,twoofthem–KansasandVirginia–providesignificantsupplementalinformationwhileadoptingthenationalguidance.CaliforniaandIowaendorsetheuseofroundaboutsasapartofaccessmanagementbutdonotprovidespecificguidanceondrivewaydistancesandintersectionspacingguidance.Michigan’sAccessManagementGuidebookstates(MDOT,2008)that“Drivewaysneedtobelocatedasafedistancefromaroundaboutwithadequatesignage.Drivewaysshouldnotbelocatedwithinaroundabout”(MDOT,2008,p.3‐29)buttheydonotprovidespecificguidanceonhowtoaccomplishthisgoal.Similarly,Wisconsindescribestheadvantageofroundaboutsintheretrofitofasuburbancommercialstripdevelopmentinanattempttominimizeconflicts.TheWisconsinreportthendescribessomeofthefactorstobeconsideredinsuchretrofits(e.g.,drivewayconsolidation,reversefrontage,coordinatedU‐turnsandleftturns,andinterconnectedparkinglots);however,theydonotprovidespecificguidanceonthelengthofthefunctionalareaaroundroundabouts.BothKansasandVirginiaadopttheunsignalizedintersectionspacingbutprovideadditionalguidance.TheKansasRoundaboutGuide:ASupplementtoFHWA’sRoundabouts(Kittelson&AssociatesandTransystemCorporation,2003)andKsDOTAccessManagementPolicy(KsDOT,2013)hasinformedandhavebeeninformedbytheNCHRP672,Roundabouts,AnInformationalGuidereport.Virginia’sAccessManagementDesignStandardsforEntrancesandIntersectionprovidesatable,shownaboveinFigure29,demonstratingthespacingfromotherintersectionsandthespacingfromotherdrivewaysorroundabouts.OnesignificantdifferencebetweenthesesetsofguidancethatmayaffecttheirinterpretationofthelengthofthefunctionalareaisthattheNCHRP672,Roundabouts,AnInformationalGuidemeasuresthefunctionalareafromthecenterlineoftheroundaboutwhileKansasmeasuresitfromtheendofthesplitterislandandVirginiameasuresfromtheouteredgeofthenearestinscribeddiameter,notthecenterline.7.3.4SummaryofFlorida’sGuidanceonRoundaboutsandAccessManagementFloridahastwomajordocumentsrelatedtoaccessmanagement:FDOTMedianHandbook(2006);andFDOTDrivewayInformationGuide(2008);andfourmajordocumentsthatincludeinformationonroundabouts:FloridaRoundaboutGuide(FDOT,1996);RoundaboutJustificationStudy(Chapter16inManualonUniformTrafficStudies,FDOT,2000);FloridaIntersectionDesignGuide2013;andBicycleandPedestrianConsiderationsatRoundabouts(FDOT,2000).FDOTMedianHandbook(2006)doesnotexplicitlymentionroundaboutdesignoraccessmanagementwhiletheFDOTDrivewayInformationGuide(2008)andtheStateHighwaySystemAccessManagementSystemandStandardsdonotmakeanyreferencetoroundabouts.

Chapter7Discussion


TheFloridaRoundaboutGuide(FDOT,1996)waspublishedearlierthanFHWA'sRoundabouts:AnInformationalGuide,1stEdition(Robinsonetal.,2000)andisintheprocessofbeingreplacedwithmorerecentdocuments.TheManualonUniformTrafficStudies,Chapter16–RoundaboutJustificationStudy(2000)justifiestheuseofroundaboutsintheStateofFlorida,andcomparesthemtothreeotheralternativestointersectioncontrols–trafficsignals,TWSC,andAWSC.TheFloridaIntersectionDesignGuide,2013,ForNewConstructionandMajorReconstructionofAt‐GradeIntersectionsontheStateHighwaySystememphasizestheneedofconsideringmodernroundaboutsforanynewroadorreconstructionprojectastheymayprovidesafetyandoperationaladvantages.ThisguidealsostatesthatFloridahasofficiallyadaptedNCHRP672,Roundabouts,AnInformationalGuide(Robinsonetal.,2010)asthemainguidefordesigningroundaboutsinFlorida.Itdescribesmanyadvantagesofbuildingroundabouts.Regardingroundaboutsandaccessmanagement,thisdocumentacceptsthatroundaboutscanbeusedaspartofanaccessmanagementplanastheycontributeinreducingdownstreamleftturns,becausevehiclescanperformU‐Turnswithintheroundaboutsandthenaccessanareabyturningright.Also,drivewaysshouldnotbeallowedinthecirculatoryroadwayunlessthereisenoughdemandtosupporttheirconstructionasadditionallegsoftheroundabout.Bicyclescanaccessaroundaboutasvehiclesusingthecirculatoryroadwayoraspedestriansusingsidewalks,sobicyclelanesshouldendatbypassrampstoallowbicyclestousethesidewalkiftheyprefer,alwaysyieldingtopedestrians.Pedestriantreatmentsatroundaboutsareconsideredthesameasinotherintersectiontypes.Incaseofbusroutesinroadswithroundabouts,busbaysshouldbeplacedcarefullytoavoidvehiclequeuesthatspillbackintothecirculatoryroadway;Busstopslocatedonthefarsideoftheroundaboutshouldhavepulloutsorbemovedfurtherdownstreamtothesplitterislandtoavoidinterruptingregulartraffic.Furthermore,theFloridaIntersectionDesignGuideadaptstheSSDformulaandtheISDrequirementsfromNCHRP672,Roundabouts,AnInformationalGuide(Equations6‐5‐6‐7,pp.6‐61‐6‐63inRodegerdtsetal.,2010).TheBicycleandPedestrianConsiderationsatRoundabouts(Shenetal.,2000)recommendsthatroundaboutsbeproperlydesignedtoaccommodatethesafetyofbicyclists,pedestriansanddrivers.Themulti‐laneroundaboutscreatemoretensionandarelesssafeforbicyclistsandpedestriansthanone‐laneroundabouts.Inadditiontotheaforementioneddocuments,FDOTpresentedaPowerPointpresentation—Roundabouts,Florida’sImplementationStrategy(PrytykaandSullivan,2012),atthe2012DesignTrainingExpowherethesupplementalaspectsfromFHWA'sRoundabouts,AnInformationalGuide(Robinsonetal.,2000)arecaptured.7.4SynthesisofFindingsoftheResearchTheStateofFloridaisintheprocessofchangingitsguidanceontheuseofroundaboutsonthestatehighwaysystem.ThechangeintheState’spolicyguidanceasdescribedintheFloridaIntersectionDesignGuide2013,FDOTMedianHandbook(2006),FDOTDrivewayInformationGuide(2008),StateHighwaySystemAccessManagementClassificationSystemandStandards(FDOT,2010),andotherguidancedocumentswilldefinehowroundaboutsareimplementedintocities,townsandcrossroadsinthestateofFlorida.WhilecommunitiesthroughoutFloridahavesignificantexperiencewithroundabouts,thelevelofexpertiseisunevenandthecontextsinwhichtheroundaboutswillbeimplementedarediverse.Theadvantagesofroundaboutsandaccessmanagementareclearlydocumentedintheliterature.Accessmanagementaffectssafety,operations,economicfactorsrelatedtoretailorcommercialmarketandpropertyvalues,landuse,andtheenvironment.Roundaboutsareseenasaformofaccessmanagementthathassimilarcharacteristicsandoperational,safety,andcostadvantagescomparedtoothertypesofintersections.Whenproperlydesignedroundaboutsandaccessmanagementcanenhancetheaestheticandenvironmentalaspectsofacorridor.Nonethelessthesameareacanexperienceeconomicdeclineandalossofcommunitylivabilitywhenaccessmanagement,includingroundabouts,ispoorlydesignedandimplemented.Theanalysiscompletedasapartofthisresearchidentifiedseveralareasdirectlyrelatedtoaccessmanagementandotherissuesthatmaybecomeapartofthestate’sstrategytoimplementchangein

Chapter7Discussion


roundaboutpolicy.Thesafetyanalysiswascompletedonallroundaboutsinthestateand,ingeneral,itshowsarelativelylowrateofcrashesnearroundaboutsbutaslightlyhigherratenearcommercialandmixedlanduses.Theoperationalanalysiswascompletedonasmallsampleof13roundaboutsidentifiedfortheirtrafficvolume,proximitytodriveways,adjacentintersections,andadjacentlanduses.Collectively,theseanalysesidentifiedacoupleofareasofconcern.Some,suchascollisionswithfixedobjectsatnight,mayrequiredesign,lighting,orsignagechanges.Others,suchasdriversstoppinginthemiddleofroundabouts,mayrequiredesignchangesordrivereducation.Stillothers,suchascrashesatmedianopening,operationalconcernsaboutleft‐turningvehicles,accesstoactivitycenters,andsafetyandoperationalconcernsaboutvulnerableroadusers,willrequiregreaterattentiontoaccessmanagementissues.RoundaboutsaredifferentfromothertypesofintersectionsbecausetheycanprovideU‐turnopportunities,allowingforareductionoffullaccesspointsalongaroadwaysegment,whileatthesametimeenhancingaccess.Theyhavedifferentoperationalcharacteristics–slowerspeedsatintersections,continuousmovementoftraffic,fewerconflictpointsbetweenvehiclesandfewersafetyissuesassociatedwithleftturningvehiclesinsidetheroundabout.Inturn,theseoperationalcharacteristicscreatechallengesforvulnerableroadwayusersandfortrucksandotherlargevehicles.Additionally,specificoperationalcharacteristicsandcontextualaspectsofroundabouts–newvs.retrofit,urbanvs.suburbanvs.rural,singlevs.multi‐lanevs.complexintersections(turbo,spiralorinvolvingoneormoresliplanes)affectthedesigncharacteristicsofroundabouts.ThisresearchinformsusaboutthesafetyandoperationsofexistingroundaboutsinthestateofFlorida.However,thetypesofroundaboutscurrentlyinusearenotrepresentativeofthetypesofroundaboutsthatarelikelytobebuiltunderthenewstateguidelines.Thesampleincludedonlyfourroundaboutsonstatehighways.Theroundaboutcorridorsthatwereevaluatedarelocatedoffthestatehighwaysystem.Roundaboutsbuiltundertheproposedguidelinesarelikelytoincludehighertrafficvolumes,morecomplexlocations,morecomplexagreementsbetweenthestateandlocalgovernment,andinthecaseofretrofits,havemorecomplexaccessmanagementissues.Assuch,roundaboutcorridors,whichwereonlyexaminedinalimitedmanner,willbecomeamoreimportantissueinthefuture.Thisraisesthequestionofhowtodesignasetofrecommendationsthataddressthecomplexityofcontextsinwhichroundaboutsarebeingimplementedinthestate.Recommendationsofthisstudyneedtospecificallyaddressthelocationofdrivewaysandintersectionsincloseproximitytoroundabouts,roundaboutsnearactivitycenters,theISDandSSDnearintersections,andtheneedsofbothvulnerableroadusersandtrucksinproximitytoroundabouts.Thefirsttwotopicsaredirectly related to accessmanagementwhile the third topic is less directly related but is an importantconsiderationinthedeploymentofroundabouts.Both the safety and operational analysis identified issues related to the location of driveway and roadswithin close proximity to the intersection. The operational analysis identified two situations wheredrivewayandroaddistancesaffectedoperations:vehiclesturning left intoan intersectionthat is locatedwithinthefunctionalareaofaroundabout,andaroundaboutlocatedtooclosetoanotherintersectionatanactivitycenter.Thesafetyanalysisshowedavarietyofsituationsinwhichleftturningvehicles,eitheronthe leg of a roundabout and/or turning onto a driveway near a roundabout may have caused a crash.However, the crash data does not indicate serious safety issueswithmedian openings in the vicinity ofroundabouts. While losing median openings located between two adjacent roundabouts could preventsome of the median opening related crashes, the location of median openings needs to be consideredwithinthecontextofoverallaccessmanagementinandaroundtheroundabout.The reviewofnational andstateguidanceon roundaboutsandaccessmanagement, and theoperationalanalysisof this study, suggest that roundaboutsaresimilar tounsignalized intersections in theway that

