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MOBILE COUNTY COMMISSIONDESIGN GUIDELINES

forImproving

Existing County MaintainedUnpaved Local Roads

2012

FORWARD

The Mobile County Commission is charged, by law, with authority in relation to theestablishment, change or discontinuance of public roads in Mobile County, Code of Alabama,Section 11-3-10. The Commission has general superintendence of public roads so as torender travel over such roads as safe and convenient as practicable, Code of Alabama,Section 23-1 -80. The Commission is vested with the authority to establish, promulgate andenforce rules and maintenance of its public roads, Code of Alabama, Section 23-1 -80.

The Commission has found that existing design policies do not recognize or adequatelyaddress the uniqueness of the unpaved road system maintained by the Mobile CountyCommission. Existing policies often require design elements not suited for conditions found inMobile County and often advocate expenditure of funds for design elements that neitherimprove the efficiency nor enhance the safety of the types of unpaved roads found in MobileCounty. Therefore, the Mobile County Commission has found it to be in the public’s bestinterest to promulgate a guide on roadway design for paving existing unpaved local roadswithin Mobile County.

Pursuant to the direction of the Mobile County Commission, the Engineering Department hasprepared a guide on roadway design for the purpose of paving existing unpaved roads. Theintent of this guide is to set the general roadway design elements for roadway constructiondirected by the Mobile County Commission.

The design elements contained within this guide come from various engineering sources,technical publications, research studies, applied engineering judgment and current publicacceptance within Mobile County. The intent of this guide is to create a safe roadway systemat a reasonable cost, to give the citizens of Mobile County the best benefit/cost ratio possible,and to serve all the areas within the county with the ultimate goal of paving all existingroadways in Mobile County. Aesthetic, historic, and cultural resource impacts of the roadwaydesign are to be principal considerations within this guide.

This guide does not apply to the general maintenance of roads, but does apply to the paving ofexisting county maintained unpaved roads.

This guide does not constitute a fixed standard, rule or regulation. As with any guide, it ismeant as a guide and is not intended to be a substitute for sound engineering judgment. Thisguide recognizes that deviations from the terms of this guide may be necessary and warrantedin the exercise of sound engineering judgment.

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This guide will go into effect upon commission approval and will be applied to any project thatis initiated after this date for the paving of an existing county maintained unpaved road in theunincorporated section of Mobile County. This guide will also be utilized for design of unpavedroadways accepted in Mobile County’s Dirt Road Program (DRP). This guide will remain ineffect pursuant to the direction of the Mobile County Commission and can only be altered,changed, or amended by a majority vote of the commission.

Approved by the Mobile County Commission:

On this 22. — day of

__________

2012

Commisserudgood

Commiss oner Connie Hudson

Commissioner Mike Dean

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Mobile County Commission Design Guidelinesfor

Paving Existing County MaintainedUnpaved Local Roads

Table of Contents

Forward

Chapter 1 11

General Terms! Elements for Design

1.1 ClearZone1 .2 Context Sensitive Solutions1.3 Drainage1 .4 Erosion Control Requirements1 .5 Functional Classification1.6 LateralOffset1.7 Roadway1 .8 Temporary Traffic Control1.9 Traffic Calming1.10 Traveled Way1.11 Typical Traveled Way Cross Section1.12 Vertical Clearance

Chapter 2 17

2. Elements of Design for Major Local Roads

2.1 Roadway Design2.2 Stopping Sight Distance2.3 Intersection Sight Distance2.4 Horizontal Alignment2.5 Vertical Alignment2.6 Cross Slope2.7 Drainage

5

Chapter325

3.ElementsofDesignforMinorLocalRoadsandResidentialRoads

3.1RoadwayDesign3.2StoppingSightDistance3.3IntersectionSightDistance3.4HorizontalAlignment3.5VerticalAlignment3.6CrossSlope3.7Drainage

Chapter433

4.ElementsofDesignforAccessRoads

4.1RoadwayDesign4.2StoppingSightDistance4.3IntersectionSightDistance4.4HorizontalAlignment4.5VerticalAlignment4.6CrossSlope4.7Drainage

Chapter541

5.ElementsofDesignforAccessWays

5.1RoadwayDesign5.2StoppingSightDistance5.3IntersectionSightDistance5.4HorizontalAlignment,VerticalAlignmentandCrossSlope5.5Drainage

6

Chapter 6. 47

6. Functional Classification

6.1 Arterial and Collector Road General Characteristics6.2 Local Road General Characteristics

6.21 Major Local Road6.22 Minor Local Road6.23 Residential Road6.24 Access Road6.25 Access Way

6.3 Reevaluation of the System

Chapter 7 59

7. Design Waiver

References 61

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CHAPTER 1

1. General Terms I Elements for Design

The following information will be included in the design for paving existing county maintainedunpaved local roads.

1.1 ClearZone

The clear zone is the traversable area provided beyond the edge of the traveledway for the recovery of errant vehicles. Any vertical obstructions in this areashall be breakaway devices or shall be shielded. For traveled ways with designspeed greater than 30 mph the clear zone shall be the same as the shoulderwidths as shown in this guide. For traveled ways with design speed 30 mph,clear zone criteria shall not apply.

