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