Chapter7Discussion


theyoperate.ThisisconfirmedbyHCM2010,p.4‐14,whereitstatesthat“[t]heoperationofroundaboutsissimilartothatoftwo‐waystop‐controlledintersections.Inroundabouts,however,enteringdriversscanonly one stream of traffic—the circulating stream—for an acceptable gap.” In HCM 2010, the servicemeasure and thresholds for roundabouts have beenmade consistent with those for other unsignalizedintersections. This is covered primarily via control delay calculation, as it is for TWSC and AWSCintersections, by adjusting for the effect of yield control. Also, “roundabouts discharge vehicles morerandomly, creating small (but not necessarily usable) gaps in traffic at downstream locations” (p. 8‐5).These gaps are different than signalized intersectionswhich create vehicle platoons but similar to gapscreatedbyotherunsignalizedintersections,suchasAWSCintersections. Assuch,roundaboutsmayhavedifferent requirements with respect to their functional area because of differences in overall speed,acceleration, deceleration and queuing. While the access management guidance recognizes thesedifferences,noresearchstudyhasexplicitlyconsideredhowcontextual factorsaffect the functionalarea.The guidance on access management, which would include roundabouts, should consider the intendedfunction of the roadway as a component of a complete transportation systemnetwork. This evaluationwould include the roadway segment’s environment,whether rural and undeveloped, urban fringe, sub‐urban,urban,anddenselydevelopedorurbancore. Itwouldalsoincludetheavailabilityofasupportingroadwaysystemtosupplyalternativeaccess,andthedesiredorappropriatebalancebetweensafetyandfrequencyofaccess(TRB,2003).Activitycentersrepresentaspecificcontext forroundabouts thatwere identified in theresearchbut forwhichnoclearguidanceonintersectionspacingandeventheirusecanbeclarified;assuch,thiscontextmay require additional research. The crashdatadidnot show increased safetyhazards at roundaboutsthat provided direct access to activity centers. Providing direct access to activity centers through adedicatedlegisdesirabletoimprovetrafficoperationsonthecorridor,aslongastheprovisiondoesnotincreasethenumberofroundaboutlegsbeyondthestandardfourlegs.Theoperationalanalysisidentifiedtwosituationsinwhichroundaboutsmayrequirespecialdesignconsiderationstoensurethecontinuousandsafe flowof traffic. First, if anadjacent intersection for circulating traffic is located tooclose to theroundabout,theoperationsoftheroundaboutandtheintersectioncanbeadverselyaffected.Second,ifaroundaboutislocatednearanurbanactivitycenter,wheretheflowofpedestriansishigh,thedesignoftheroundaboutshouldincorporateconvenientandaccessibledrop‐offandpick‐uplocationsincloseproximitytotheroundabout.AnotheraccessmanagementissueassociatedwithroundaboutsforwhichtheresearchcouldnotprovideclearguidancerelatestotheSSDandtheISD.Driversenteringandexitingaroundaboutneedtoseeandreacttothedriversinfrontofthemwithchangesintheirspeed;assuchtheSSDandISDareanimportantpartofensuringthatthefunctionalareaofaroundaboutisadequatetoensurethesafetyandefficiencyforallusersaroundroundabouts.Bothoftheseissueswereidentifiedinthesafetyanalysis,butthecrashdatashowsthatdownstreamdrivewaycornerclearanceshaveagreatersafetyimpactthanupstreamdrivewaycorner clearances. Longerdownstreamcorner clearances aredesirablebecause theyprovideadditionaltimefordrivewayvehiclesthatexperiencereducedvehiclegapsandhigherapproachvehiclespeedfromupstreamroundabouts.AlthoughtheISDandSSDwereshowntoberelatedtothesafetyoftheoperationsoftheroundabout,thesampleofroundabouts(n=37)isrelativelysmall.Theoperationalanalysisdidnotprovide any additional insights into how the ISD and SSD affect the capacity and operation of theroundabouts. However,theISDandSSDneedtobeconsideredinthedesignoftheroundaboutbecausetheycandirectlyaffectsafetyandtheoperationsofaroundaboutinitsfunctionalarea.Drivewayslocatedatorneartheroundaboutcancreateconflictswiththecirculatoryroadway,duetoaccelerationanddecelerationalongthecorridor.Yetbecauseoftheslowerspeeds,drivewaysmayposelessofachallengeforaccessmanagementthanforothertypesofintersectionincludingunsignalizedintersections.However,alongmanypartsofthestatehighwaysystem,theexistingdrivewaysmayposeachallengewhenaroundaboutisretrofittedintoanurbanenvironment.Insomesituations–forexample,if

Chapter7Discussion


thedrivewayhaslowtrafficvolumes–accesswasprovidedpriortotheinstallationoftheroundabout.Inthiscase,noalternativeaccesspointsareavailable.Thedrivewayisproperlydesignedtoallowvehiclestoturnaroundandexitfacingforward–thedrivewayscouldbelocatedinthefunctionalareaofaroundaboutifitincludesadequateISDandSSD.Wheredrivewaysarelocatedinorneararoundabout,thedesignshouldgiveaclearvisualindicationthatprivatedrivewaysareadjacenttotheroundaboutandarenotforpublicuse.Accessmanagementintheproximityofaroundaboutislargelyconnectedtotheoperationinthefunctionalareaaroundtheroundabout,whichisinfluencedbytheISDandtheSSD,thelocationofdriveways,andthedistancetotheclosestintersectionorroundabout.Thesafetyanalysissuggeststhatthedownstreamfunctionalareaneedstobelongerforthedownstreamlegthanfortheupstreamlegbecausedriversarelikelytobedeceleratingastheydrivetowardsaroundabout.Figure29,above,showsthatthetypicaldimensionsforleft‐turnaccessnearroundaboutsshouldbeaminimumof275ft.,subjecttolocalconditions.Inadditiontothedistancefromthecenteroftheroundabouttoitslegs,thisso‐calledfunctionalareaincludesthedistancefromthecenterfortheroundabouttotheedgeofthesplitterisland,aminimumof50ft.toclear,75ft.toallowforleftturningmovements,and90ft.fordeceleration.NCHRP672,Roundabouts,AnInformationalGuideisclearaboutthecomponentsofthefunctionalarea,butdifferentstatesmeasurethatdistancedifferentlysoitisimportanttobeclearabouthowtomeasurethedistance.7.5RecommendationsThesynthesisoftheresearchfindingssuggeststhat,whilesignificantresearchhasbeencompletedonroundaboutsandonaccessmanagement,additionalresearchisneededonthecombinationofroundaboutsandaccessmanagementindifferentcontextsandconditions.Roundaboutshavegenerallybeenconsideredsimilartounsignalizedintersections,buttheyhavedifferentoperationalcharacteristicsrelatedtothedownstreamflowofvehicles,andthespeedwithwhichvehiclesenterthem.Roundaboutscanbeseenasapartofaccessmanagement,likemedianswhentheyfacilitateU‐turns,or,astheyaregenerallycategorized,asatypeofintersection.However,theyhavedesignconsiderationsthatdifferfromdrivewaysandleft‐turnmedians.Irrespectiveofhowtheyarecategorized,andthecontextinwhichtheyareimplemented,roundaboutsneedtobedesignedinamannerthatensurestheoperationalefficiencyoftheintersectionandthesafetyofallusers.Guidancethatresultsinroundaboutswithlengthyqueuinglanescouldundulydecreasethenumberofroundaboutsthatareimplemented,whilepoorlydesignedguidancecouldcreateunsafedrivingconditionsforroadwayusersandreducetheaccessandeconomicviabilityofbusinessesonadjacentland.Inthissection,threetypesofrecommendationsaremaderegardingaccessmanagementaroundroundabouts.ThefirstsetofrecommendationsprovidesdirectionfortheFDOTonupdatingtheirguidanceonroundaboutsandaccessmanagement,includingaccessmanagementtools,theMedianHandbook,theDrivewayInformationGuide,andthesoftwareusedtoanalyzeroundabouts.Nextasetofrecommendationsismadeforfutureresearchregardingroundaboutsandaccessmanagement.Inparticular,recommendationsaremadetoproposeanNCHRPProjectonroundaboutsandaccessmanagement,abefore‐and‐afterstudyoftheproposedroundaboutretrofitinDowntownSarasota,andastudytoestablishFlorida‐specificparameterstousewiththeHCSandothersoftwareemployedtoanalyzethecapacityofroadwaysonwhichroundaboutsareproposed.7.5.1RecommendationsforFlorida’sGuidanceonRoundaboutsandAccessManagementAsFloridaincorporatesroundaboutsintoitspractices,allpolicyguidanceneedstoprovideaconsistentsetofguidanceontheuseofroundaboutsthataddressthediversesituationsunderwhichroundaboutsareimplemented.Essentialtothisguidanceisconsiderationofthedifferencesbetweenroundaboutsandothertypesofintersections,aswellasothertypesofaccessmanagement,suchasdriveways,andmedians,whicharediscussedinlatersections.Thedesignspeedsforroundaboutsissignificantlylowerthanthedesign