1.2 Context Sensitive Solutions

Context sensitive solutions (CSS) are theoretical and practical approaches totransportation decision-making and design that takes into consideration thecommunities and lands which streets and roads pass through (“the context”).These streets and roads should fit the physical setting and preserve scenic,aesthetic, historic, natural, and environmental resources, while maintaining safetyand mobility (being “sensitive” to location). Projects involve efficient and effectiveuse of resources (time, budget, community) with minimal disruption to thecommunity. All stakeholders involved in a project should communicate andcollaborate to achieve a design that meets most expectations.

1.3 Drainage

All drainage design shall conform to the Mobile County Flood DamagePrevention Ordinance. The purpose of this ordinance is to promote public health,safety, and general welfare, and to minimize public and private losses due toflood conditions.

1.4 Erosion Control Requirements

All projects shall include the current requirements of NPDES. Reference thecurrent edition of the Alabama Handbook for Erosion Control, Sediment Control,and Stormwater Management on Construction Sites and Urban Areas. Alldevices shall be clearly defined and quantified, including all applicable directions,notes and associated references to perform the construction.

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1.5FunctionalClassification

Functionalclassificationisthegroupingofstreets,roads,andwaysbythecharacterofservicetheyprovide.Thefunctionalclassificationofeachroadshallbenotedontheplans.

1.6LateralOffset

Lateraloffsetistheareaadjacenttothetraveledwayclearofallverticalobstructionssuchassigns,utilitypoles,luminairesupports,firehydrants,etc.,includingbreakawaydevices.Forthisguidethelateraloffsetshallbe1.5feetfromthetraveledway.

1.7Roadway

Theroadwayisthetotalcompositionofalldesignelementsincludingthetraveledway,shoulder,anddrainagesystem.

I8TemporaryTrafficControl

AllprojectsshallincludeatemporarytrafficcontrolplanandwillconformtothecurrenteditionoftheMUTCD,ManualonUniformTrafficControlDevices,PartVI.Alldevicesshallbeclearlydefinedandquantified,includingallapplicabledirections,notesandassociatedreferencestoperformtheconstruction.

1.9TrafficCalming

Trafficcalminginvolvesdesignelementsthatincorporatechangesinstreetalignment,installationofbarriersandotherphysicalmeasurestoreducetrafficspeedsand/orcut-throughvolumes,intheinterestofstreetsafety,livability,andotherpublicpurposes.

1.10TraveledWay

Traveledwayistheportionoftheroadway,exclusiveoftheshouldersanddrainagesystem,whichisusedforthemovementofvehicles.

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1.11 Typical Traveled Way Cross Section

Roadways for this guide may include the following cross section types:

Open DitchCurb and GutterRoll CurbAsphalt WingInverted CrownNo-Crown I Single Slope

1.12 Vertical Clearance

Shoulders and traveled way should be clear of obstructions 16 feet above theirsurface. Any locations that cannot achieve the required vertical clearance shallbe marked with appropriate traffic control devices. For traveled ways with designspeed 30 mph the vertical clearance criteria shall not apply.

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Chapter 2

Elements of Designfor

Major Local Roads

CHAPTER 2

2. Elements of Design for Major Local Roads

The following roadway design criteria will be used for existing county maintained unpavedroads functionally classified as Major Local Roads.

The design speed for this classification should be in the 25 — 45 mph range. Under warrantedsituations lower design speeds may be used only with approval by Mobile County EngineeringDepartment.

Right-of-way width should be a minimum of 60 feet, unless the environment which it servesrestricts the acquisition to a lesser width.

2.1 Roadway Design

On some open-ditch type roadways I - 2 feet of paved shoulder may be approved by MobileCounty Engineering Department. If approved, this paved area shall have the same crossslope as the traveled way, but would be considered as part of the shoulder width.

I -750 ADT

Design Speed Traveled Way Width Shoulder Width Clear Zone(mph) (feet) Minimum (feet) Minimum (feet)

15to30 18 4 n/a

35to45 18 4 4

For roadways with design speeds 30 mph clear zone shall not apply (n/a).Curb and gutter section roadways shall have a 1.5 feet clear zone measured from the face ofcurb.

751 - 2000 ADT

Design Speed Traveled Way Width Shoulder Width Clear Zone(mph) (feet) Minimum (feet) Minimum (feet)

15to30 18 4 n/a

35to40 20 4 4

45 22 4 4

For roadways with design speeds 30 mph clear zone shall not apply (n/a).Curb and gutter section roadways shall have a 1.5 feet clear zone measured from the face ofcurb.

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>2000ADT

DesignSpeedTraveledWayWidthShoulderWidthClearZone(mph)(feet)Minimum(feet)Minimum(feet)

15to30204n/a

352244

402266

Forroadwayswithdesignspeeds30mphclearzoneshallnotapply(n/a).Curbandguttersectionroadwaysshallhavea1.5feetclearzonemeasuredfromthefaceofcurb.

