Low Cost Safety ImprovementsPooled Funds Study

Safety of Lane/Shoulder WidthCombinations on Two-Lane Rural Roads

Dr. Frank Gross, Vanasse Hangen Brustlin (VHB), IncDr. Paul P. Jovanis, Penn State University

Overview

Introduction

Objective

Study Design

Methodology

Data Collection

Preliminary Results

Conclusions

Future Research

Background on Strategy Shoulder Paving/Widening

Proven strategy

Pavement Width What lane/shoulder width

produces lowest crash odds? Identified at Technical Advisory

Committee June 2006

Target crashes Head-on Run-off-road Sideswipe

Potential Difficulties Confounding Variables

Key to Success Flexible modeling approach

Literature Review Crash Modification Factors in Highway Safety Manual

Key studies: Zegeer et al. (1981); Zegeer et al. (1988); Griffin and Mak (1987)

0.80

0.90

1.00

1.10

1.20

1.30

1.40

1.50

1.60

0 500 1000 1500 2000 2500

Average Daily Traffic Volume (veh/day)

0' Shoulder

This factor applies to single-vehicle run-off-the-road, multiple-vehicle same direction sideswipe accidents, and multiple-vehicle opposite direction sideswipe accidents.

2' Shoulder

4' Shoulder

6' Shoulder

8' Shoulder

Recommended CMF for Shoulder Width (Harwood et al., 2000)

Literature Review

Few Studies Address Allocation of Total Width “Road diets” change total number of lanes

• Burden and Lagerwey (2001); Welch (1999) Reallocation of width on urban freeways

• Add lane by reducing lane and shoulder width

• McCasland (1978); Urbanik and Bonilla (1987)

Evaluate Re-allocation Without Other Changes

Objective

Estimate Safety Effectiveness For a given pavement width, what allocation of

lane/shoulder width produces the lowest crash odds?

Secondary Questions of Interest Do effects vary by:

• Traffic volume?• Speed limit?

For a given lane width, do effects vary as shoulder width increases?

Is the treatment economically feasible?

Methodology Case-Control Methodology

Cases: crash-involved segments for a given year Controls: non-crash-involved segments for a given year

Matching Variables ADT and Segment Length

Additional Covariates Speed, District, Unpaved Shoulder, Curvature, and Grade

Methodology

Pair ADTSegment length

SW: 0

SW: 1

SW: 2

SW: 3

SW: 4

SW: 5

LW: 9

LW: 10

LW:11

LW:12

Outcome

1 2 5 0 0 1 0 0 0 0 1 0 0 1

1 2 5 0 0 0 0 0 1 0 0 0 1 0

2 4 3 1 0 0 0 0 0 1 0 0 0 1

2 4 3 0 1 0 0 0 0 0 1 0 0 0

SW means shoulder width and LW means lane width

Methodology

Case-Control Methodology Allows answer to primary and secondary questions Regression-to-the-mean not an issue Accounts for confounding variables

• Matched design

• Model covariates

Study Design

Required Sample Size Minimum: 15,094 segment-years

• Detect 10 percent reduction in total crashes with 90 percent confidence

Desirable: 57,576 segment-years• Detect 5 percent reduction in total crashes with 90 percent

confidence

Assumption 50 percent discordant pairs

How Does Assumption Hold? PA discordant pairs: 70 percent (LW) and 80 percent (SW) WA discordant pairs: 66 percent (LW) and 84 percent (SW)

Data Collection-1

Data Collection-2

Crash Data 5 years of PA data 6 years of WA data

Roadway Data

(PA and WA) Number of Lanes Area Type AADT Segment Length Speed Limit Surface Width Paved Shoulder Width

(WA only) Horizontal Curvature Vertical Curvature

(PA only) Unpaved Shoulder Width District

Data Collection-3

 Variable

PA Cases (total crashes)

PAControls (total crashes)

WACases (total crashes)