Chapter7Discussion


speedforunsignalizedintersections,withadesignspeedof20to30mphand25to35mphforasingle‐laneandmulti‐laneroundabout,respectively.Theguidanceshouldaddressthedifferencesinoperationalconsiderationsbetweenroundaboutsandotherformsofaccessmanagement,anddifferencesintheoperationofthefunctionalarea,includingqueuing,decelerationandacceleration,accommodationofpedestrians,andotheraspectsofthemovementofvehicleswithinthefunctionalarea.Thefindingsalsoidentifiedtwospecificissuesrelatedtoroundaboutsthatshouldbeaddressedintheaccessmanagementguidance:theuseofroundaboutstoprovideaccesstoactivitycenters,andtheaccommodationofallusersaroundasingleoracorridorofroundabouts.TheFloridastateguidanceonaccessmanagementneedstoreinforcetheexistingprocessforimplementingaccessmanagementasroundaboutsareincorporatedintotheaccessmanagementguidance.Inparticular,TRB’sAccessManagementManualrecommendsthreebasicstepstoimplementaccessmanagementonaroadway:definingaccesscategories,establishingaccessmanagementstandards,andassigningcategoriestotheroadwaysorroadwaysegments.Initialfactorstobeconsideredarethedegreeofroadwayimportance,roadwaycharacteristics,landuseandgrowthmanagementobjectives;andthecurrentandpredictedflowsofgeneraltransit,aswellaspedestrianandbicycletraffic.Fourgeneralaspectsofdevelopingaccessmanagementstandardsincludemedians,degreeofurbanization,speed,andsafety(TRB,2003,p.71).Finally,theassignmentofcategoriesinroadwaysystemsneedstotakeintoaccountthefollowingfactors:theintendedfunctionoftheroadwayasacomponentofacompletetransportationsystemnetwork;theroadwaysegment’senvironment(ruralandundeveloped,urbanfringe,sub‐urban,urban,anddenselydevelopedorurbancore);theavailabilityofasupportingroadwaysystemtosupplyalternativeaccess;andthedesiredorappropriatebalancebetweensafetyandfrequencyofaccess(TRB,2003,p.71).Totheextentpossible,thestateshouldconsidertheuseoflocallydevelopedparametersforvariousaspectsofdesignandoperationalanalysisofroundabouts.Otherstateshavedevelopedlocalparametersthatrelatetotheinfluenceofdriverbehaviorasitaffectscapacityandoperationalcharacteristicsofroundabouts.ThestatehasalreadyadoptedNCHRP672,Roundabouts,AnInformationalGuideforitsguidanceonroundabouts,andguidanceonthefunctionalareashouldbeincludedinthestateguidance.Differencesintheoperationswithinthefunctionalareashouldbehighlighted.Theguidanceneedstobeexplicitaboutthedefinitionofthefunctionalareaofaroundabout,especiallyifitdeviatesfromtheguidanceprovidedinNCHRP672,Roundabouts,AnInformationalGuide.Establishingthelengthsofthefunctionalareabaseduponthefunctionalclassificationoftheroadwayiscomplex.Whilemuchoftheguidanceisbuiltontheassumptionthatroundaboutsoperatelikeunsignalizedintersections,thespeedwithwhichvehiclesenteraroundaboutismuchslowerthanunsignalizedintersections.Assuch,thismightsuggestthatthefunctionalareaofaroundaboutisshorter.TheexistingguidanceforunsignalizedintersectionsandVirginia’sMinimumSpacingStandardsforCommercialEntrances,Intersections,andCrossovers,asshowninFigure29,shouldbereviewedtoestablishinitialguidanceforlocalgovernmentstouseastheybegintoexploretheiroptionsforroundaboutsandaccessmanagement.ItisnoteworthythattheintersectionspacingstandardsforthestateofVirginia,asshowninthelastcolumninFigure29,arecloserthantheintersectionspacingforunsignalizedintersections.Additionally,guidanceondrivewayandintersectionspacingneedstoaddressthefactthatthespeedsnearroundaboutsaresignificantlylowerthanthe45mphusedintheexistingguidance.7.5.1.1AccommodationofAllUsersAroundRoundabouts.Asthestatebeginstoimplementroundaboutsinagreatervarietyoflocations,theneedsofallroadwaysusers,includingbicyclists,pedestrians,andlargevehicles,needtobeaccommodated.Thefindingsofboththesafetyandoperationalanalysisidentifytheneedtoaccommodatebicyclistsandpedestriansaroundroundabouts.Becauseofthelowerspeedsassociatedwithroundabouts,experiencedbicyclistsmaybeabletomergewithmotoristsastheynavigatethroughtheroundabouts.Becauseofthesplitterislandandthelocationofthecrossing

Chapter7Discussion


behindthevehicleenteringtheroundabout,driversmayencounterlessdelaythanvehiclesatunsignalizedintersections.However,becauseofthecontinuousmovementthroughroundabouts,pedestrians,andinparticularvisuallyimpairedpedestrians,areatgreaterriskatroundaboutsthanatotherunsignalizedintersections.Additionally,asdiscussedbelow,roundaboutspresentaparticularchallengetopedestriansnearactivitycentersifpick‐upanddrop‐offisnotproperlyhandled.Whilethesafetyandoperationalanalysisofthisstudydidnotidentifysignificantproblemswithtrucksandotherlargevehicles,theyarelikelytobecomeanissueasroundaboutsaremorewidelyusedalongstateroadways,whichcanhavemoretrucktraffic.During2007‐2011,atotalof18crashesinvolvedheavyvehiclesatthe131commercialroundabouts.Theguidanceneedstobedesignedtoaccommodatetrucksasapartofaccommodatingallusersinthesystem.Whenroundaboutsareimplementedengineersandlocalofficialsmaybelievethattheycanremoveorrestrictmovementatmediansorotheraccessmanagementdevicesbasedupontheideathatleft‐turningmovementscanbeaccommodatedattheroundabout.TheU‐turnalternativemayincreasethenumberofsideswipecrashesatroundabouts,especiallyforlargevehicles.Largetrucksandbusesoftenfinditdifficulttonegotiateasmallerroundabout.Inparticular,lackofadequatelateralclearancecouldresultinheavyvehiclessideswipingothervehiclesorbecominginvolvedinacollisionwithafixedobject,usuallywiththeroundaboutcenterisland.Whileasingleroundaboutmaynotbeabletoaccommodatetrucks,theymaybemoreeasilyaccommodatedalongaroundaboutcorridororthroughalternative,parallelaccessthatallowstruckstoreachcommercialdestinations.Furthermore,forplaceswherethepercentageofheavyvehiclesishigh,thedesignoftheroundaboutsshouldtaketheradiusintoconsideration.Whenthelackofspacepreventstheinstallationofalargeroundabout,itisrecommendedthatothertypesofintersectionarepreferred.7.5.1.2UseofRoundaboutsNearMajorActivityCenters. Theresultsofthisresearchshowconflictingresultswithrespecttotheuseofroundaboutsattheentrancetomajoractivitycenters.Accessaroundactivitycenterscanbecomplexduetotheneedtoprovideaccesstoavarietyofdestinationswithinashortdistance.Becauseroundaboutsallowacontinuousflowoftraffic,theymaybeseenasamoreefficientsolutionthanusingcontinuousrightandleftturnlaneswithdirectionmediansandotherformsofaccessmanagement.Thesafetyanalysisfoundthatroundaboutswiththreeorfourlegsattheentrancetoactivitycentersarejustassafeasroundaboutsinothercommerciallocations.Theoperationalanalysisfoundthatifaroundaboutislocatedtooclosetoanadjacentintersectionspilloverandadecreaseincapacitycantakeplace.Assuch,thestateshouldconsiderdevelopingguidanceontheuseofroundaboutsatornearmajoractivitycenters.Thisguidanceshouldconsiderwhethertheactivitycenterislocatedinanurban,suburbanorruralcontext;howtheactivitycenterissituatedwithinthestreetnetwork;andhowtrucksareaccommodatedinthevicinityoftheroundabout.Forexample,cantruckshaveaccesstothestoresforloadingandunloadingofdeliveriesusingaparallelroadway?Inanurbancontextwhereactivitycentersarelocatedalongaroad,aroundaboutcouldpotentiallyprovidebetteraccesstotheactivitycenter.Withmedianclosingandtheuseofaseriesofroundaboutsinacorridor,safeoperationandaccesstoactivitycenterscanbothbeguaranteed.Ifroundaboutsarenotproperlydesignedtoaccommodatepick‐upsanddrop‐offsnearmajoractivitycenters,driversmayneedtomaneuveraroundstoppedvehiclesorstopinthemiddleoftheroundabout.Additionally,largepedestrianvolumesatcrosswalkswithintheroundaboutcanalsocauseaqueuewithintheroundabout.Theguidanceforroundaboutlocationrecommendsagainsttheuseofroundaboutswheretherearehighpedestrianvolumes.However,otherpropertiesofroundabouts,suchasaestheticsandlandscaping,mayjustifytheirusageeveninlocationswithhighpedestrianvolumes.Ifaroundaboutisusedinhighpedestrianareas,pedestrianscouldbeaccommodatedwithunderpassesoroverpasses,orwithsidewalksfurtherfromthecirculatoryroadway.Regardlessofwhethertheroundaboutislocatedinanurbanorsuburbancontext,nosignificantimpactonoperationisshown.

Chapter7Discussion


7.5.1.3RecommendationsontheSoftwareforAnalysisofRoundabouts.Softwareforanalysisofroundaboutsneedstobeavailableforavarietyofapplicationsincludingplanninglevelsizing,preliminarydesign,analysisofpedestriantreatments,systemsanalysis,andpublicinvolvement.Generally,theseneedscanbeaddressedwithHCS.Otherdeterministicsoftwarecanconducttheplanning‐levelandpreliminarydesignreview,whilesimulationsoftwarecanbeusedforthesystemsanalysis,publicinvolvementandanalysisofpedestriantreatments.Deterministicsoftware,suchasHCS,Synchro,SIDRA,RODELandARCADY,canperformqueuinganalysisandprovideusefulinformationrelatedtoaccessmanagement,especiallyforplacingdriveways.Simulationsoftware,suchasVISSIM,canbeusedtoevaluatetheoperationofroundaboutsandtheinteractionbetweentrafficflowsatroundaboutsandadjacentdrivewaysbyconductingmicroscopicanalysis.Itisclearfromthisanalysisthatdeterministicsoftwarecanprovideguidanceonwherethedrivewayshouldbeplacedbeforetheconstructionofintersections,whilesimulationcanbeusedtoevaluatetheimpactofdrivewayandotheraccessmanagementissuesonroundaboutoperation. ThenewversionofHCS2010providesaviabletooltoconductqueuinganalysisforroundabout,whichcanbeusedtodeterminethelocationofaccesspointandthelengthoffunctionalarea.CORSIM,whichisusedforotherapplicationsinFlorida,whencomparedtoothersimulationsoftwarepackages,requiressomemodificationinordertoaccuratelyreplicateroundaboutoperations.RoundaboutsshouldbemadeavailableinCORSIMbyallowingmultiplenodestobegroupedtogetherasoneroundabout,andfollowuptimeandcriticalgapshouldbemadeapproach‐based.7.5.2RecommendationsforAdditionalResearchWhilethenumberofroundaboutshasincreasedsignificantlyoverthepastcoupleofdecades,researchhasnotkeptupwithourunderstandingofthedifferencesbetweenthesafetyandoperationalcharacteristicsofroundaboutsastheyhavebeenimplementedinadiversityofsituations.NCHRP672,Roundabouts,AnInformationalGuideprovidesguidanceonavarietyofaspectsoftheanalysisanduseofroundaboutsanditcharacterizesthesimilaritiesbetweenroundaboutsandothertypesofintersections.However,itdoesnotprovidedetailedguidanceonroundaboutsandaccessmanagement.TheFDOTshouldconductitsownresearchandworkwithAASHTOandotherpartnerstoensurethatguidance,includingroundaboutsasacomponentofaccessmanagement,beincorporatedintopractice.Inthissection,threeseparateresearchinitiativesareidentifiedbasedupontheresearchconductedinthisstudyincluding:nationalresearchonroundaboutsandaccessmanagement,abefore‐and‐afterstudyofproposedroundaboutsintheUS41corridorinSarasota,andstudiesonthedevelopmentoflocalvariablesforparametersintheanalysistoolsforassessmentofroundabouts.Thefirstresearchwouldbeproposedforanationalstudy,whilethelasttwowouldberecommendedforFDOTfunding.7.5.2.1NationalResearchEffortonRoundaboutsandAccessManagementThroughoutthisresearchithasbecomeincreasinglyclearthatlittleresearchhasbeenconductedonroundaboutsincombinationwithotherformsofaccessmanagementandroundaboutsasaformofaccessmanagement.Roundaboutscanbeseenasaformofaccessmanagementbecausetheycanaccommodateleft‐turnsandallowtheremovalofdirectionalleft‐turnlanes,yettheyfunctionasintersections.Thedifferencesinsafetyandoperationalcharacteristicsfromothertypesofaccessmanagementandotherintersectionsmeansthatthesitedistances,stoppingdistances,functionalareacharacteristics,andintersectionanddrivewayspacingmaybedifferentforroundabouts.Furthermore,theuseofroundaboutsinavarietyoftransportationandlandusecontextsmaymeanthatthesefactorsdifferbycontext.WhileNCHRP672,Roundabouts,AnInformationalGuideprovidesagreatstartonthisresearch,aprojectisneededthatspecificallyfocusesonguidanceonaccessmanagementformajorarterialsandothersimilarroadwaysfoundinthestatehighwaysystem.7.5.2.2Before‐and‐AfterStudyoftheSarasotaRoundabouts

