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PPT version for Printing
Low Cost Safety Improvements Pooled Funds Study
Two-way Left-Turn Lanes on Two-lane Roads Results
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| Craig Lyon, Persaud and Lyon, Inc |
Overview
- Background
- Literature Review
- Objective
- Data Collection
- Results
- Economic Analysis
- Conclusions
- Four to Three Lane Conversions
Background on Strategy
- Tried, moderate, long implementation
- Evaluating 2 to 3 lane conversion
- Two installation methods
- IA study also evaluated 2-3 lane
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Literature Review
- Highway Safety Design Model Draft (Chapter 8) provides Accident Modification Factor function for Two-way left turn lane
- Accident rates for 2U and 3T California and Michigan roads (NCHRP Report 282)
| | Accidents/million vehicle miles 2U | Accidents/million vehicle miles 3T | "Accident Modification Factor" ratio |
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| Non-Intersection Commercial | 2.39 | 1.56 | 0.65 |
| Non-Intersection Residential | 1.88 | 1.64 | 0.87 |
| Un-signalized intersection Commercial | 2.11 | 2.43 | 1.15 |
| Un-signalized intersection Residential | 2.88 | 1.91 | 0.66 |
| Total- Commercial | 4.50 | 3.99 | 0.89 |
| Total- Residential | 4.76 | 3.55 | 0.75 |
Objective
- To estimate the safety effectiveness of installation of two-way left-turn lanes on two-lane roads
- Target crashes
- Questions of interest
Data Collection
| | AR | CA | IL | NC |
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| Total Mileage | 13.2 | 6.8 | 6.0 | 21.3 |
| Crashes/site-year before | 7.3 | 9.7 | 50.9 | 5.1 |
| Crashes/site-year after | 5.7 | 6.2 | 35.1 | 4.9 |
| Injury crashes/site-year before | 1.0 | 2.1 | 7.9 | 0.9 |
| Injury crashes/site-year after | 0.7 | 1.1 | 3.2 | 0.6 |
| Rear-end crashes/ site-year before | 2.5 | 4.1 | 28.3 | 1.9 |
| Rear-end crashes/ site-year after | 1.3 | 2.0 | 12.7 | 1.4 |
Aggregate Evaluation Results
| States | Percent reduction in Rear-end crashes | Percent reduction in Injury crashes | Percent reduction in Total crashes |
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| AR | 49.9 | 38.1 | 22.5 |
| CA | 49.4 | 27.5 | 34.1 |
| IL | 42.0 | 53.1 | 12.6 |
| NC | 21.7 | -1.9 | 15.7 |
| All | 38.7 | 26.1 | 20.3 |
| State | Standard errorof
Rear-end crashes | Standard error of
Injury crashes | Standard error of
Total crashes |
|---|
| AR | (7.3) | (11.0) | (5.8) |
| CA | (7.3) | (8.7) | (5.7) |
| IL | (7.6) | (11.9) | (7.3) |
| NC | (7.7) | (14.7) | (4.8) |
| All | (4.0) | (6.8) | (3.0) |
Disaggregate Evaluation Results
| Disaggregate Group | Sites | Estimate of percent reduction
(standard error) |
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| Arkansas – rural | 15 | 51.2 (7.1) |
| Arkansas – urban | 10 | 3.8 (8.3) |
| California – rural | 21 | 50.8 (5.7) |
| California – urban | 10 | -2.8 (13.4) |
| Illinois – rural | 5 | 16.7 (10.5) |
| Illinois – urban | 5 | 9.4 (10.0) |
| North Carolina – rural | 38 | 27.3 (5.5) |
| North Carolina – urban | 40 | -5.0 (8.8) |
Economic Analysis
Comparison of construction costs and accident savings for two-way left- turn lanes (TWLTL) projects
| State | Initial | Initial converted to annual | Cost per mile year of rear end accidents saved Low (unsignalized intersection) | Cost per mile year of rear end accidents saved High non-intersection) |
|---|
| Arkansas | $440,000 | $31,882 | $18,363 | $41,657 |
| California | $500,000 | $36,230 | $25,697 | $58,410 |
| Illinois | $1,780,000 | $128,979 | $124,715 | $283,479 |
| North Carolina | $424,000 | $30,733 | $5,138 | $11,680 |
Cost/mile calculation (based on a 7% discount rate and a 50 year life)
Conclusions
- Significant reductions in total and rear-end crashes
- Observed reductions in head-on, intersection and non-intersection crashes
- Rural sites more effective than urban
- Research needed to answer where urban treatments are most effective
Four to Three Lane Conversions
- Four to Three Lane conversions studied under NCHRP 17-25
- Iowa data for urban conversions
- Previous study using HSIS data
- Empirical Bayes Before-After study employed
- Iowa
- 15 treated sites
- 296 reference sites
- HSIS data
- 30 sites in 8 cities from CA and WA
- 51 reference sites
Results
| Dataset | Mean(θ) | standard error |
|---|
| Iowa | 0.777 | 0.028 |
| HSIS | 0.811 | 0.025 |
| All | 0.801 | 0.019 |
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