2.2StoppingSightDistance

DesignStoppingSightStoppingSightStoppingSightSpeedDistanceforDistanceforDistancefor

LevelRoadwaysDowngradesUpgrades(feet)(feet)(feet)

(mph)<3%3%6%9%3%6%9%

1580808285757473

20115116120126109107104

25155158165173147143140

30200205215227200184179

35250257271287237229222

40305315333354289278269

45360378400427344331320

Distancesshouldbeadjustedforgradessteeperthanthoseshown.

d=

30((3:2)±G)

Formulasource:AASHTO,AGuideonGeometricDesignofHighwaysandStreets(3-3)

Stoppingsightdistanceshallbecalculatedonadrivereyeheightof35feettoanobjectheightof2feet.Theshoulderareashallnotbeutilizedtodeterminethesightdistancelength.Whereasitespecificlocationhasastoppingsightdistancelessthanthecalculateddistancebasedonthedesignspeed,trafficcontroldevicesshallbeinstalledtoalertthedriverofthecondition.

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2.3 Intersection Sight Distance

(mph) (feet)

15 170

20 225

25 280

30 335

35 390

40 445

45 500

50 555

55 610

Distances shown are for a passenger (P) design vehicle making a left turn maneuver onto amain road and may require additional distance for a larger design vehicle. For reference use:AASHTO, A Guide on Geometric Design of Highways and Streets, Chapter 9, Intersections,Table 9-6.

Speed shown (mph) is the speed the vehicle is traveling along the main road and the distanceshown (feet) is from the side road to the vehicle on the main road. Intersections that do notmeet minimum distance may require an advisory intersection and speed sign.

Distances shall be adjusted for grades more than 3 %. For reference use: AASHTO, A Guideon Geometric Design of Highways and Streets, Table 9-4, Adjustment Factors for SightDistance Based on Approach Grade.

2.4 Horizontal Alignment

Rmin = V2

15 ( O.Olemax+fmax)

Formula source: AASHTO, A Guide on Geometric Design of Highways and Streets (3-8)

Centerline Radius (feet) Minimum

Design Speed (mph) 15 20 25 30 35 40 45

fmax .31 .26 .22 .20 .18 .16 .15Cross Slope (%)

-2.0 55 115 210 340 515 765 1,040

-2.2 55 115 215 345 520 775 1,060

-2.5 55 120 220 350 530 795 1,085

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Superelevation

emax=.06(UrbanArea)emax.08(RuralArea)

CenterlineRadius(feet)Minimum

DesignSpeed(mph)15202530354045

fmax.31.26.22.20.18.16.15CrossSlope(%)

8.04080140220320450590

7.04585145225330465615

6.04585150235345490645

5.04590160245360510680

4.04590165255375535715

3.04595170265390565755

2.55095175270400580775

2.250100175275410590790

2.050100180280415595800

2.5VerticalAlignment

DesignSpeedKValueforCrestKValueforSag(mph)VerticalCurvesVerticalCurves

15310

20717

251226

301937

352949

404464

456179

Rateofverticalcurvature,K,isthelength(L)ofcurvedividedbythepercentalgebraicdifferenceinintersectinggrades(A).K=LIA

Maximumgradefortraveledwayshallbe12%.

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2.6 Cross Slope

Pavement Cross SlopeOpen Ditch 2.0% - 2.5%Curb and Gutter 2.0% - 2.5%Roll Curb 2.0% - 2.5%Asphalt Wing 3.33% (typical)

Grass Shoulder Cross Slope 4% - 6%2% if adjacent to sidewalk

Front Slope Desirable 3: 1Maximum 2: 1

Back Slope Desirable 3: 1Maximum I : 1

2.7 Drainage

Side Drain or Lateral Storm Sewer Minimum 10 yr. storm

Crossing (Closed) Storm Sewer Minimum 10 yr. storm

Sump Crossing (Closed) Storm Sewer Minimum 25 yr. storm

Culvert (Open) Cross Drainage Minimum 25 yr. storm

Bridge or Bridge Culvert * Minimum 50 yr. storm

* All Bridge or Bridge Culvert requires HS-20 Structural Design.

Drainage structures located within a federally designated 100 year base elevation should bedesigned to pass a minimum of a 100 year storm without raising the base flood elevation morethan 1 foot.

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CHAPTER 3

3. Elements of Design for Minor Local Roads and Residential Roads

The following roadway design criteria will be used for existing county maintained unpavedroads functionally classified as Minor Local Roads and Residential Roads.

The design speed for these classifications should be in the 10 — 30 mph range. Traffic calmingelements should be incorporated to keep roadway users from traveling at speeds higher thanthe design speed. Under warranted situations for Minor Local Roads only, higher designspeeds >30 mph may be used, only with approval by Mobile County Engineering Department.Design speed for Minor Locals Roads >30 mph shall follow the guidelines of Chapter 2,Elements of Design for Major Local Roads.

Context sensitive solutions should be incorporated in areas that are highly developed or thathave an isolated location that should be preserved.

Right-of-way width should be a minimum of 30 feet, unless the environment which it servesrestricts the acquisition to a lesser width. The following criteria are intended to be designedwithin the existing right-of-way, but may require additional right-of-way at site-specific locationsfor roadway elements and/or for utility relocations adjacent to the roadway.

3.1 Roadway Design

*Design Speed for Minor Locals Roads >30 mph

shall follow the guidelines of Chapter 2, Elements of Design for Major Local Roads

Design elements such as horizontal and vertical curves, traffic calming devices, short tangentlengths, etc., should be incorporated to reduce high running speeds.

Some locations may require additional pavement width such as on the inside of horizontalcurves with small radii or at site-specific locations that will reduce the need for futuremaintenance (drainage outfalls, driveways, intersections, etc.).

