four-lane to three-lane conversions in iowa: a summary results from several research studies of the...
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Four-Lane to Three-Lane Four-Lane to Three-Lane Conversions In Iowa:Conversions In Iowa:
A SummaryA SummaryResults from Several Research Results from Several Research
Studies of the Impacts on Studies of the Impacts on SafetySafety
Iowa Department of TransportationIowa Department of Transportation
Office of Traffic and SafetyOffice of Traffic and Safety
Iowa State UniversityIowa State University
Center for Transportation Research and EducationCenter for Transportation Research and Education
PrefacePreface
The research results I am presenting The research results I am presenting belong to others.belong to others.
Their permission in doing so is greatly Their permission in doing so is greatly appreciated.appreciated.
The ResearchersThe Researchers
Michael Pawlovich, Iowa DOTMichael Pawlovich, Iowa DOTAlicia Carriquiry, ISU Statistics Alicia Carriquiry, ISU Statistics DepartmentDepartmentWin Lee, ISU Statistics DepartmentWin Lee, ISU Statistics DepartmentTom Stout, CTRETom Stout, CTREKeith Knapp, formerly CTREKeith Knapp, formerly CTREKaren Giese, formerly CTREKaren Giese, formerly CTRETom Welch, Iowa DOT (sponsor)Tom Welch, Iowa DOT (sponsor)
Presentation OutlinePresentation Outline
4 lane to 3 lane basics4 lane to 3 lane basics
Results from “traditional” safety studiesResults from “traditional” safety studies
Results from Bayesian safety studyResults from Bayesian safety study
ConclusionsConclusions
The BasicsThe Basics
• Converting an existing 4-Converting an existing 4-lane roadway to a 3-lane roadway to a 3-
lane roadway lane roadway typically typically 2-lanes with a center 2-lanes with a center
continuous two-way left-continuous two-way left-turn lane (TWLTL)turn lane (TWLTL)
• The addition of the turn The addition of the turn lane is the critical lane is the critical ingredient for improved ingredient for improved traffic flow and safetytraffic flow and safety
• May free up enough room May free up enough room for bike lanes or parking for bike lanes or parking
Source: Huang et al./HSIS 2002,2004,2005
The Iowa 4 Lane To 3 Lane The Iowa 4 Lane To 3 Lane ExperienceExperience
Iowa DOT sees these conversions as positive in Iowa DOT sees these conversions as positive in terms of both safety and quality of lifeterms of both safety and quality of life
Before
After
Conversion GuidelinesConversion Guidelines
When conversions seem to make sense When conversions seem to make sense ADT < 20,000ADT < 20,000 Urban or small city arterial or collector with a Urban or small city arterial or collector with a
posted speed of 30 to 45 mphposted speed of 30 to 45 mph History of left-turn related crashesHistory of left-turn related crashes
Broadside and rear end collisionsBroadside and rear end collisions Significant commercial and/or residential Significant commercial and/or residential
driveway densitydriveway densityQuestion: is there a density that is too high?Question: is there a density that is too high?
Significant left-turning volumesSignificant left-turning volumes
Two Example 4-LanesTwo Example 4-LanesSource: Iowa DOT RoadView
Storm Lake, IA – US 7 EB !!!
