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NCHRP Project 17-35: Evaluation of Safety Strategies at Signalized Intersections

 

 

Evaluations of Low Cost Safety Improvements Pooled Fund Study

 

PPT version for Printing

Safety of Lane/Shoulder Width Combinations on Two–Lane Rural Roads

Header image –Picture shows series of three scenarios: a vehicle on a meandering road, safety personnel at work, and a car that is very badly damaged after it appears to have collided into a telephone pole.

Dr. Frank Gross, Vanasse Hangen Brustlin (VHB), Inc
Dr. 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)
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  Recommended CMF for Shoulder Width (Harwood et al., 2000)
  • 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
Pair ADT Segment 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

  • 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

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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) PA Controls (total crashes) WA Cases (total crashes) WA Controls (total crashes) PA Cases (target crashes) PA Controls (target crashes) WA Cases (target crashes) WA Controls (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
Lane Width (ft) 11.21 11.19 11.55 11.61 11.02 11.10 11.49 11.61
Shoulder Width (ft) 2.96 3.14 4.95 5.23 2.82 3.02 4.81 5.33
 

Data Collection –4

Pavement Width Lane Width Shoulder Width 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)

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Evaluation Results (PA Target Crashes)

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Evaluation Results (WA Total Crashes)

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Evaluation Results (WA Target Crashes)

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Example Odds Ratio

Total Paved Width (ft) LW (ft) SW (ft) Odds Ratio Standard Error P value Lower Confidence Level. Lower Confidence 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
 
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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

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