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3.4HorizontalAlignment

Rmin=V2

15(O.Olemax+fmax)

Formulasource:AASHTO,AGuideonGeometricDesignofHighwaysandStreets(3-8)

CenterlineRadius(feet)Minimum

DesignSpeed(mph)1015202530

fmax.38.31.26.22.20 CrossSlope(%)

-203555115210340

-223555115215345

-253555120220350

-3.03560120225355

-3.33560120225365

-3.53560120230370

-4.03560125235375


DesignSpeedKValueforCrestVerticalCurvesKValueforSagVerticalCurves (mph)

10310

15310

20717

251226

301937

Rateofverticalcurvature,K,isthelength(L)ofcurvedividedbythepercentofalgebraicdifferenceinintersectinggrades(A).K=L/A


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3.6 Cross Slope

Pavement Cross Slope Open Ditch 2.0% - 2.5%Curb and Gutter 2.0% - 2.5%Roll Curb 2.0% - 2.5%Asphalt Wing 3.33% (typical)Inverted Crown 2.0% - 4.0%No-Crown I Single Slope 4.0% (max)

Grass Shoulder Cross Slope 4% - 6%2% if adjacent to sidewalkShould match pavement cross slope forNo-Crown I Single Slope

Front Slope: Desirable 3 : 1Maximum 2: 1

Back Slope: Desirable 3 : 1Maximum 1 : 1

3.7 Drainage


Crossing ( Closed ) Storm Sewer Minimum 10 yr. storm


Culvert ( Open ) Cross Drainage Minimum 25 yr. storm




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CHAPTER 4

4. Elements of Design for Access Roads

The following roadway design criteria will be used for existing county maintained unpavedroads functionally classified as Access Roads. Roads for this classification are typically two-directional one-lane roads and should have an ADT of 400.

The design speed for this classification should be in the 10 — 25 mph range. Traffic calmingelements should be incorporated to keep roadway users from traveling higher than the designspeed.

Context sensitive solutions should be incorporated in areas that are highly developed or thathave an isolated location that should be preserved. In context sensitive areas, especiallywhere an established path has been used by the community that it serves, right-of-way widthshould conform to the surroundings.

Right-of-way width in some areas may be the same as the roadway width, and may onlyinclude the prescriptive area of the roadway. The following criteria are intended to bedesigned within the existing right-of-way currently available, but may require additional right-ofway at site-specific locations for roadway elements and/or for utility relocations adjacent to theroadway.

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4.1RoadwayDesign

DesignSpeedTraveledWayWidthShoulderWidth(mph)(feet)Minimum(feet)Minimum

10-15102

20-25124

Theshoulderareamayhavepavedturnoutsthatparallelthetraveledwayatsharpcurves,atspacingintervalsofapproximately500-1000feet.,oratotherappropriateareaslikedriveways,forthepurposeofallowingvehiclestopasswithminimalbacking.

Widershouldersshouldbeusedwhereverpossibleunlessright-of-wayconstraints,orcontextsensitivesolutionsrestricttheiruse.

Designelementsalongtheroadwaysuchashorizontalandverticalcurves,trafficcalmingdevices,short-tangentlengths,etc.,shouldbeconsideredtoreducehighrunningspeeds.

Somelocationsmayrequireadditionalpavementwidthsuchasontheinsideofsmall-radiihorizontalcurvesoratsite-specificlocationsthatwillreducetheneedforfuturemaintenance(drainageoutfalls,driveways,intersections),etc.Considerationshouldbegivenatintersectionstowidentotwo-lanewidthswithappropriateradiitoaccommodatethevehicledesign-type.

4.2StoppingSightDistanceL

StoppingSightSightDistanceSightDistanceDistanceforAdjustmentforAdjustmentfor

LevelRoadwayDowngradesUpgradesDesignSpeed(feet)(feet)(feet)

(mph)Level3%6%9%3%6%9%

1066757576595959

15115134136138107107107

20143178181184130129129

25194248253258173172171——————

StoppingSightDistancesarecalculatedbasedonthedriver’seyeheightof3.5feet.toanobjectheightof2feet.Theshouldershallnotbeusedaspartofthesightdistancelength.Underextremecircumstances,whereasite-specificlocationhasastoppingsightdistancelessthanthedesignspeedfortheentireproject,trafficcontroldevicesmaybeusedtoalertthedriverofthiscondition.

Forgradessteeperthanthoseshownadjustmentshouldbemade.c

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Formula source: AASHTO, A Guide on Geometric Design of Highways and Streets (3-3)

4.3 Intersection Sight Distance

Posted Speed Limit Intersection Sight Distance(mph) (feet)

10 110

15 170

20 225

25 280

30 335

35 390

40 445

45 500

50 555

55 610

Distances shown are for a passenger (P) design vehicle making a left turn maneuver onto amain road and may require additional distance for larger design vehicles. For reference use:AASHTO, A Guide on Geometric Design of Highways and Streets, Chapter 9, Intersections,Table 9-6.

Speed shown (mph) is the speed the vehicle is traveling along the main road and the distanceshown (feet) is from the side road to the vehicle on the main road. Intersections that do notmeet minimum distance may require an advisory intersection and speed sign.