Left-turning vehicle slows van
Clear Lake, IA – US 18 EB (west of Clear Lake)
Two Example 3-LanesTwo Example 3-LanesSource: Iowa DOT RoadView
Storm Lake, IA – US 7 EB
Storm Lake, IA – US 7 EB
Left-turning vehicle has no effect
Drive Through:Drive Through:
Sioux Center, IASioux Center, IA
US 75 NBUS 75 NB
Source: Iowa DOT RoadView
1/8
Long transition length
Multiplesigns
2/8
Source: Iowa DOT RoadView
From45 mph
3/8
Source: Iowa DOT RoadView
Passed after signage
4/8
Source: Iowa DOT RoadView
Plenty of time to change lanes
5/8
Source: Iowa DOT RoadView
Left-turning van
Opposing flow queue headways
4-lane to 3-lane transition
6/8
Source: Iowa DOT RoadView
ParallelParking
Multiple left-turning pickups
Opposing flow queue headways
7/8
Source: Iowa DOT RoadView
Signing and traffic markings
8/8
Source: Iowa DOT RoadView
3-lane to 4-lane transition
Drive Through:Drive Through:
Sioux Center, IASioux Center, IA
US 75 SBUS 75 SB
1/8
Source: Iowa DOT RoadView
Longtransition
length
Multiplesigns
From45 mph
to35 mph
2/8
Source: Iowa DOT RoadView
Multiplesigns
3/8
Source: Iowa DOT RoadView
3-lane section begins
4/8
Source: Iowa DOT RoadView
ParallelParking
Multiple left-turning vans,pickup to turn right-on-red soon
5/8
Source: Iowa DOT RoadView
ParallelParking
Van finishes left movement,pickup turns right-on-red
6/8
Source: Iowa DOT RoadView
ParallelParking
Right-turning car,van waiting for opposing left,opposing thru traffic not hindered
$1.52/gallon!!!
7/8
Source: Iowa DOT RoadView
More lefts and thru traffic
Pedestrian signs
ParallelParking
8/8
Source: Iowa DOT RoadView
3-lane to 4-lane transition
Analysis HistoryAnalysis History Treatments in Iowa (All Potential Case Studies)Treatments in Iowa (All Potential Case Studies)
15 from 1993 – 200315 from 1993 – 2003 2001: Guidelines (CTRE) 2001: Guidelines (CTRE)
4 to 3 Conversion Safety Impact Studies4 to 3 Conversion Safety Impact Studies 2002: Huang et al. (FHWA/HSIS)2002: Huang et al. (FHWA/HSIS)
Showed small reduction in crash density and no reduction in Showed small reduction in crash density and no reduction in crash rate (this study is the “odd duck” here)crash rate (this study is the “odd duck” here)
Fall 2004: Matched-Pair Before/After (CTRE)Fall 2004: Matched-Pair Before/After (CTRE) Spring 2005: Bayesian Before/After (ISU Statistics Spring 2005: Bayesian Before/After (ISU Statistics
Department)Department)
Yoked Pair ComparisonYoked Pair Comparison
4-3 conversion segments were compared 4-3 conversion segments were compared with similar “control” segmentswith similar “control” segments
City population within 20%City population within 20%
Corridor traffic (AADT) within 20%Corridor traffic (AADT) within 20%
Physically similar (length, land use, access Physically similar (length, land use, access management, etc.)management, etc.)
Yoked Pair ResultsYoked Pair Results
Net effect – 27 to 28% reduction in crash Net effect – 27 to 28% reduction in crash raterate
Crash Rate AnalysisCrash Rate Analysis
Before – 8.86 per MVMBefore – 8.86 per MVM
After – 4.45 per MVMAfter – 4.45 per MVM
Older Driver AnalysisOlder Driver Analysis
How did older drivers fare?How did older drivers fare?
Age 65 and older – average crash rate of Age 65 and older – average crash rate of 7 before (on the 4 lane), 2.9 after (on the 3 7 before (on the 4 lane), 2.9 after (on the 3 lane)lane)
Age 75 and older – average of 3.3 before, Age 75 and older – average of 3.3 before, 1.5 after1.5 after
Younger Driver AnalysisYounger Driver Analysis
How about younger drivers?How about younger drivers?
Age 25 and younger – average of 28.1 Age 25 and younger – average of 28.1 before, 13.3 afterbefore, 13.3 after
If you have teenagers, now you know why If you have teenagers, now you know why their insurance rates are so high!their insurance rates are so high!
