Low Cost Safety Improvements Pooled Funds Study

Increasing Retroreflectivity of STOP Signs Results

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

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 20percent reduction in right angle crashes Desirable 2,036 intersection years per period to detect a

10percent reduction in all crashes Assumes 0.44 crashes per intersection per year before

strategy of which 0.17 are right angle crashes

Data Collection

Data Collection

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

Data Collection

Variable Connecticut (231)

South Carolina

(108)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

Aggregate Evaluation Results(Continued)

State

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 Group SitesPercent reduction

of all crashes

SC urban 47 13.7SC rural 61 -2.0CT urban 190 -2.2CT rural 41 15.4

Disaggregate Evaluation Results: Urban versus Rural

Disaggregate Group Sites Percent reduction

CT 3-legged 172 4.1CT 4-legged 59 -11.6CT 3-legged, rural 29 23.1CT 4-legged, rural 12 -0.2SC 3-legged 48 15.9SC 4-legged 60 -5.3SC 3-legged, urban 20 26.3SC 4-legged, urban 27 0.05

Disaggregate Evaluation Results 3-leg versus 4-leg

Disaggregate Group Sites Percent reduction

SC < 1200 minor AADT 42 24.9SC > 1200 minor AADT 66 -3.4CT < 1000 minor AADT 90 14.3CT >1000 minor AADT 141 -5.1

Disaggregate Evaluation: Effect of Minor ADT

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 (17.5percent) 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

Conclusions

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

QUESTIONS???