Distances should be adjusted for grades more than 3 %.

Intersection of the side road to the main road should be widened to allow two-lanes ofoperation for ingress and egress (to the side road). If both roadways are two-directional onelane roadways all directions should be considered to widen to two-lane widths, allowing aplace to pull over and pass each other before continuing.

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4.4HorizontalAlignment

CenterlineRadius

DesignSpeed•‘152025

(mph)

MinimumRadius100300450750

(feet)Minimum——

AdditionalPavementWidthRequiredforCurves

Iftheroadwillhaveaschoolbus,garbagetruck,otherlargesingleunitvehicles,oravehiclepullingatrailer(boat,utility,etc)onaregularbasis,additionalwidthofpavementwillbenecessarytokeepoff-trackingontotheshouldertoaminimum.Theadditionalpavementwidthshouldbeconsideredtobeplacedontheinsideareaoftheradius.

Radius100101-500500-750751-950>950

(feet)

lOft.Lane4321nla

12ft.Lane21nlanlanla


DesignSpeedKValueforCrestKValueforSag(mph)VerticalCurvesVerticalCurves

102n/a

15410

201017

251626

Rateofverticalcurvature,K,isthelengthofcurvedividedbythepercentalgebraicdifferenceinintersectinggrades(A).K=LIA


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4.6 Cross Slope

Pavement Cross Slope Match existing topography (4.0% max)

Grass Shoulder Cross Slope Match pavement cross slope

Front Slope Desirable 3 : 1Maximum 2: 1

Back Slope Desirable 3: 1Maximum I : I

4.7 Drainage

Drainage may match existing topography if determined that it would be a contextsensitive solution for the area. Some site-specific drainage improvements may bedetermined and incorporated into the design of the roadway by the engineer without acomprehensive drainage system.

If drainage is used, the following should apply:








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Chapter 5

Elements of Designfor

Access Ways

CHAPTER 5

5. Elements of Design for Access Ways

The following roadway design criteria will be used for existing county maintained unpavedroads functionally classified as Access Ways.

An Access Way shall be paved only if it connects directly to an existing paved road. It may bepaved if associated jointly with a roadway paving project that will create a continuous pavedroute to an existing paved road.

Roads for this classification are typically two-directional one-lane roads and should have anADT of 200. ADT for this classification of roadway is an aggregate of repeated trips by thelocal property users and their guests. If the purpose of any property adjacent to this type ofroadway is going to encourage users unfamiliar with this area to travel with frequency, differentdesign criteria should be considered.

A moderate to high housing density should be associated with this type of roadway and shouldaverage 1 house for about every 125 linear feet of roadway, or less (maximum of 18 houses).Road length should be no greater than about 1/3 mile (1700 feet). An Access Way may divideto access different properties, or loop back onto itself. These segments of the roadway mayhave different road names given for addressing purposes, and may constitute a lesser housingdensity. These segments of the roadway provide additional adjacent property access andshould be considered part of the Access Way.

The design speed for this classification should be 10mph.

Context sensitive solutions should be incorporated for this type of roadway. The secludednature and access to property that this type of road serves should be preserved. In contextsensitive areas, with a minimal crash history, especially where an Access Way has beenestablished by the community that it serves, right-of-way width should conform to thesurroundings.

Right-of-way width in some areas may be the same as the roadway width, and may onlyinclude the prescriptive area of the Access Way. Prescriptive right-of-way roadways shouldretain the width that Mobile County has maintained. The following criteria are intended to bedesigned within the existing right-of-way, but may require additional right-of-way at site-specificlocations for roadway elements and/or for utility relocations adjacent to the way.

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5.1RoadwayDesign

DesignSpeedTraveledWayShoulderWidthMaximumWidth

10mphExistingTravelWidthExistingWidth

(Maximum14feet)

AreasalongtheAccessWaymayincludeadditionalsite-specificpavedareaswiderthanthemaximumshowntoaidpossibleconflictingtrafficorroadwaymaintenancelocations.Somelocationsmaybeatsharpcurves,drainagestructures,intersections,driveways,oratotherappropriateareas.Someareasmaybeforthepurposeofallowingvehiclestopasswithminimalbacking.

5.2StoppingSightDistance

StoppingSightDistancesarecalculatedbasedonthedriver’seyeheightof3.5feettoanobjectheightof2feet.Theshoulderareamaybeutilizedtodeterminethesightdistancelength.Theminimumstoppingsightdistanceshallbe40feet.Sitespecificlocationsmayhavetrafficcontroldevicesinstalledtoalertthedriver.

5.3IntersectionSightDistance

Intersectionsshallbeevaluatedbasedontheirfunctionalclassificationandpostedspeedlimit.AnAccessWaythatintersectsanotherAccessWayshouldhavethesameintersectionsightdistanceasstoppingsightdistance,40feet.OnAccessWaysthatintersectroadswhicharefunctionallyclassifiedotherthananAccessWay,thefollowingintersectionsightdistancetableshouldbeused.

PostedSpeedLimitMainRoadIntersectionSightDistanceAlongMainRoad(mph)(feet)

10110

15170

20225

25280

30335

35390

40445

45500

50555

55610

Distancesshowninthetableareforapassenger(P)designvehiclemakingaleftturnmaneuverontoamainroadandmayrequireadditionaldistanceforlargerdesignvehicles.