Chapter7Discussion


Anotherareawherefurtherresearchisneededisrelatedtounderstandingthedifferencesinoperationalcharacteristicsbetweencorridorsusingroundaboutsandotherstandardintersections.Thecontractor’sreportonNCHRP3‐100,whichevaluatestheuseofroundaboutsalongcorridors,islikelytoincreaseourunderstandingofthesedifferences.However,thisstudyisacross‐sectionalstudy,whichmaynothaveacompletesetofoperationaldatathatallowsforacomprehensiveunderstandingofthesedifferences.FDOThasauniqueopportunitytocompletesuchastudyontheUS41corridorinSarasotawheretworoundaboutsareproposedinaportionofthedowntownarea.ThisprojectiscurrentlyscheduledinthelateryearsoftheregionalTransportationImprovementProgram(TIP).Assuch,theFDOThastheopportunitytocompleteabefore‐and‐afterstudybycollectingthebeforedatawithinthenexttwoyearsandthenattwopointsafterwhentheprojectiscompleted.Asecondsetofdatacouldbecollectedtounderstandtheadjustmentofroadwayuserstothenewroundaboutandotheraccessmanagementfeatures,whilethethirdsetofdatacouldbecollectedafterdrivershaveadjustedtothechangeinthecorridor.Tocompletesuchanevaluationwouldrequirethecollectionofthefollowingtypesofdata:

Existinggeometry(numberoflanes,typesoflanes,etc.).FDOTshouldbeabletoprovideas‐builtplans.Thesecanthenbeverifiedthroughfieldobservation.

Traveltime.Thiscanbeverifiedusinganinstrumentedvehiclemakingnumerousrunsalongthecorridor.Eachrunwouldbevideo‐recordedsothattheresearcherscanaccuratelyidentifysourcesofvariationinthetraveltimes.

Trafficvolumes.ThisdatacouldcomefromstationaryvideocamerasorexistingFDOTsensorinfrastructure,ifitexists.

Turningmovementpercentages(right,through,left,U‐turn).Again,thiscouldcomefromstationaryvideocamerasorexistingFDOTsensorinfrastructure,ifitexists.

Intersectionapproachlegaveragequeuelengths(thiscanbeestimatedfromvideorecordings). Signaltimings(assumingtherearecurrentlysignalizedintersectionsalongthiscorridor).These

datashouldbeabletobeprovidedbyFDOT.Theycanbeverifiedthroughfieldobservation.7.5.2.3FocusedStudiesonState‐specificlocationsguidanceAmajorchallengewiththeuseofnationalguidance,orguidancefromotherstates,isthatdriversinFloridamayresponddifferentlytodifferentformsofaccessmanagement,theymayhavedifferentreactiontimesandtheymaydrivecloserorfurtherfromotherdriversastheyenterintersectionsandroundabouts.Theroundaboutsguidanceinseveralstatesprovidesdocumentationofuseoflocally‐developedparametersforvariousaspectsofdesignandoperationalanalysis(e.g.,Californiaforaccelerationanddecelerationeffects;MichiganforSPFsandCMFs;andWashingtonforcornerclearance,parallelroundabouts,U‐Turns,parkingandtransitstops,andWisconsinforlocationofdrivewaysandsitedistancebetweenusers).Thesefactorsmayinfluencethecalculationoftheentryflowrate,conflictingflowrateandexitflowrateofroundabouts.TotheextentthatFloridadriversbehavedifferentlythandriversinotherstates,theFDOTshouldfundresearchtojustifytheuseofdifferentparametersforsoftwareandotheranalyticaltoolsforplanning‐leveldesign,preliminarydesign,analysisofpedestriantreatments,andsystemsanalysis.

Chapter8Conclusions


ChapterEight:ConclusionsThisFDOTresearchprojectfocusedonprovidingadviceonhowtoevaluatetheuseofroundaboutsasaformofaccessmanagementandconsequentlyonwhatshouldbeincludedintheFDOT’sMedianHandbook,andDrivewayInformationGuide.Inordertoaccomplishthisgoalitisimportanttounderstandtheconnectionbetweenroundaboutsandaccessmanagementandotherformsoftrafficcontrol.Therefore,thisprojectincludedthreeprimarycomponents:areviewandassessmentofnationalandstateguidancerelatedtoroundaboutsandaccessmanagement;asafetyanalysisofall283roundaboutsinFlorida;andanoperationalanalysisofselectedroundabouts.Thischaptersummarizestheconclusionsofthisresearcheffort.8.1ConclusionsoftheReviewofNationalandStateGuidanceThereviewofnationalguidanceonroundaboutandaccessmanagementshowedthatthereareonlyfivenationalreportsthatrefertoroundabouts:AASHTOGreenBook(2011),NCHRPReport672,NCHRPReport572,NCHRPReport674,andNCHRPSynthesis264,ofwhichonlytheformerthreearerelevanttothisstudy.NCHRPReport672,Roundabouts,AnInformationalGuidereferstoaccessmanagementinthecontextofroundaboutsandreinforcestheideathatmanyoftheaccessmanagementprinciplesappliedtoconventionalintersectionscanbeappliedtoroundaboutsaswell.TheAASHTOGreenBook(2011)explainsaccessmanagementconsiderationsforroundabouts.NCHRPProject3‐100,currentlyinprogress,evaluatesthePerformanceofCorridorswithRoundaboutsandwillsoonproduceanothernationalreportwhichwillberelevanttothisproject.Regardingstateguidanceonroundabouts,fromthefiftystatesandtheDistrictofColumbia,twenty‐sixstateshavewebsitesonroundaboutswithvaryingdegreesofinformation.MostofthesestatesadoptthenationalguidancefromNCHRPReport672,Roundabouts,AnInformationalGuide(Rodegerdtsetal.,2010).Infourteenstatesguidanceontheuseoflocallydevelopedparametersforvariousaspectsofdesignandoperationalanalysisisprovided.Minnesota,WisconsinandNewHampshirerecommendspecificsoftwarefortheassessmentoftheuseofroundaboutsforanintersectiondesign.Threeotherstates,Wisconsin,Virginia,andKansas,addressthecoordinationofroundaboutsandaccessmanagement.Regardingaccessmanagementguidance,forty‐threestateshaveincorporatedaccessand/oraccessmanagementintotheirplanninganddesignpolicies.Nineteenstateshaveaccessmanagementmanuals,separatefromgeneraldesignmanualsandelevenstateDOTsmentionaccessmanagementordesignmanuals.AnothersixteenDOTshaveotherdocumentswithvariousnames.However,onlysevenstatesincorporateroundaboutsintotheiraccessmanagementguidance:Kansas,Virginia,California,Iowa,Michigan,Wisconsin,andWashington.Generally,whenitcomestoroundaboutsandaccessmanagement,onlyKansasandVirginiaprovidesignificantsupplementalinformationtoNCHRP672,Roundabouts,AnInformationalGuide,whilemostoftheotherstatessimplyadoptedtheguidancewithoutsupplementation.Floridahasthreemajordocumentsrelatedtoaccessmanagement.TheFDOTMedianHandbook(2006)addressessomedesignconsiderationsrelatedtoroundaboutsbutitdoesnotprovideinformationaboutroundaboutdesignoraccessmanagement.Theothertwodocumentsdonotrefertoroundabouts.8.2ConclusionsAboutSafetyAnalysisofRoundaboutsinFloridaDuring2007‐2011,atotalof2,941crasheswerefoundtohaveoccurredwithin500ft.ofthe283roundabouts.Policereportsofthesecrashesweredownloadedandreviewed.Crashlocationsofthese2,941crashesweremanuallyverifiedandtheincorrectlocationswerecorrected.Intersection‐relatedcrashesandthosethatdidnotoccurontheroundaboutsandtheirapproachlegswereexcluded.Finally,atotalof1,882crashesthatoccurredwithin500ft.ofthe283roundaboutswereincludedintheanalysis.

Chapter8Conclusions


Thefollowingpotentialsafetyconcernsassociatedwithroundaboutsincommercialareaswereinvestigated:

Impactofdrivewaycornerclearancesonroundaboutsafety. Safetyimpactofmedianopeningsinthevicinityofroundabouts. Safetyatroundaboutsthatprovidedirectaccesstoactivitycenters. Safetyofvulnerableroadusersincludingpedestriansandbicyclists.

Basedontheresultsfromthesafetyanalysis,thefollowinggeneralrecommendationsrelatedtotheaccessfeaturesinthevicinityofroundaboutsaremade:

Crashdatashowthatdownstreamdrivewaycornerclearanceshaveagreatersafetyimpactthanupstreamdrivewaycornerclearances.Longerdownstreamcornerclearancesaredesirabletoprovideadditionaltimefordrivewayvehiclesthatexperiencereducedvehiclegapsandhigherapproachvehiclespeedfromupstreamroundabouts.

Crashdatadidnotindicateserioussafetyissueswithmedianopeningsinthevicinityofroundabouts.However,closingmedianopeningslocatedbetweentwoadjacentroundaboutscouldpreventsomeofthemedianopeningrelatedcrashesandisdesirableifthecorridorisdesignedtoservelowheavyvehiclevolumesoriftheroundaboutsaresufficientlylargetosafelyaccommodateU‐turnsbyheavyvehicles.

Crashdatadidnotshowanincreasedsafetyhazardatroundaboutsthatprovidedirectaccesstoactivitycenters.Providingdirectaccesstoactivitycentersthroughadedicatedlegisdesirabletoimprovetrafficoperationsonthecorridoriftheprovisiondoesnotincreasethenumberofroundaboutlegstobeyondthestandardfour.

