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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 Evaluation of STOP AHEAD Pavement Markings

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

Overview

  • Introduction
  • Objective
  • Study Design
  • Data Collection
  • Results
  • Economic Analysis
  • Conclusions

Background on Strategy

  • STOP AHEAD
    • Tried
    • Low cost
    • Short–term implementation
  • Target crashes
    • Right–angle
    • Rear–end
    • Other STOP sign violation crashes
  • Potential Difficulties
    • Visibility in winter
    • Low coefficient of friction
  • Key to Success
    • Maintenance of markings
 View Alternative text

Literature Review

  • STOP AHEAD pavement markings
    • No available literature
  • STOP AHEAD signs
    • Several studies
    • Not particularly effective (Zwahlen, 1988)
View Alternative text

Objective

  • Estimate Safety Effectiveness
    • Total crash frequency
    • Target crash frequency
      • Right–angle collisions
      • Rear–end collisions
      • Injury collisions
  • Questions of Interest
    • Do effects vary by:
      • Traffic volumes?
      • Area type?
      • Number of approach legs?
      • Number of stop–controlled approaches?
    • Is the treatment economically feasible?

Study Design

  • Required Sample Size
    • Minimum: 53 intersection–years
      • Detect 20 percent reduction in total crashes with 90 percent confidence
    • Desirable: 260 intersection–years
      • Detect 10 percent reduction in total crashes with 90 percent confidence
  • Assumptions
    • Number of reference sites = number of strategy sites

Data Collection

View Alternative text

Data Collection – Arkansas

Total number of intersections used = 8

Variable Mean Minimum Maximum
Months before 102.9 82.3 127.3
Months after 32.1 16.7 49.7
Crashes/site–year before 1.60 0.13 7.78
Crashes/site–year after 1.36 0.00 9.00
Injury crashes/site–year before 0.86 0.00 4.44
Injury crashes/site–year after 1.08 0.00 7.50
Right–angle crashes/site–year before 0.82 0.00 5.00
Right–angle crashes/site–year after 0.81 0.00 6.50
Rear–end crashes/site–year before 0.32 0.00 1.78
Rear–end crashes/site–year after 0.04 0.00 0.34
Total Entering AADT before 5,330 407 11,284
Total Entering AADT after 5,588 513 11,850
 

Data Collection – Maryland

Total number of intersections used = 9

Variable Mean Minimum Maximum
Months before 79.0 44.7 107.9
Months after 41.0 12.1 75.3
Crashes/site–year before 3.71 0.22 7.19
Crashes/site–year after 2.83 0.00 7.00
Injury crashes/site–year before 2.18 0.11 4.29
Injury crashes/site–year after 1.34 0.00 3.20
Right–angle crashes/site–year before 1.49 0.00 3.81
Right–angle crashes/site–year after 1.06 0.00 3.60
Rear–end crashes/site–year before 0.58 0.00 1.68
Rear–end crashes/site–year after 0.59 0.00 2.00
Total Entering AADT before 8,094 1,627 14,043
Total Entering AADT after 8,856 1,663 17,263
 

Data Collection – Minnesota

Total number of intersections used = 158

Variable Mean Minimum Maximum
Months before 117.0 36.0 120.0
Months after 26.1 24.0 96.0
Crashes/site–year before 0.04 0.00 0.67
Crashes/site–year after 0.01 0.00 0.63
Injury crashes/site–year before 0.02 0.00 0.40
Injury crashes/site–year after 0.00 0.00 0.20
Right–angle crashes/site–year before 0.01 0.00 0.30
Right–angle crashes/site–year after 0.00 0.00 0.33
Rear–end crashes/site–year before 0.00 0.00 0.10
Rear–end crashes/site–year after 0.00 0.00 0.17
Total Entering AADT before 756 80 6,076
Total Entering AADT after 858 88 6,310
 

Evaluation Results

Aggregate Analysis

States Percent reduction in Right–angle crashes Percent reduction in Rear–end crashes Percent reduction in Injury crashes Percent reduction in Total crashes
Combined Results (AR and MD) –3.6 29.0 21.6 31.1
AR Results 42.1 90.3 31.7 52.3
MD Results –39.0 –1.6 17.6 22.9
MN Results 66.9 67.9 82.2 34.1

Note: A negative sign indicates an increase in crashes.
Bold numbers indicate a statistically significant effect (95% confidence level).

States Standard Error for Right–angle crashes Standard Error for Rear–end crashes Standard Error for Injury crashes Standard Error for Total crashes
Combined Results (AR and MD) (18.1) (18.0) (12.0) (8.0)
AR Results (17.5) (9.5) (18.1) (10.8)
MD Results (31.1) (28.7) (15.4) (10.5)
MN Results (23.4) (32.1) (12.6) (19.3)

Note: A negative sign indicates an increase in crashes.
Bold numbers indicate a statistically significant effect (95 percent confidence level).

Disaggregate Analysis

Disaggregate Group Sites Estimate of Percent Reduction (standard error)
Injury crashes:3–legged 5 54.7 (16.4)
Injury crashes:4–legged 12 11.9 (15.0)
Injury crashes: All–way stop–controlled 7 42.3 (14.9)
Injury crashes: All–way stop–controlled/ Two–way stop–controlled 10 7.7 (17.5)
Total crashes:3–legged 5 60.1 (11.2)
Total crashes: 4–legged 12 23.0 (9.9)
Total crashes: All–way stop–controlled 7 55.9 (9.1)
Total crashes: All–way stop–controlled/ Two–way stop–controlled 10 12.8(12.2)

Note: A negative sign indicates an increase in crashes.
Bold numbers indicate a statistically significant effect (95% confidence level).

Economic Analysis

Determine the Annual Cost of Installation

  • States Provided Installation Costs and Service Life
    • Latex: $140 per approach (2 year service life)
    • Thermoplastic: $1,500 per approach (5 year service life)
  • Convert Installation Cost to Annual Cost
    View Alternative text
    • Latex: $78 per approach/year
    • Thermoplastic: $366 per approach/year

Estimate Crash Costs

  • FHWA Unit Crash Cost Data (Council et al., 2005)
    • $58,832 for undefined collision
    • Includes "hard dollar" and "non–monetary" costs
  • Assume 2:1 Benefit–Cost Ratio
    View Alternative text
  • Required Crash Reduction (2:1 Benefit–Cost Ratio)
    • Latex: 0.005 (Two–way stop– controlled) or 0.011 (All–way stop– controlled)
    • Thermoplastic: 0.025 (Two–way stop– controlled) or 0.050 (All–way stop– controlled)

Conclusions

  • Total Crashes
    • Significant reduction in AR, MD, and overall
  • Right–angle and Rear–end Crashes
    • Significant reduction in AR
  • Injury Crashes
    • Significant at 10 percent level overall
  • Disaggregate Analysis
    • Greater effect for 3–legged (highly significant)
    • Greater effect for All–way stop–controlled (highly significant)
    • Some variation by AADT
  • General Reduction in Crashes
    • Results supported by MN data
      View Alternative text
  • Economically Feasible
    • Low cost strategy
    • Modest reduction to achieve 2:1 Benefit–Cost ratio
    • Necessary reduction is easily achievable

Benefit >> Cost

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