Speedshown(mph)isthespeedthevehicleistravelingalongthemainroadandthedistance

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shown (feet) is from the centerline of side road to the vehicle on the main road. Intersectionsthat do not meet minimum distance may require an advisory intersection and speed sign to beposted on the main road.

5.4 Horizontal Alignment, Vertical Alignment and Cross Slope

Access Ways should match existing topography.

5.5 Drainage

Drainage should match existing topography. Typically no drainage improvements are required.If determined that context sensitive drainage solutions for the area should be included, site-specific drainage improvements may be incorporated.

If the drainage system is improved, the design should accommodate the following drainagecriteria:








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CHAPTER 6Functional Classification

The classification of local roadways into different functional classes is necessary forcommunication among engineers, local agencies, and the general public. Differentclassifications apply for different purposes in both the rural and urban environment. Functionalclassification, the grouping of streets / roads by the character of service they provide, is themain purpose in planning, designing, meeting social needs, and establishing funding. Thefunctional classification of roads and streets is an important management tool in theestablishment of realistic improvements both for individual roadways and for the county roadsystem as a whole. The idea is to combine adequacy with economy and to attain a rural roador urban street system giving the highest overall level of service per dollar of investment. Acomplete functional roadway system provides the motoring public with a series of distinct travelmovements. From main movement at the national level, to the termination movement at anisolated location at the rural level, the function of each type of roadway should be designedand planned in a distinctly different manner.

According to the guidelines as established in AASHTO, A Guide on Geometric Design ofHighways and Streets, Current Edition, the current concept of a complete highway systemconsists of six stages or movements. In most cases these stages are main, transition,distribution, collection, access, and termination. In some cases, the termination of a trip mayexclude some of these movements. (See fig. 6.1)

Main movement involves the transportation of the highest volumes of vehicular traffic onfreeway and primary arterial systems at national and state levels. Examples of this includeroads like the Interstate System (1-10 and 1-65), U.S. Highway System ( U.S. Hwy 45, U.S.Hwy 98, State Hwy 188). This level requires the highest design standards for mobility andsafety, while limiting access points throughout its entire system.

Transition movement is the first step in directing traffic from a national, state, or inter-countylevel roadway system to the local environment level. The transition movement includes minorarterial and collector systems. Examples of this include roads like Lott Rd., Airport Blvd.,Schillinger Rd., and Cottage Hill Rd. Transition movement design requires the designer orplanner to start allocating for the need of public access to property. However mobility at thislevel is still the primary focus.

Distribution movement consists of a broad spectrum of vehicular traffic trying to access a widevariety of major and minor collector and various local roads in a county-wide system.Examples of this include roads like Prine Rd., Spice Pond Rd., Tanner Williams Rd., andBellingrath Rd. The demand for public access to property becomes varied and is the mainfocus in design. Mobility is restricted at different levels and for different purposes.

The collection movement, access movement and termination movement constitute the rest ofthe various local road systems. Transportation at this level involves the movement of a widevariety of vehicular types to and from a wide variety of destinations. Access is the main focusfor the design, blending with the movement with the various types of design vehicles. Suchmovement can range from high-density work or school traffic, to a single vehicle destined for aboat ramp on an isolated rural road (way). The design of these roads may and will varyconsiderably.

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MAINMOVEMENTFREEWAYS-ARTERIALS

HighMobUity-LimitedtoRestrictedAccess

MODERATEACCESSTERMINATIONPOINT

ACCESSMOVEMENTMINORLOCALROADSRESIDENTIALROADS

ACCESSROADSACCESSWAYS

MinimaltoRestrictedMobility-HighAccess

COLLECTIONMOVEMENTVARIOUSMAJOR,MINORLOCALROADS&RESIDENTIALROADS

ModeratetoMinimalMobility-HightoModemteAccess

HIGHACCESSTERMINATIONPOINT

MINORLOCALROADSRESIDEN11ALROADS

ACCESSROADSACCESSWAYS

MINIMALACCESSTERMINATIONPOINT

TRANSITIONMOVEMENTARTERIALS-COLLECTORS

HightoModerateMobility-MinimaltoLimitedAccess

DISTRIBUTIONMOVEMENTCOLLECTORS-MAJORLOCALROADS

ModerateMobility-ModeratetoMinimalAccess

MAINACCESSTERMINATIONPOINT

HIGHWAYSYSTEMOFMOVEMENT (Hg.61)

RelationshipofFunctionallyClassifiedSystems

afficMobiIity

48

The first step in developing road classifications is to determine limits of urban and ruralroadway boundary areas. AASHTO, A Guide on Geometric Design of Highways and Streets,Current Edition, defines urban areas as all areas which have a population greater than 5,000or more. Urban areas are further subdivided into urbanized areas (population of 50,000 andover) and small urban areas (population between 5,000 and 50,000). All other areas whichhave a lesser population are rural. The Alabama Department of Transportation has establishedby code (Section 101 of Title 23, U.S. Code) the boundary of Mobile County that has anurbanized area. In both the rural or urban areas all the vehicular movement levels occur, butthe vehicular densities and the access to properties vary considerably. The quantity ofvehicular traffic does not determine an urban area iust as the use of land does not determine arural area.