8.3ConclusionsAboutOperationalAnalysisofRoundaboutsinFloridaTheconclusionsfromtheoperationsanalysisofroundaboutsinFloridaaredescribedinthisparagraph.Theroundabouts’operationalanalysisconductedinFloridashowedthatconflictscanoccurinthefunctionalareaofaroundaboutwhendrivewaysorotherintersectionsarelocatedtooclosetoaroundabout.Thefunctionalareaofaroundaboutmaybedifferentfromconventionalintersections,especiallyincaseswherethespeedissignificantlylowerthanmostun‐signalizedintersectionscurrentlyoperate.Inordertoavoidsuchconflicts,geometricdesignshouldtakeintoconsiderationthetrafficqueuethatcouldbedevelopduringroundaboutoperationsastheycanaffectprocesseswithintheroundaboutorwiththesurroundingintersections.Duringtheoperationalanalysis,highpedestrianandbicyclesvolumescanaffectthecapacityandtheeffectiveoperationsofroundabouts.Theoperationalanalysisalsoindicatederroneousdriverbehaviorsuchasstoppinginthemiddleoftheintersectiontopickupordropoffpedestrians,causingqueueswhichusuallyhappeninareaswithhighpedestrianandbicyclevolumes.Thisconflictswiththesafetyanalysis,whichreinforcedtheadvantagesofusingroundaboutsforaccesstoactivitycentersbecausetheyreducedthechallengesofaccessthroughopenmediansortheplacementofanAWSCintersectionincloseproximitytotheroundabout.Anotherconcernisspillbackintotheroundaboutfromadownstreambottleneck,whichwouldresultincompletelylockingtheroundabout.8.4FinalRemarksAsFloridastartsincorporatingroundaboutsintoitspracticesmoreoften,consistentguidanceontheuseofroundaboutsthataddressthediversesituationsunderwhichtheyareimplementedshouldbeprovided.Essentialtothisguidanceisconsiderationofthedifferencesbetweenroundaboutsandothertypesofintersectionsandothertypesofaccessmanagement,suchasdriveways,andmedians.Roundaboutshave

Chapter8Conclusions


generallybeenconsideredsimilartounsignalizedintersectionsbuttheyhavedifferentoperationalcharacteristicsrelatedtothedownstreamflowofvehicles,andthespeedwithwhichvehiclesenterthem.Irrespectiveofhowtheyareconsidered,andthecontextinwhichtheyareimplemented,roundaboutsneedtobedesignedinamannerthatensurestheiroperationalefficiencyandthesafetyofallusers.Thefindingsofboththesafetyandoperationalanalysisidentifytheneedtoaccommodatebicyclistsandpedestriansaroundroundaboutsbecausepedestrians,andinparticular,visuallyimpairedpedestrians,areatgreaterriskatroundaboutthanatotherunsignalizedintersectionsduetothecontinuousmovementthroughthem.Additionally,roundaboutspresentaparticularchallengetopedestriansnearactivitycentersifpick‐upsanddrop‐offsarenotproperlyhandled.Theresultsofthisresearchshowconflictingresultswithrespecttotheuseofroundaboutsattheentrancetomajoractivitycenters.Roundaboutsallowacontinuoustrafficflowsotheymaybeseenasamoreefficientsolutionthanusingcontinuousrightandleftturnlaneswithdirectionmediansandotherformsofaccessmanagement.Thesafetyanalysisfoundthatroundaboutswiththreeorfourlegsattheentrancetoactivitycentersarejustassafeasroundaboutsinothercommerciallocations.However,theoperationalanalysisfoundthatifaroundaboutislocatedtooclosetoanadjacentintersection,spilloverandadecreaseincapacitymayhappen.Assuch,thestateshouldconsiderdevelopingguidanceontheuseofroundaboutsatornearmajoractivitycentersandconsiderthecontextwheretheactivitycenterislocated,howtheactivitycenterissituatedwithinthestreetnetwork,andiftrucksanddeliveryvehiclesareproperlyaccommodatedinthevicinityoftheroundabout.Ifaroundaboutisconstructedinhighpedestrianareas,pedestrianscouldbeaccommodatedwithunderpassesoroverpassesorwithsidewalksfurtherfromthecirculatoryroadway.Whilethesafetyandoperationalanalysisofthisstudydidnotidentifysignificantproblemswithtrucksandotherlargevehicles,theyarelikelytobecomeanissueasroundaboutsaremorewidelyusedalongstateroadways.Thesecanhavemoretrucktrafficandlargetrucksandbusesmayfinditdifficulttonegotiateasmallroundabout.Therefore,theroundaboutdesignshouldaccountforadequatelateralclearanceandalargerradius.FloridahasalreadyadoptedNCHRP672,Roundabouts,AnInformationalGuidebuttotheextentpossible,thestateshouldconsidertheuseoflocallydevelopedparametersforvariousaspectsofdesignandoperationalanalysisofroundabouts.Otherstateshavedevelopedlocalparametersthatrelatetotheinfluenceofdriverbehaviorasitaffectscapacityandoperationalcharacteristicsofroundabouts.Differencesintheoperationswithinthefunctionalareashouldbehighlighted.TheguidanceneedstobeexplicitaboutthedefinitionofthefunctionalareaofaroundaboutespeciallyifitisdifferentfromtheonespecifiedinNCHRP672,Roundabouts,AnInformationalGuide.Inordertoestimateandexaminetheeffectsandoperationsofaroundabout,simulationandanalysissoftwareshouldbeavailable.Sofar,HCSandotherdeterministicsoftwaresuchasHCS,Synchro,SIDRA,RODELandARCADYcanconducttheplanning‐level,preliminarydesignanalysis,queuinganalysisandprovideinformationrelatedtoaccessmanagementandlocationofdriveways.SimulationsoftwaresuchasVISSIMcanbeusedforthetrafficnetworkanalysis,publicinvolvementandpedestriantreatmentsanalysis.Notallthesimulationprogramscanadequatelysimulaterealworldapplicationssotheplannersandengineeringshouldpayattentiontowhichsoftwaretheyuseandwhichparameterstheyconsiderintheanalysisofroundaboutsordrivewayplacementinthevicinityofroundabouts.Finally,thisresearchdidnotshowsignificantimpactsoftheroundaboutlocation,whetherinanurbanorsuburbancontext,ontrafficoperations.8.5AdditionalResearchNeedsTheresearchfindingsofthisprojectsuggestthatwhilesomeresearchhasbeencompletedonroundabouts,additionalresearchisneededonthecombinationofroundaboutsandaccessmanagementindifferent

Chapter8Conclusions


contextsandconditions.NCHRP672,Roundabouts,AnInformationalGuide,themainnationalguidebookonroundabouts,doesnotprovidedetailedguidanceonroundaboutsandaccessmanagement.TheFDOTshouldconductitsownresearchandworkwithAASHTOandotherpartnerstoensurethatguidance,includingroundaboutsasacomponentofaccessmanagement,isincorporatedintopractice.ThedevelopmentoflocalvariablesforparametersintheanalysistoolsforassessmentofroundaboutsisnecessarybecauseusingnationalguidanceorguidancefromotherstatesmaynotcapturethewayinwhichdriversinFloridarespondtodifferentformsofaccessmanagement.Theymayhavedifferentreactiontimesordrivecloserorfurtherfromotherdriversastheyenterintersectionsandroundabouts.Theroundaboutsguidanceinseveralstatesprovidesdocumentationofuseforlocally‐developedparametersforvariousaspectsofdesignandoperationalanalysis(e.g.,Californiaforaccelerationanddecelerationeffects;MichiganforSPFsandCMFs;Washingtonforcornerclearance,parallelroundabouts,U‐Turns,parkingandtransitstops;andWisconsinforlocationofdrivewaysandsitedistancebetweenusers).Thesefactorsmayinfluencethecalculationoftheentryflowrate,conflictingflowrateandexitflowrateofroundabouts.TotheextentthatFloridadriversbehavedifferentlythandriversinotherstates,FDOTshouldfundresearchtojustifytheuseofdifferentparametersforthesoftwareandotheranalyticaltoolsforplanning‐leveldesign,preliminarydesign,analysisofpedestriantreatments,andsystemsanalysis.Also,inordertoenhanceunderstandingoftheeffectsofroundaboutsontrafficconditions,safety,andtrafficnetworkoperations,thereisaneedtoconductnationalresearchonroundaboutsandaccessmanagementthatspecificallyfocusesonaccessmanagementformajorarterialsandothersimilarroadwaysfoundonthestatehighwaysystem.Throughoutthisresearchithasbecomeincreasinglyclearthat,whilemuchresearchhasbeenconductedaboutroundaboutsandaboutaccessmanagement,littleresearchhasbeenconductedonroundaboutsincombinationwithotherformsofaccessmanagementandroundaboutsasaformofaccessmanagement.Roundaboutscanbeseenasaformofaccessmanagementbecausetheycanaccommodateleft‐turnsandallowtheremovalofdirectionalleft‐turnlanes,yettheyfunctionasintersections.Thedifferencesintheirsafetyandoperationalcharacteristicsfromothertypesofaccessmanagementandotherintersectionsmeansthatsitedistances,stoppingdistances,functionalareacharacteristics,andintersectionanddrivewayspacingmaybedifferentforroundabouts.Furthermore,theuseofroundaboutsinavarietyoftransportationandlandusecontextsmaymeanthatthesefactorsdifferbycontext.Additionally,thereisalackofresearchonaccessmanagementandroundaboutsoraseriesofroundaboutsincorridors.NCHRP3‐100,whichevaluatestheuseofroundaboutsalongcorridors,isonprogressanditislikelytogivesomeinsightofthedifferencesbetweenroundaboutsandconventionalintersections.However,thisstudymaynothaveacompletesetofoperationaldatathatcanallowforamorecomprehensiveunderstandingofthesedifferences.Recently,thecityofSarasotaproposedaseriesofroundaboutsonUS41.Conductingabefore‐and‐afterstudytherewouldgiveabetterunderstandingoftheoperationalandsafetycharacteristicsofcorridorswithroundaboutsinsteadofconventionalintersections.Therefore,FDOThasauniqueopportunitytocompletearealdatastudyontheUS41corridorinSarasotawheretworoundaboutsareproposedinaportionofthedowntown.

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Prytyka,G.&Sullivan,F.(2012).Roundabouts,Florida’sImplementationStrategy.Tallahassee,Florida.Retrievedfromwww.dot.state.fl.us/structures/designExpo2012/Presentations/Roundabouts_Final‐Expo‐2012.pdf

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Retting,R.A.,Persaud,B.N.,Garder,P.E.,&Lord,D.(2001).CrashandinjuryreductionfollowinginstallationofroundaboutsintheUnitedStates.AmericanJournalofPublicHealth,91(4),628.

Richfield,V.,&Hourdos,J.(2013).EffectofSignsandStripingonRoundaboutSafety:AnObservationalBefore/AfterStudy.InTransportationResearchBoard92ndAnnualMeeting(No.13‐4568).

Robinson,B.W.,Rodegerdts,L.,Scarborough,W.,Kittelson,W.,Troutbeck,R.etal.(2000).Roundabouts:AnInformationalGuide.(FHWA‐RD‐00‐067).Retrievedfromhttp://www.fhwa.dot.gov/publications/research/safety/00067/00067.pdf

Rodegerdts,L.,Blogg,M.,Wemple,E.,Myers,E.,Kyte,M.,Dixon,M.,etal.(2007).RoundaboutsintheUnitedStates(Vol.572).TransportationResearchBoardNationalResearch.Retrievedfromhttp://onlinepubs.trb.org/onlinepubs/nchrp/nchrp_rpt_572.pdf

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AppendixAStatePolicies


AppendixA:RoundaboutsFeaturesandDimensionsKeyFeaturesofaModernRoundaboutSource:(Rodegerdtsetal.,2010,6)TableA.1.KeyFeaturesofaModernRoundabout

Feature DescriptionCentralisland Thecentralisland istheraisedareainthecenterofaroundaboutaround

whichtrafficcirculates.Splitterisland Asplitterislandisaraisedorpaintedareaonanapproachusedto

separateenteringfromexistingtraffic,deflectandslowenteringtraffic,andprovidestoragespaceforpedestrianscrossingtheroadintwostages.

Circulatoryroadway Thecirculatoryroadwayisthecurvedpathusedbyvehiclestotravelcounterclockwisearoundthecentralisland.

Apron Anapronisaraisedsectionofpavementaroundthecentralislandadjacenttothecirculatoryroadwaythatcanaccommodatethewheeltrackingoflargervehiclesonsmallerroundabouts.

Yieldline Ayieldlineisapavementmarkingthatdesignatesthepointofentryfromanapproachintothecirculatoryroadwayandisgenerallyplacedalongtheinscribedcircle.Enteringvehiclesmustyieldtoanycirculatingtrafficcomingfromtheleft,beforecrossingthislineintothecirculatoryroadway.