Injury Crashes OnlyInjury Crashes Only
Net benefit, 27% reduction in injury Net benefit, 27% reduction in injury crashescrashes
ConclusionsConclusions
Overall crash frequency reduction of 21%Overall crash frequency reduction of 21%
Improved crash rate (27-28% reduction)Improved crash rate (27-28% reduction)
Improvements for both older and younger Improvements for both older and younger driversdrivers
Reduced injury numbers and rateReduced injury numbers and rate
Bayesian Before/After StudyBayesian Before/After Study
Objective: Assess whether 4 lane to 3 lane Objective: Assess whether 4 lane to 3 lane conversions appear to result in crash reductions on conversions appear to result in crash reductions on Iowa roadsIowa roads
Control for “regression to the mean” phenomenonControl for “regression to the mean” phenomenon Sometimes, crash rate improvements are simply random Sometimes, crash rate improvements are simply random
Much more rigorous study than the “traditional” study Much more rigorous study than the “traditional” study just describedjust described
Bayesian Before/AfterBayesian Before/After
DataData 15 treatment and 15* control15 treatment and 15* control 23 years (1982-2004)23 years (1982-2004) Volumes: 2,030 to 15,350 (1982-2004) Volumes: 2,030 to 15,350 (1982-2004)
* One control site served as a matched pair for two treatment sites.
Data Data 3-lane (treatment sites) 3-lane (treatment sites)SID CITY LITERAL CIPOP ADT
2000 2000
1 Storm Lake Iowa 7 from Lake Ave. to Lakeshore Dr. 10,076 7,503
2 Clear Lake US 18 from N 16 St. W to N 8th St. 8,161 10,403
3 Mason City Iowa 122 from West intersection of Birch Drive to a Driveway 29,172 7,800
4 Osceola US 34 from Corporate limits on east side to where highway divides to 4 lanes on west side 4,659 8,172
5 Manchester Iowa 13 from River St. to Butler St. 5,257 9,400
6 Iowa Falls US 65 from City Limits - ? to Park Ave. 5,193 10,609
7 Rock Rapids Iowa 9 from S Greene St. to Tama St. 2,573 4,766
8 Glenwood US 275 from MP 36.2 to MP 37.42 5,358 6,410
9 Des Moines Beaver Ave from Urbandale Ave. to Aurora Ave. 198,682 13,695
10 Council Bluffs US 6 from McKenzie Ave. west 1300 ft. 58,268 11,000
11 Blue Grass Old US 61 from Oak Lane to 400' W of Terrace Drive 1,169 9,155
12 Sioux Center US 75 from 200' South of 10th St. S. to 250' North of 9th St. NW 6,002 8,942
13 Indianola Iowa 92 from South R St. to Jct. of US 65/69 12,998 13,288
14 Lawton US 20 from 100' east of Co. Rd. Eastland Ave. to 1130' West of Co. Rd. Emmet Ave. 697 9,237
15 Sioux City Transit Ave. from Vine Ave. to just west of Paxton St. at curve 85,013 9,608
Data Data 4-lane (control sites) 4-lane (control sites)SID CITY LITERAL CIPOP ADT
2000 2000
18 Storm Lake Iowa 7 from Lake Ave. to Barton St 10,076 8,790
19 Le Mars US 75 from 0.01 miles north of 3rd St NW to 0.36 miles SW of 12th St SW 9,237 10,880
20 Cedar Falls Green Hill Road from 0.10 miles east of IA 58 to 0.09 miles west of Cedar Heights Dr. 36,145 2,768
21 Jefferson Iowa 4 from National Ave to 0.13 miles north of 250th Ave 4,626 5,685
22 Harlan Iowa 44 from US 59 to 6th St 5,282 6,981
23 Norwalk Iowa 28 from 0.03 miles south of Gordon Ave to 0.04 miles south of North Ave 6,884 7,679
24 Belmond US 69 from 0.38 miles north of Main St to 0.58 miles south of Main St 2,560 3,734