WAControls (total crashes)

PACases (target crashes)

PAControls (target crashes)

WACases (target crashes)

WAControls (target crashes)

Length (ft) 2598 2578 1936 1874 2628 2609 2059 1996

AADT 3921 3701 4693 4398 3530 3363 4133 3970

Speed(mph)

47.36 48.20 51.04 51.18 47.61 48.26 51.76 51.65

LaneWidth (ft)

11.21 11.19 11.55 11.61 11.02 11.10 11.49 11.61

ShoulderWidth (ft)

2.96 3.14 4.95 5.23 2.82 3.02 4.81 5.33

Data Collection -4

PavementWidth

LaneWidth

ShoulderWidth

PA Sample (Total crashes)

WA Sample (Total crashes)

PA Sample (Target crashes)

WA Sample (Target crashes)

26' 10‘ 3' 4,838 352 3,550 60

26' 11' 2' 4,443 2,001 3,134 522

26' 12' 1' 196 173 148 53

28' 10' 4' 4,024 225 2,803 20

28' 11' 3' 6,756 2,581 4,601 686

28' 12' 2' 1,485 591 994 161

30' 10' 5' 567 84 393 17

30' 11' 4' 10,156 2,388 6,622 526

30' 12' 3' 2,156 1,479 1,420 429

32' 10' 6' 406 63 250 12

32' 11' 5' 2,960 778 1,932 206

32' 12' 4' 4,859 2,358 3,107 640

34' 10' 7' 84 4 54 1

34' 11' 6' 2,677 1,190 1,667 277

34' 12' 5' 1,242 906 720 242

36' 10' 8' 75 61 42 14

36' 11' 7' 294 403 188 115

36' 12' 6' 1,577 1,691 954 454

Total     48,795 17,328 32,579 4,435

Evaluation Results (PA Total Crashes)

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

2.0

10 11 12

Lane Width Category (ft)

Crash Risk 26' PW

32' PW

36' PW

34' PW

30' PW

28' PW

Evaluation Results (PA Target Crashes)

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

2.0

10 11 12

Lane Width Category (ft)

Crash Risk26' PW

32' PW

36' PW

34' PW

30' PW

28' PW

Evaluation Results (WA Total Crashes)

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

2.0

10 11 12

Lane Width Category (ft)

Crash Risk 26' PW

32' PW

36' PW

34' PW

30' PW

28' PW

Evaluation Results (WA Target Crashes)

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

2.0

10 11 12

Lane Width Category (ft)

Crash Risk

26' PW

32' PW

36' PW

34' PW

30' PW

28' PW

Example Odds Ratio

Total Paved

Width (ft)LW(ft)

SW(ft)

OddsRatio

StandardError

P value Lower

Confidence Level.

LowerConfidence

Level

32 10 6 1.000 * * * *

32 11 5 1.419 0.197 0.012 1.081 1.863

32 12 4 1.366 0.185 0.021 1.047 1.783

)β,Cov(β*2-)Var(β)Var(β)βse(β j1

i1

j1

i1

j1

i1

)βexp(βRatio(ij) Odds j1

i1

)]βse(β*1.96)βexp[(βC.I. 95% j1

i1

j1

i1

Conclusions

Within Pavement Width PA: Not many significant changes

• Particularly for total crashes WA: General decrease in crashes for narrow lane and wide

shoulder• Target crashes in particular• Be aware of small samples

Within Lane Width General decrease in crashes as shoulder increases

• Supports model results (consistent with prior studies)

Preliminary Results! Need to explore outliers Answer secondary questions

Future Research

Field-verify sites in PA Preliminary data verification using PA video logs

Evaluate Anomalies Met with PennDOT to discuss results Review PennDOT and WSDOT design guides

Secondary Questions Do effects vary by:

• Traffic volumes?• Speed limit?

Economic analysis

QUESTIONSor

COMMENTS