Accessiblepedestriancrossing

Accessiblepedestriancrossingsshouldbeprovidedatallroundabouts.Thecrossinglocationissetbackfromtheyieldline,andthesplitterislandiscuttoallowpedestrians,wheelchairs,strollers,andbicyclestopassthrough.

Bicycletreatments Bicycletreatmentsatroundaboutsprovidebicycliststheoptionoftravellingthroughtheroundabouteitherasavehicleorasapedestrian,dependingonthebicyclist’slevelofcomfort.

Landscapingbuffer Landscapingbuffersareprovidedatmostroundaboutstoseparatevehicularandpedestriantrafficandtoencouragepedestrianstocrossonlyatthedesignatedcrossinglocations.Landscapingbufferscanalsosignificantlyimprovetheaesthetics.

DimensionsSourcefrom:(Rodegerdtsetal.,2010,7)TableA.2.DimensionsofRoundabouts

Dimension DescriptionInscribedcirclediameter

Theinscribedcirclediameteristhebasicparameterusedtodefinethesizeofaroundabout.Itismeasuredbetweentheouteredgesofthecirculatoryroadway.

Circulatoryroadwaywidth

Thecirculatoryroadwaywidthdefinestheroadwaywidthforvehiclecirculationaroundthecentralisland.Itismeasuredasthewidthbetweentheouteredgeofthisroadwayandthecentralisland.Itdoesnotincludethewidthofanymountableapron,whichisdefinedtobepartofthecentralisland.

Approachwidth Theapproachwidthisthewidthoftheroadwayusedbyapproachingtrafficupstreamofanychangesinwidthassociatedwiththeroundabout.

AppendixAStatePolicies


Dimension DescriptionTheapproachwidthistypicallynomorethanhalfofthetotalwidthoftheroadway.

Departurewidth Thedeparturewidthisthewidthoftheroadwayusedbydepartingtrafficdownstreamofanychangesinwidthassociatedwiththeroundabout.Thedeparturewidthistypicallylessthanorequaltohalfthetotalwidthoftheroadway.

Entrywidth Theentrywidthdefinesthewidthoftheentrywhereitmeetstheinscribedcircle.Itmeasuresperpendicularlyfromtherightedgeoftheentrytotheintersectionpointoftheleftedgelineandtheinscribedcircle.

Exitwidth Theexitwidthdefinesthewidthoftheexitwhereitmeetstheinscribedcircle.Itismeasuredperpendicularlyfromtherightedgeoftheexittotheintersectionpointoftheleftlineandtheinscribedcircle.

Entryradius Theentryradiusistheminimumradiusofcurvatureoftheoutsidecurbattheentry.

Exitradius Theexitradiusistheminimumradiusofcurvatureoftheoutsidecurbattheexit.

AppendixBStatePolicies


AppendixB:StatePolicies

Thissectionsupportsthestates’reviewofroundaboutinformation,accessmanagement,anddrivewayspacingguidancewithadditionaldetailnotincludedinChaptersFourandFive.Thissectionisbrokendownbystate.

TableB.3.StateWebsitesandGuidanceonRoundaboutsandAccessManagement

State Roundabout AccessManagementAlabama searchengine:accessmanagementAlaska http://www.alaskaroundabouts.com/index.

htmlArizona http://www.azdot.gov/CCPartnerships/Rou

ndabouts/index.asphttp://www.azaccessmanagement.com/

California http://www.dot.ca.gov/hq/oppd/roundabt/Colorado http://www.coloradodot.info/projects/i70e

dwardsinterchange/area‐roundabout‐history.html

Connecticut http://www.ct.gov/dot/cwp/view.asp?a=4109&q=467780&PM=1

Delaware http://deldot.gov/information/community_programs_and_services/roundabouts/index.shtml

Florida SearchEngine:roundabout(containsmuchinformationaboutroundabouts)

http://www.dot.state.fl.us/planning/systems/sm/accman/

Georgia http://www.dot.ga.gov/travelingingeorgia/trafficcontrol/roundabouts/Pages/default.aspx

http://www.dot.ga.gov/doingbusiness/permits/Pages/AccessManagement.aspx

Indiana http://www.in.gov/indot/2512.htmIowa http://www.iowadot.gov/roundabouts/roun

dabouts.htmhttp://www.iowadot.gov/traffic/access/index.html

Kansas http://www.ksdot.org/burTrafficEng/Roundabouts/roundabout.asp

http://www.ksdot.org/accessmanagement/

Kentucky http://transportation.ky.gov/congestion‐toolbox/pages/roundabouts.aspx

SearchEngine:accessmanagement

Louisiana http://www.dotd.la.gov/administration/public_info/projects/roundabouts/

SearchEngine:accessmanagement(Brochure)

Maine http://www.maine.gov/mdot/ppp/accessmgmt/index.htm

Maryland http://www.marylandroads.com/Pages/Roundabouts.aspx

http://roads.maryland.gov/index.aspx?pageid=320&d=95

Michigan http://michigan.gov/mdot/0,1607,7‐151‐9615_53039‐‐‐,00.html

http://www.michigan.gov/mdot/0,1607,7‐151‐9621_11041_29705‐‐‐,00.html

Minnesota http://www.dot.state.mn.us/roundabouts/ http://www.dot.state.mn.us/accessmanagement/

Mississippi SearchEngine:accessmanagementMissouri PerLocalDistrict:KansasCity,Northeast, http://www.modot.org/safety/AccessMan



State Roundabout AccessManagementSouthwest agement.htm

Montana http://www.mdt.mt.gov/travinfo/roundabouts/about.shtml

http://www.mdt.mt.gov/research/toolkit/m1/pptools/ds/am.shtml

Nevada http://www.nevadadot.com/Traveler_Info/Safety/Roundabouts.aspx

http://www.nevadadot.com/Content.aspx?id=6274&terms=access%20management

NewJersey SearchEngine:accessmanagementNewYork https://www.dot.ny.gov/main/roundabouts

/backgroundOhio SearchEngine:roundabout http://www.dot.state.oh.us/districts/D01

/PlanningPrograms/trafficstudies/Pages/Access‐Management.aspx

Oregon http://www.oregon.gov/ODOT/hwy/engservices/Pages/roundabout_home.aspx

http://www.oregon.gov/ODOT/HWY/ACCESSMGT/Pages/index.aspx

Pennsylvania http://www.dot.state.pa.us/Internet/web.nsf/Secondary?openframeset&frame=main&src=RoundaboutContactInfo?readform

SearchEngine:accessmanagement

RhodeIsland http://www.dot.ri.gov/engineering/trafficdesign/roundabouts.asp

SouthDakota http://www.sddot.com/transportation/highways/management/Default.aspx

Vermont http://vtransplanning.vermont.gov/vam

Virginia http://www.virginiadot.org/info/faq‐roundabouts.asp

http://www.virginiadot.org/info/access_management_regulations_and_standards.asp

Washington http://www.wsdot.wa.gov/Safety/roundabouts/default.htm

Wisconsin http://www.dot.wisconsin.gov/safety/motorist/roaddesign/roundabouts/index.htm



TableB.4.RoundaboutGuidelinesinDrivewayorHighwayManuals

No Date State DocumentTitle Description1 2000 Florida ManualonUniform

TrafficStudies,Chapter16‐Roundabouts

WrittenbyFDOTandpublishedin2000,this16‐pagereportisthelastchapterintheFDOTManualonUniformTrafficStudies(MUTS).TheMUTSestablishesminimumstandardsforconductingtraffic‐engineeringstudiesonroadsnearthejurisdictionoftheFDOT.ThischapteronroundaboutsjustifiestheiruseintheStateofFlorida,andcomparesthemtothreeotheralternativestointersectioncontrol–trafficsignals,two‐waystopcontrol(TWSC),andall‐waystopcontrol(AWSC).Thischaptercitesthe1996FDOTFloridaRoundaboutGuideforspecificguidelinesonroundaboutlocation,design,andoperation.

2 2007 NewHampshire

NHDOTSupplementalDesignCriteria

WrittenbyNHDOT,the5‐page supplementaldesigncriteriamentionstheconsiderationsforroundaboutdesign,includingoperation(withattachedcapacityworksheet,andRODELsetting),andgeometricdesign.Designvehiclereceivesadditionalattentioninthisdocument.FHWARoundabouts,AnInformationalGuide(Robinsonetal.2000).

3 2009 Iowa DesignManualChapter6GeometricDesign,6A‐3ModernRoundabout

WrittenbyIowaDOT,Chapter6oftheGeometricDesignmanualincludesa16‐pagesectiononmodernroundaboutsforIowa.Thechapteroutlineshowroundaboutsareclassifiedincomparisonwithothertrafficintersections,keyfeaturesandgeometricelementsofroundabouts,roundaboutoperationsanddesign,inadditiontosectionsonroundabouteducationandsafety.Asignificantportionofthechapteraddressesconsiderationsandfeasibilityofroundaboutimplementation,takingintoaccountregionalcontext,accessmanagementissues,andsafetyfactors.

3 2009 Minnesota MnDOTRoadDesignManual:Chapter12,DesignGuidelinesforModernRoundabouts

WrittenbyMinnesotaDOT,thisdesignguidelinedocumentshowsanenhancementtableoftypicalinscribedcirclediameterwithdailyservicevolume,intersectioncontrolevaluationandsiterequirementsections,andspecialdesignstoaccommodatespecificlanduses.Additionally,thisdocumentsuggestsRODELandARCADYastoolstoexamineintersectioncontrolevaluations.

4 2011 Maryland MarylandDesignGuidelines:Chapter3C:RoundaboutMarkings

WrittenbytheMarylandStateHighwayAdministration,this16‐pagechapterincludesdesignguidelinesforpavementmarkingsinroundaboutsinMaryland.Itincludesmarkingsforone‐,two‐,andthree‐laneroundabouts,aswellasforcrosswalk,pedestrian,andbicyclistmarkingsthroughroundabouts.

5 2011 Washington DesignManual22.01.08:Chapter1320‐Roundabouts

WrittenbyWashingtonStateDOT,the50‐pagesectiongivesinformationabouttheprocedurestodesignaroundaboutataspecificstatewidelevel.Thisdocumentexplainsmultipleaccesscirculationinsection1320.11includingaccess,parkingandtransitfacilities.Informationaboutaccess:“Noroadapproachconnectionstothecirculatingroadwayareallowedatroundaboutsunlesstheyaredesignedaslegstotheroundabout.Itisdesirablethatroadapproachesnotbelocatedontheapproachordeparturelegswithinthelengthofthesplitterisland.”(WSDOT,2011,pp.1320‐21).Fordriveways,“iftheparceladjoinstwolegsoftheroundabout,it



isacceptabletoprovidearight‐in/right‐outdrivewaywithinthelengthofthesplitterislandsonbothlegs.Thisprovidesforallmovements;designbothdrivewaystoaccommodatetheirdesignvehicle.”(WSDOT,2011,pp.1320‐21)

6 2010 Kentucky DesignGuidanceforRoundaboutIntersections

WrittenbytheDirectoroftheDivisionofHighwayDesignintheKentuckyDOT,this29‐pagereportgivesspecificexplanationsofhowKentuckymayreviewandapproveroundaboutdesigns.Thedocumentalsoincludesguidesforwarrantandoperationalanalysis.Thisoperationalanalysisincludestherelationtocapacityaspectintheroundabout.