25 Harlan Iowa 44 from US 59 to 6th St 5,282 6,981
26 Des Moines Hickman Road - 40th Place east to 0.07 miles west of W 18th St 198,682 13,953
27 Ames 13th Street from 0.09 miles east of Stange Road to 0.07 miles west of Crescent Circle Dr. 50,731 10,711
28 Mapleton Iowa 141 from 0.02 miles north of Sioux St. to 0.08 miles south of Oak St. 1,322 3,007
29 Algona US 169 from 0.07 miles south of US 18 to 0.23 miles south of Irvington Rd. 5,741 7,263
30 Oskaloosa Iowa 92 from 0.12 miles east of IA 432 to 0.07 miles west of Hillcrest Dr 10,938 11,143
31 Merrill US 75 from 0.05 miles north of 2nd St to 0.18 miles north of Jackson St 754 7,774
32 Sioux City S. Lakeport from 4th Ave to Lincoln Way 85,013 15,333
Data Data Pairings* Pairings*
SID ROUTE LENGTH
1 IA 7 1.41
2 US 18 1.51
3 IA 122 1.78
4 US 34 2.04
5 IA 13 0.35
6 US 65 1.23
7 IA 9 0.35
8 US 275 1.09
9 Local 1.19
10 US 6 0.20
11 Local 0.72
12 US 75 1.52
13 IA 92 1.57
14 US 20 0.64
15 Local 0.77
COMPYEAR
1993
2003
2001
2001
2001
2002
1998
1998
1999
2000
1999
1999
1999
2000
2000
Treatment ControlSID ROUTE LENGTH
18 IA 7 0.71
19 US 75 1.80
20 Local 1.80
21 IA 4 2.40
22 IA 44 1.20
23 IA 28 0.80
24 US 69 0.90
25 IA 44 1.20
26 Local 1.50
27 Local 0.33
28 IA 141 0.70
29 US 169 2.00
30 IA 92 1.50
31 US 75 0.50
32 Local 1.20
* Not essential for Bayesian, more for discussion purposes
Monthly Crash DensitiesMonthly Crash Densities
Treatment
Control
Monthly Crash RatesMonthly Crash Rates
Intervention
Modelling/StatisticsModelling/Statistics
Crashes: random, rare count eventsCrashes: random, rare count events
Model: Poisson w/ log mean expressed as a piece-Model: Poisson w/ log mean expressed as a piece-
wise linear function of:wise linear function of:
- time period (months)- time period (months)
- seasonal effects- seasonal effects
- random effect for each site- random effect for each site
Adopted a Bayesian statistical framework for Adopted a Bayesian statistical framework for
estimation of model parametersestimation of model parameters
Results/DiscussionResults/Discussion
25.2% (23.2% - 27.8%) in 25.2% (23.2% - 27.8%) in crashes/milecrashes/mile
25%25%
18.8% (17.9% - 20.0%) in 18.8% (17.9% - 20.0%) in crash ratecrash rate
19%19%
Results/DiscussionResults/Discussion
Agrees with:Agrees with: CTRE Matched-pair Before/After studyCTRE Matched-pair Before/After study Why? Prior Sioux Center speed studyWhy? Prior Sioux Center speed study
4-5 mph in 85th percentile free flow4-5 mph in 85th percentile free flow
speedspeed 12-14% in percentage of vehicles 12-14% in percentage of vehicles
traveling more than 5 mph over the speed limit traveling more than 5 mph over the speed limit
(i.e., vehicles traveling 35 mph or higher). (i.e., vehicles traveling 35 mph or higher).
ConclusionsConclusions
4 lane to 3 lane conversions appear to be 4 lane to 3 lane conversions appear to be successful treatments for calming traffic successful treatments for calming traffic under the right conditionsunder the right conditions
They can be very effective in improving They can be very effective in improving traffic safetytraffic safety
They are not especially problematic for They are not especially problematic for either elderly or young driverseither elderly or young drivers