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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

Increasing Retroreflectivity of STOP Signs Results

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. Bhagwant Persaud, Persaud and Lyon, Inc


Overview

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

Background on Strategy

  • Identified by TAC, not in Guides
  • Low–cost, short–term implementation
  • Target crashes
    • Right–angle
    • Other STOP sign violation
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Literature Review

  • Legibility effects of increasing the retroreflectivity of freeway guide signs (Carlson and Hawkins)
  • No studies on increased retroreflectivity levels of STOP signs

Objective

  • To estimate the safety effectiveness of increasing the retroreflectivity of STOP signs as measured by crash frequency
  • To assess cost–effectiveness
  • Questions of interest
    • Do effects vary by traffic volumes?
    • Do effects vary by land use (i.e., urban/rural)
    • Do effects vary by type of interest (i.e., 3 versus 4–leg)

Study Design

  • Sample Size
    • Minimum 1,076 intersection years per period to detect a 20 percent reduction in right angle crashes
    • Desirable 2,036 intersection years per period to detect a 10 percent reduction in all crashes
    • Assumes 0.44 crashes per intersection per year before strategy of which 0.17 are right angle crashes

Data Collection

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Variable Connecticut (231) South Carolina(108)
Months Before 59.7 100.7
Months After 46.2 42.1
Crashes/site–year before 1.9 2.1
Crashes/site–year after 2.4 2.0
Injury crashes/site–year before 0.7 0.7
Injury crashes/site–year after 0.8 0.6
Right–angle crashes/site–year before 0.5 0.8
Right–angle crashes/site–year after 0.6 0.7
Rear–end crashes/site–year before 0.9 0.7
Rear–end crashes/site–year after 1.4 0.7
Daytime crashes/sites–year before 1.4 1.7
Daytime crashes/site–year after 1.8 1.6
Nighttime crashes/site–year before 0.5 0.4
Nighttime crashes/site–year after 0.6 0.4
Major road AADT before 7,690 9,847
Minor road AADT before 2,033 2,017
Major road AADT after 8,021 10,414
Minor road AADT after 2,122 2,139
 

Aggregate Evaluation Results

States Percent reduction in Right Angle Crashes Percent reduction in Rear–end Crashes Percent reduction in Night Crashes Percent reduction in Day Crashes Percent reduction in Injury Crashes Percent reduction in Total Crashes
CT –5.8 –9.7 6.6 –3.2 6.0 –0.2
SC 7.6 17.5 –4.4 9.1 9.4 5.4
ALL –1.2 –2.2 4.4 –0.1 6.7 1.2
 
States Percent reduction in crashes (standard error)
Standard Error of Right Angle Crashes Standard Error of Rear–end Crashes Standard Error of Night Crashes Standard Error of Day Crashes Standard Error of Injury Crashes Standard Error of Total Crashes
CT 6.2 5.7 5.5 3.6 4.8 3.1
SC 7.6 7.3 10.8 5.3 8.1 4.9
ALL 5.3 4.8 6.0 2.7 4.5 2.7
 

Disaggregate Evaluation Results: Urban versus Rural

Disaggregate Group Sites Percent reduction of all crashes
SC urban 47 13.7
SC rural 61 –2.0
CT urban 190 –2.2
CT rural 41 15.4
 

Disaggregate Evaluation Results 3–leg versus 4–leg

Disaggregate Group Sites Percent reduction
CT 3–legged 172 4.1
CT 4–legged 59 –11.6
CT 3–legged, rural 29 23.1
CT 4–legged, rural 12 –0.2
SC 3–legged 48 15.9
SC 4–legged 60 –5.3
SC 3–legged, urban 20 26.3
SC 4–legged, urban 27 0.05
 

Disaggregate Evaluation: Effect of Minor ADT

Disaggregate Group Sites Percent reduction
SC ≤ 1200 minor AADT 42 24.9
SC > 1200 minor AADT 66 –3.4
CT ≤ 1000 minor AADT 90 14.3
CT >1000 minor AADT 141 –5.1
 

Economic Analysis

  • FHWA cost per crash for unsignalized intersections
    • $13,238 for rear–end
    • $61,114 for right angle
  • $66/year crash savings per intersection required for a 2:1 benefit cost ratio
    • Requires 0.005 rear–end crashes saved per intersection per year
    • Target seems easily achievable – especially under favorable circumstances identified in the disaggregate analysis

Conclusions

  • Significant reduction (17.5 percent) in rear–end crashes in South Carolina
  • Strategy is more effective at lower volumes on the minor approaches
  • Urban versus rural – Strategy tended to be more effective at:
    • Rural installations in Connecticut
    • Urban installations in South Carolina
  • Strategy was more effective at 3–legged intersections
  • No detectable effects for nighttime crashes
  • Strategy has potential to reduce crashes cost–effectively, particularly in situations identified

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