7 2011 Wisconsin FacilitiesDevelopmentManual,Chapter11,Section26:Roundabouts

WrittenbytheWisconsinDOTin2011,the79‐pagesectionshowsthecompletedesignprocessofaroundaboutandothersupplementalaspects.Thefirstsupplementisontheguidanceofshared‐usepathsforbicyclists.Inregardtoaccessmanagement,thisguidelineconsidersthreeaspectstolocateadrivewayontheroundaboutentryorexit:volumeofdriveways,operationalimpact,andsightdistancebetweenusers.

TableB.5.SpecificManualsonRoundaboutGuidance

No Date State DocumentTitle Description1 1996 Florida Florida

RoundaboutGuide

WrittenbytheFDOTandpublishedin1996,the109‐pagereportoutlinesroundaboutdesignandguidanceinFlorida.ThisdocumentwaspreparedearlierthanFHWA'sRoundabouts,AnInformationalGuide(Robinsonetal.,2000).ThemainwaythisdiffersfromtheFHWAdocumentisthejustificationofwhytobuildaroundabout.AnothersupplementalaspectistheexplanationaboutSIDRAsoftwareutilization.Inaddition,thisdocumentalsoconsidersothersoftware,suchasARCADY,andRODEL.Thisdocumentincludestheformstodeterminecapacityandotherrequireddocumentsforroundaboutjustification.

2 2000 Florida BicycleandPedestrianConsiderationsatRoundabouts

WrittenbyFDOTandpublishedin2000,thisreportexaminesspecificconcernsaboutbicyclistsandpedestriansattheroundabouts.Theresultsofthisstudyarethathighbicyclecrashratesthanthoseoncarandpedestrian,themultilaneroundaboutsprovidealesssafeenvironmentforbicyclistsandpedestriansthanone‐laneroundabouts.Recommendationsincludebuildinganadditionalbicyclefacilityoutsidetheroundabout(ifspaceisavailable),crossingprovisions,andpropersignage.

3 2012 Florida Roundabouts,Florida’sImplementationStrategy

WrittenbytheDesignTrainingExpoandpublishedin2012,thisPowerPointpresentationcapturessupplementalaspectsfromFHWA'sRoundabouts,AnInformationalGuide(Robinsonetal.,2000),especiallyinregardtopedestrians,trucks,andmarkinginformation.

4 2003 Arizona Roundabouts:AnArizonaCaseStudyandDesignGuidelines

WrittenbyLeeEngineeringandKittelson&Associates,the260‐pagereportisacasestudyofroundaboutsinArizona.(Leeetal.,2003).

5 2003 Kansas KansasRoundaboutGuide:ASupplementtoFHWA’s

WrittenbytheKansasDOT,Kittelson&Associates,andTransystemCorporationin2003,theKansasRoundaboutGuideisa176‐pagereportthatshowssupplementalaspects,suchasdifferentiatingtrafficcirclesandroundaboutswith



Roundabouts examplesfromKansasroundabouts.Italsospecifiestheroundaboutselectionguidance;addingtheroundaboutcategoriesonadesigncharacteristictable(whetherurbanandruralroundaboutsaresingleordoublelane),detailingindesignprocess,givingexamplesoffiveprojectsinKansasforcurbandpavementdesign,detailingthedrawingofsignageonurban,suburban,multilane,andshowingtheluminanceforintersectionbasedonpavementclassification(thePortlandcementconcretesurfaceandtypicalasphaltsurface),androadwayclassification.

6 2007 Pennsylvania GuidetoRoundabouts

WrittenbyPennsylvaniaDOT,the236‐pagereportsupplementsthepedestrianaspectofFHWA'sRoundabouts,AnInformationalGuide(Robinsonetal.,2000),byshowingdetailedrequirementsfordetectablewarningsurfacesandotherpedestrianfeatures.

7 2008 Iowa Planning‐LevelGuidanceforModernRoundabouts

WrittenbyHallmarketal.,this32‐pagereportprovidestheIowaDOTwithinformationanddirectiononroundaboutpolicies,designguidelines,andpubliceducation.Theprojectdevelopsaroundabouttaskforce,documentsbestpracticesofstateswithsuccessfulroundaboutprograms,developsimplementationguidelines,developsdraftroundaboutpolicies,andassistsinpubliceducationaboutroundabouts.

8 2011 Michigan EvaluatingthePerformanceandSafetyEffectivenessofRoundabouts

WrittenbytheMichiganDOT,thisreportstudiessafetyperformanceonroundabouts.Usingthesimplebefore‐afterandEmpiricalBayesanalysiswithasamplesizeof58roundaboutsinMichigan,thisresearchfindsthat“Singlelanehas60.55crashesperyearreduction;Doublelane;18.56crashesperyearreduction;Triplelane;94.76crashesperyearincrease;andFatal&A‐Level;5.39crashesperyearreduction”(MDOT,2011,pp.81or7‐1).ThisresearchalsosuggestsadditionalaspectsofroundaboutstobeconsideredinthenextMichiganStateRoundaboutGuide.Onesuggestionaboutroundaboutsthathascorrelationtoaccessmanagementwouldbeto“considerrestrictingleftturnsintoandoutofdrivewaysnearroundabouts.Thiswouldreducethenumberofconflictpointsandallowvehiclestoutilizetheroundabouttomakeanindirectleftturn.”(Bagdadeetal.,2011,pp.86).

9 2012 Maryland RoundaboutDesignGuidelines

WrittenbytheMarylandStateHighwayAdministration,this32‐pagereportincludesguidelinesforroundaboutdesignandoperations.

10 2007 California RoundaboutGeometricDesignGuidance

WrittenbytheCaltrans DivisionofResearchandInnovation,this113‐pagedocumentincludesthreemaintopics:operation,roundaboutfordifferentusers,andgeometricdesign.



TableB.6.StateGuidanceonAccessManagementManuals

No. States NameofDocuments Year RetrievedFrom Pages

1 Alabama AccessManagementManual January,2013

http://www.dot.state.al.us/maweb/doc/ALDOT%20Access%20Management%20Manual.pdf

65

2 Arizona RoadwayDesignGuidelines May,2012 http://www.azdot.gov/highways/Roadway_Engineering/Roadway_Design/Guidelines/Manuals/PDF/RoadwayDesignGuidelines.pdf

412

3 California HighwayDesignManual May7,2012 http://www.dot.ca.gov/hq/oppd/hdm/hdmtoc.htm

(web)

4 Colorado StateHighwayAccessCode 1998(revisedMarch2002)

http://www.coloradodot.info/business/permits/accesspermits/references/601_1_accesscode_march2002_.pdf/view

70

5 Connecticut HighwayDesignManual 2003(revisedFebruary.2013)

http://www.ct.gov/dot/lib/dot/documents/dpublications/highway/cover.zip

630

6 Delaware StandardsandRegulationsforSubdivisionStreetsandStateHighwayAccess

2011 http://regulations.delaware.gov/register/june2011/proposed/14%20DE%20Reg%201323%2006‐01‐11.pdf

136

7 DistrictofColumbia(Washington,DC)

ThePolicyandprocessforAccesstotheDistrictofColombiaInterstateandFreewaySystem

2010 http://ddot.dc.gov/DC/DDOT/Projects+and+Planning/Standards+and+Guidelines/Interstate+and+Freeway+Access+Process/Policy+and+Process+for+Access+to+the+DC+Interstate+and+Freeway+System

(web)

8 Florida StateHighwaySystemAccessManagement

2009 https://www.flrules.org/gateway/ChapterHome.asp?Chapter=14‐97 (web)

9 Georgia RegulationforDrivewayandEncroachmentControl

2009 http://www.dot.ga.gov/doingbusiness/PoliciesManuals/roads/Encroachment/DrivewayFull.pdf

101

10 Idaho AccessManagement:StandardsandProceduresforHighwayRight‐of‐WayEncroachments

April,2001 http://itd.idaho.gov/highways/ops/Traffic/PUBLIC%20FOLDER/Access/Idaho%20AM%20Standards%20and%20Procedures.pdf

93

11 Illinois Chapter35‐AccessControl/AccessManagement

September2010

http://dot.state.il.us/desenv/BDE%20Manual/BDE/pdf/Chapter%2035%20Access%20Control‐Access%20Management.pdf

52

12 Indiana AccessManagementGuide 2009 http://www.in.gov/indot/files/guide_total.pdf

178

13 Iowa IowaPrimaryHighwayAccessManagementPolicy

2012 http://www.iowadot.gov/traffic/pdfs/AccessPolicy.pdf

47




14 Kansas AccessManagementPolicy January,2013

http://www.ksdot.org/accessmanagement/Access_Management_Policy_Jan2013.pdf

300

15 Louisiana AccessConnectionPolicy November,2012

http://www.dotd.la.gov/highways/maintenance/maintmgt/documents/AC_Policy_Manual.pdf

81

16 Maine AccessManagementRules March18,2005

http://www.maine.gov/mdot/ppp/accessmgmt/amrules.htm

(web)

17 Maryland StateHighwayAccessManual

2004 http://roads.maryland.gov/ohd/accesspermits.pdf

232

18 Massachusetts HighwayDesignChapter15AccessManagement

2006 http://www.massdot.state.ma.us/Portals/8/docs/designGuide/CH_15_a.pdf

25

19 Michigan AccessManagementGuidebook

October1,2001

http://www.accessmanagement.info/pdf/GuidebookMI.pdf

164

20 Minnesota AccessManagementManual 2008 http://www.dot.state.mn.us/accessmanagement/resources.html

(web)

21 Mississippi AccessManagementManual 2012 http://sp.mdot.ms.gov/RoadwayDesign/Documents/MISSISSIPPI%20Access%20Management%20Guide_v2_Feb2012.pdf

36

22 Missouri AccessManagementGuidelines

2003 http://www.modot.org/newsandinfo/documents/AccessMgmtGuidelines_1003.pdf

51

23 Montana Chapter8‐AccessManagement

March,2007 http://www.mdt.mt.gov/other/rw/external/manual/chapter_8.pdf

21

24 Nebraska AccessControlPolicytotheStateHighwaySystem

March1,2006

http://www.transportation.nebraska.gov/roway/pdfs/accesscontrol.pdf

24

25 Nevada AccessManagementSystemandStandards

1999 http://www.nevadadot.com/uploadedFiles/TrafEng_AccesMgtSysStandards.pdf

38

26 NewHampshire

DrivewayPermit March10,2000

http://www.nh.gov/dot/org/operations/highwaymaintenance/documents/DrivewayPolicy.pdf

43

27 NewJersey StateHighwayAccessManagementCode

2013 http://www.state.nj.us/transportation/business/accessmgt/NJHAMC/

89

28 NewMexico StateHighwayAccessManagementRequirements

October15,2001

http://dot.state.nm.us/content/dam/nmdot/Infrastructure/Access_management_Manual.pdf

197

29 NewYork HighwayDesignManualChapter6‐Interchanges;

July16,2002

https://www.dot.ny.gov/divisions/engineering/design/dqab/hdm/hdm‐repository/chapt_06_new_07162002.pdf

18




30 NorthCarolina PolicyonStreetandDrivewayAccesstoNorthCarolinaHighways

July,2003 https://connect.ncdot.gov/resources/safety/Congestion%20Mngmt%20and%20Signing/Congestion%20Management/Policy%20on%20Street%20and%20Driveway%20Access%20to%20North%20Carolina%20Highways%20Current%20Edition%20July%202003.pdf

90

31 NorthDakota DesignManual‐DrivewaysandAccessManagement

July8,2009 http://www.dot.nd.gov/manuals/design/designmanual/DM‐TOC‐Master_tag.pdf