The next step in developing road classifications is to determine the general characteristics ofthe road. Some of the general characteristics would include linkage with other roads, theservice it provides to the area, design speeds, access to property, truck usage, and level ofservice. The current roadway system, as recognized by NACE and defined by AASHTO,designates four basic classifications: Principal Arterial, Minor Arterial, Collector (Major andMinor) and Local. (See fig.6.2)

Functional Classification System

Urban Area UrbanMileage

(Percent)

Principal Arterial System 5 - 10

Principal Arterial plus 15 - 25Minor Arterial System

Collector System 5 - 10

Local System 65 - 80

Rural Area RuralMileage

(Percent)

Principal Arterial System 2 — 4

Principal Arterial plus 6—12Minor Arterial System

Collector System 20 -25

Local System 65 — 75

(Source: AASHTO, A Guide on Geometric Design of Highways and Streets, Table 1-1 and Table 1-2)

(Fig. 6.2)

6.1 Arterial and Collector Road General Characteristics

N6.11 Principal Arterial

A. Major link from state to state

B. Provide high mobility for longer trips

C. Access is limited to other roads only and rarely to property

D. Design speeds are high, 45-70 mph

E. Truck usage is high and will accommodate the largest vehicle type

F. Level of Service B desirable

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6.12MinorArterial

A.Majorlinkwithinstateorcountylevel

B.Servesashightrafficcorridortoprimaryarterials

C.Providehightomoderatemobilityformoderatetrips

D.Accesstoabuttingpropertyshouldbelimitedandsometimesrestricted

E.Designspeedsaremoderatetohigh,35-70mph

F.Truckusageishighwithpossiblerestrictionsinvehicletype

G.LevelofServiceCdesirable

6.13Collector

A.Majorlinkwithincounty

B.Servesasmajortrafficgeneratornotdirectlyservedbyarterials

C.Spacedatintervalsthataccumulatetrafficforlocalroadsystems

D.Providemoderatemobilitywithincounty

E.Accesstoabuttingpropertyisminimaltolimited

F.Designspeedsaremoderatetolow20-60mphNote:lowspeeds20-30mphshouldonlybeusedinmountainousterrain

G.Truckusageismoderatewithsomerestrictionsinvehicletype

H.LevelofServiceCtoDdesirable

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6.2 Local Road General Characteristics

The Local Road functional classification is the next level and constitutes approximately 65-80%of the total roadway mileage. Based on AASHTO this classification has the same generaldesign criteria characteristics. However, not all Local Roads have or should have the samecharacteristics in design, function, and environment. Because of the diversity incharacteristics, the following local road classifications have been established by MobileCounty. (See fig. 6.3)

Local Road Functional Classification

Local Roads Urban RuralMileage Mileage

(Percent) (Percent)

Major Local Roads 10 —30 20 - 30

MinorLocaiRoads 5—15 5-20

Residential Roads 25 — 50 15 - 35

Access Roads 5—20 5—20

Access Ways 1 — 5 1 — 5

(Fig.6.3)621 Major Local Road

This classification of roadway is the highest local level that links to equal or higherfunctional classification roadways at each end. It may also include roadways that linkisolated (developed or planned) areas like subdivisions, industrial or recreational sites inurban areas or commercial farms, timberlands, small communities, residential areas,industrial or recreational sites in rural areas to higher functional classification roadways.

A. Serves as a link between equal or higher functional classificationroadways, or may loop into an isolated area and reconnect to the sametype of roadway

B. Serves as a link to communities or significant areas within a jurisdiction, orserves as typically the only connecting roadway from a higherclassification roadway

C. Access to abutting property is moderate to high

D. Design speeds are moderate, 25 - 45 mphUnder some conditions a lower design speed may be considered

E. Right-of-way width should be a minimum of 60 feet, unless theenvironment which it serves restricts the acquisition to a lesser width

F. Truck usage is moderate to high

G. The level of service should provide moderate traffic flow except duringtimes of heavy congestion and may have passing restrictions

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6.22MinorLocalRoad

Thisclassificationofroadwaylinksisolatedareastoahigherfunctionalclassificationroadway.Isolatedareasmaybeareaswheretheroadextendsto;abusiness,recreationalarea,scenicplace,agriculture/landmanagementareas,industrialsite,ortoaminimalnumberofhouses.

A.Linksisolatedareastohigherfunctionalclassificationroadways

B.Mayserveastheonlyaccesspoint

C.Accesstoabuttingpropertyishigh(mainfunction)

Intheurbangeographicalenvironment(city/urbancommunity)itcharacteristicallyconsistsofsmallparcelsoflandservingvariousbusinesses

Intheruralgeographicalenvironment(country/ruralcommunity),itcharacteristicallyconsistsofpossiblelargeparcelsoflandservingalowpopulationdensityand/orbusiness

D.Designspeedsarelow,10-30mph

Forsomelocations,wheregreaterthan30-mphoperationalspeedsmayneedtobeimpeded,trafficcalmingelementsshouldbeconsideredinthedesign

Undersomewarrantedsituations,onlywhenthedistancetotravelislengthy,higherdesignspeeds>30mphshouldbeconsidered

E.Right-of-wayshouldbeaminimumof30feet

Incontextsensitiveareas,especiallywhereanestablishedroadhasbeenusedbythecommunitythatitserves,right-of-waywidthshouldconformtothesurroundings

F.Truckusagemaybehighinbusinessareas

G.Thelevelofserviceforshortdistancetravellengthshouldcharacteristicallyprovideslowtraffic-speedflows

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6.23 Residential Road

This classification of roadway links developed or planned subdivisions, orisolated dwelling areas to a higher functional classification roadway. Isolateddwelling areas may be areas where the road extends to a number of houses thatmay be associated with a business or an agriculture / land management area.

