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PPT version for Printing
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
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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
| 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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