3

32 Ohio StateHighwayAccessManagementManual

2001 http://www.dot.state.oh.us/Divisions/Engineering/Roadway/AccessManagement/Documents/State%20Highway%20Access%20Management%20Manual%20March%202008.pdf

66

33 Oregon HighwayApproachPermitting,AccessControl,andAccessManagementStandards

June29,2012

http://www.oregon.gov/ODOT/HWY/ACCESSMGT/docs/pdf/734‐051_Perm_Rule.pdf

91

34 SouthCarolina ARMS—AccessandRoadsideManagementStandards

2008(latestrevisiononSept26,2012)

http://www.scdot.org/doing/technicalpdfs/publicationsmanuals/trafficengineering/arms_2008.pdf

130

35 SouthDakota Chapter17—AccessManagement

http://sddot.com/business/design/docs/rd/rdmch17.pdf

22

36 Texas AccessManagementManual July,2011 http://onlinemanuals.txdot.gov/txdotmanuals/acm/acm.pdf

46

37 Utah AccommodationofUtilitiesandtheControlandProtectionofStateHighwayRightofWay

January,2006

http://www.udot.utah.gov/main/uconowner.gf?n=6599114996078154

100

38 Vermont AccessManagementProgramGuidelines

July1,1999(LastRevision:July22,2005)

http://vtransengineering.vermont.gov/sites/aot_program_development/files/documents/rightofway/UandPAccManProgGuidelinesRev072205.pdf

33

39 Virginia AccessManagementDesignStandardsforPrincipalArterial/MinorArterials,Collectors,andlocalstreets/EntrancesandIntersection

2012/2012/2007

http://www.virginiadot.org/info/access_management_regulations_and_standards.asp

18/19/116

40 Washington AccessControl June,2009 http://www.wsdot.wa.gov/publications/manuals/fulltext/m22‐01/520.pdf

8

41 WestVirginia ManualonRulesandRegulationsforConstructingDrivewaysonStateHighwayRightsofway

May,2004 http://www.transportation.wv.gov/highways/traffic/Documents/DrivewayManual.pdf

94




42 Wisconsin AccessControl—FacilitiesDevelopmentManual

June19,2013

http://roadwaystandards.dot.wi.gov/standards/fdm/07‐00toc.pdf

(web)

43 Wyoming RulesandRegulations andpolicyforAccessestoWyomingStateHighways

March,2005 http://www.dot.state.wy.us/files/live/sites/wydot/files/shared/Traffic/WYDOT%20Access%20Manual.pdf

48



TableB.7.OtherDocumentsRelatedtoAccessManagement


1 Idaho AccessManagementToolkit August18,2008

http://www.compassidaho.org/documents/planning/studies/AcMgtTlkt_08Cover_Electronic.pdf

94

2 Oregon AccessManagementManual(web‐based)

Various(1996to2004)

http://www.oregon.gov/ODOT/HWY/ACCESSMGT/Pages/accessmanagementmanual.aspx

(web)

3 Michigan MichiganAccessManagementProgramEvaluation

May,2010 http://www.michigan.gov/documents/mdot/Final_MDOT_Access_Management_Evaluation_Report_by_TTI_May_2010_324062_7.pdf

112

4 NewYork ProjectDevelopmentManualAppendix8:InterstateandOtherFreewayAccessControlandModification

January7,2002

https://www.dot.ny.gov/divisions/engineering/design/dqab/dqab‐repository/pdmapp8.pdf

19

5 Kentucky AccessManagementforKentucky(Stamatiadisetal.,2004)

February,2004

http://transportation.ky.gov/Congestion‐Toolbox/Documents/KTC%20Access%20Management%20Report.pdf

170

6 AccessManagementImplementationinKentuckyTechnicalSupportDocumentandStatusReport

May,2008 http://transportation.ky.gov/Congestion‐Toolbox/Documents/Access%20Management%20Implementation%20Report%202008.pdf

111

7 Utah AssessingtheSafetyBenefitsofAccessManagementTechniques

May,2006 http://www.udot.utah.gov/main/uconowner.gf?n=7861430698992951

150

8 SouthCarolina

SouthCarolinaStrategicCorridorSystemPlan

http://www.scdot.org/inside/pdfs/planning/strategiccorridorplan.pdf

126

9 SouthDakota

ReviewofSDDPT'sHighwayAccessControlProcess

February,2000

http://sddot.com/business/research/projects/docs/SD1999_01_Final_Report.pdf

214

10 Washington RightofWayManual March,2013 http://www.wsdot.wa.gov/publications/manuals/fulltext/M26‐01/M26‐01.10Revision.pdf

62

Date State DocumentTitle Description2006 Florida MedianHandbook

TheFDOTMedianHandbookisan81‐pagereportthatborrowed“heavily”fromtheAccessManagementManual,publishedbytheTransportationResearchBoard;aswellasTransportationandLandDevelopment(VergilStover)publishedbyITE.Whilethehandbookaddressesseveraldesignconsiderationsrelatedtoroundabouts,itdoesnotexplicitlydetailanythingaboutroundaboutdesignoraccessmanagement.

2008 Florida DrivewayInformationGuide

TheFDOTDrivewayInformationGuideisa94‐pagereportthataddressesseveraldesignguidelinesfordrivewaydesigninFlorida,suchassightdistanceatdriveways,drivewaylocation,andpedestrianfactors,butdoesnotmakeanyreferencetoroundaboutsatall.

AppendixCAccessManagementTechniquesinStateGuidelines


AppendixC:AccessManagementTechniquesinStateGuidelinesTableC.8.SpacingRequirements

(Source:GluckandLorenz,2010,pp.47)



TableC.9.AccessManagementElementsontheStates(GluckandLorenz,2010,page48)



TableC.10.AccessManagementTechniquesappliedbytheStateDOTs(GluckandLorenz,2010,pages49‐50)

AppendixDSiteSelection


AppendixD:SiteSelectionThefigurebelowshowsthedatacollectionoftheClearwaterroundabout,whichhasbeendebatedforyears.Fourcameraswereplacedonfouroutofthesixlegsofthisroundabouttorecordtrafficinteractionbetweendrivewaysandapproachinglanes.Thisroundaboutislocatedclosetoatouristattractionarea;thereforetrafficwassignificantatthetimeofdatacollection.Pinellas Causeway Blvd and Mandalay Ave 3/22: 3pm-5:30pm Problems: Huge traffic, lots of spill backs into circulating lanes.

FigureD.1.CameraLocationofRoundaboutatCausewayBlvdandMandalayAve



ThefigurebelowshowsaroundaboutinGainesville,Florida.Thissiteisanidealintersectionforresearcherstoobserveconflictbetweentrafficonapproaching/exitlanesanddrivewayssincethedistancebetweendrivewaysandtheroundaboutisveryclose.Alachua SW 2nd Ave and SW 6th St. 4/5: 3pm-5:30pm Problems: Driveway is too close to the roundabout

FigureD.2.CameraLocationofRoundaboutatSW2ndAveandSW6thSt.



ThefigurebelowshowsaroundaboutsiteinOsceolaCounty.Althoughthereisadrivewayclosetotheroundabout,wedidn’tobservesignificantconflictatthesite.Osceola MLK Blvd. and N. Central Ave. 4/5: 11am-12pm Problems: NA

FigureD.3.CameraLocationofRoundaboutatMLKBlvd.andN.CentralAve.



ThefollowingfigureshowsaroundaboutinOrangeCounty,Florida.Thissiteisclosetoashoppingmallsowepickedaweekendtoconductdatacollection.Orange Eagle’s Reserve Blvd and Dyer Blvd 4/14: 12pm-1pm Problems: Design is abnormal

FigureD.4.CameraLocationofRoundaboutatEagle’sReserveBlvdandDyerBlvd



ThefollowingfigureshowsaroundaboutsiteinJacksonville,Florida.ItislocatedintheCBDareaandabusinesscenterwassituatedrightnexttotheroundabout.Trafficattractedandgeneratedbythebusinesscentercausedsignificantimpactonroundaboutoperation.Duval Independent Dr. and S. Laura St. 4/23: 11am-2pm Problems: Huge pedestrian flow, business center right next to roundabout.

FigureD.5.CameraLocationofRoundaboutatIndependentDr.andS.LauraSt.



ThefigurebelowshowsaroundaboutinSt.Lucie,Florida.Althoughseveraldrivewaysarelocatedneartheroundabout,wedidn’tobservedmanyconflictsatthissite.St. Lucie CR-707 and Ave A 5/9: 1pm-3pm Problems: Driveway too close to roundabout

FigureD.6.CameraLocationofRoundaboutatCR‐707andAveA



ThefigurebelowshowsaroundaboutinSt.Johns,Florida.Ashoppingcenterwaslocatedneartheroundabout.St. Johns CR-210 and Mickler Rd. 5/9: 1pm-3pm Problems: NA

FigureD.7.CameraLocationofRoundaboutatCR‐210andMicklerRd.



ThenextsiteisaroundaboutinHomestead,Florida.Aswecanseefromthefigurebelow,thereisanAWSCintersectionnorthoftheroundabout.Miami-Dade NE 10th Ct. & SW 152nd Ave. 5/13: 5pm-7:20pm Problems: You cannot see queue in the driveway from camera 2 due to the high hedges along the roadway.

FigureD.8.CameraLocationofRoundaboutatNE10thCt.andSW152ndAve.



ThenexttworoundaboutsformaseriesofroundaboutsinMiami,Florida.Oneofthefeaturesofthesetworoundaboutsisonstreetparkingisevidentinthesesites.Miami-Dade Greenway Dr. and Sagovia St. 5/14: 4:50pm-7:10pm Problems: Long queue build-up on Coral Way westbound on easternmost leg.

FigureD.9.CameraLocationofRoundaboutatGreenwayDr.andSegoviaSt.&CoralWay



Miami-Dade Biltmore Way and Sagonia St. 5/15: 4:50pm-7:15pm Problems: Easternmost leg had heavy traffic traveling east with some spill back into the roundabout.

FigureD.10.CameraLocationofRoundaboutatBiltmoreWayandSagoviaSt.



ThemapbelowshowsaroundaboutinBrowardCounty,Florida.Thisareaismostlyresidentialwithsomefactionofmixed‐usedparcel.Broward Holmberg Rd. & Parkside Dr. 5/16: 3:25pm-5:30pm Problems: NA

FigureD.11.CameraLocationofRoundaboutatHolmbergRd.andParksideDr.



ThemapbelowshowsaroundaboutsiteinMiami,Florida.Thissiteisinterestingbecausethedesignisabnormalcomparedtootherroundaboutsinourlist,andyettheaccessissuestillpredominatesatthissite.Miami-Dade Ponce De Leon Blvd and Ruiz Ave 5/21: 4:50pm-7:05pm Problems: For camera 3 we could not place the camera in the median due to the median being covered with bushes and trees. We had to place it across the street. There is some difficulty seeing the access point because of the cars crossing through our line of site.

FigureD.12.CameraLocationofRoundaboutatPonceDeLeonBlvd.andRuizAve.



ThemapbelowshowsaroundaboutinBrowardCounty,Florida.Thissitewasonthetopofourlistsincealotofaccesspointswerefoundateachlane.Broward Margate Blvd and NW 58th Ave 5/23: 7:40am-9:40am Problems: NA

FigureD.13.CameraLocationofRoundaboutatMargateBlvd.andNW58thSt.