A. Links residential areas to higher functional classification roadways, or linksisolated areas

B. May be part of an internal grid of residential roadways, has a linkagesystem to parcels, or serves as the only access point

C. Access to abutting property is high (main function)

In the urban geographical environment (city / urban community) it consistsof small parcels of land and a high population density

In the rural geographical environment (country I rural community), itconsists of possible large parcels of land and a low population density

D. Design speeds are low, 10 - 30 mph.

For some locations, where greater than 30-mph operational speeds mayneed to be impeded, traffic calming elements should be considered in thedesign

E. Right-of-way should be a minimum of 30 feet

In context sensitive areas, especially where an established road has beenused by the community that it serves, right-of-way width should conform tothe surroundings

F Truck usage is low

G. The level of service should provide slow traffic-speed flows

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6.24AccessRoad

Thisclassificationofroadwaylinksisolatedaccessareastoahigherfunctionalclassificationroadway.Isolatedaccessareasmaybeareaswherethepathextendstoaminimalnumberofhouses.Roadsforthisclassificationaretypicallytwo-directionalone-laneroadsandshouldhaveanADTof400.Thisclassificationisnotintendedtoaccommodatefuturedevelopments.

A.Linksisolatedareastoanyfunctionalclassofroadway

B.Servesastheonlyaccesspoint

C.Accesstopropertiesistheonlyfunction

D.Designspeedsareverylow,10-25mph.

E.Trafficcalmingelementsshouldbeconsideredinthedesign

F.Right-of-waymaybethesameastheroadwaywidth,andmayonlyincludetheprescriptiveareaoftheroadway

Incontextsensitiveareas,especiallywhereanestablishedpathhasbeenusedbythecommunitythatitserves,right-of-waywidthshouldconformtothesurroundings

G.Truckusageisnegligibleandmayrequirebackingitsentirelength.

H.Thelevelofserviceshouldprovideslowtraffic-speedflows

Forsometypesofdesignthedrivermayberequiredtostoptoallowopposingtraffictopass,ormayrequirebackingtoallowopposingtraffictopass

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6.25 Access Way

This classification of roadway links limited housing access areas to a higherfunctional classification roadway. Limited housing access areas are propertyingress I egress areas where the pathway has a very low-volume of traffic, localuser traffic only, 200 ADT, with housing density no greater than 18 units over1/3 mile (1,700 feet)of travel. Travel on this type of roadway may divide, or loopunto itself to reach properties. This classification is not intended toaccommodate future developments.

A. Links limited housing areas to any functional class of roadway

B. Serves as the only access point

C. Access to properties is the only function

D. Design speeds should be 10 mph maximum

E. Traffic users are the property users of the road and their guest

F. Right-of-way may be the same as the roadway width, and may onlyinclude the prescriptive area of the roadway

In context sensitive areas, especially where an established path has beenused by the community that it serves, right-of-way width should conform tothe surroundings

G. Truck usage is negligible and may require backing its entire length

H. The local property users should provide very slow traffic-speed flows

I. For some alignment (types of design) the driver may be required to stop,use turnouts, and/or back to allow opposing traffic to pass

6.25 Reevaluation of the System

The roadway functional classification system should be developed at a local level by thoseindividuals who have the knowledge about the distribution and location of the variousenvironmental, industrial, residential, recreational, and commercial areas within theirjurisdictional boundaries. In most cases this would be those having jurisdiction and control fordesign, construction and maintenance of the local roadway system. These local agenciesshould have good information as to the frequency and usage by the traveling public. Thereevaluation of the functional classification system as a management tool should be updatedon an approximate ten to twenty-year cycle. Change to an existing functionally classified roadcan occur at any time if the environment associated along the roadway dictates.

55

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CHAPTER 7

Design Waiver

Design waivers shall be permitted in accordance with the following procedure. The engineershall submit a written proposal that will detail the design change and outline the specificreasons and consequences of the change to the County Engineer. The County Engineer willsubmit the waiver with his recommendation to the County Commission for their consideration.

For clear zone and lateral offset, the criteria in this design guide should be treated as guidanceand not as a county standard requiring a design waiver if not numerically met.

59

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References

1. AASHTO. A Guide on Geometric Design of Highways and Streets.Washington, D. C.: AASHTO, 2011.

2. Cost Effective Local Road Safety PlannIng and Implementation, NationalAssociation of County Engineers (NACE) and American Traffic Safety ServicesAssociation (ATSSA), 2011

3. NCHRP Report 500: Guide Series, National Cooperative Highway ResearchProgram, 2009

4. Transportation Research Board, National Research Council. Determination ofStopping Sight Distances, NCHRP Report 400. Washington, D.C.: 1997.

5. AASHTO, Guidelines for Geometric Design of Very Low-Volume Local Roads(ADT 400). AASHTO, 2001

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