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Highway accident severities and the mixed logit model: an exploratory empirical analysis
John C Milton1, Venky N Shankar, Fred L Mannering
1Washington State Department of Transportation, Urban Corridors, PO Box 47330, Olympia, WA 98504, United States. miltonj@wsdot.wa.gov
Accident; Analysis and Prevention
|January 25, 2008
Summary
This study introduces a new model to analyze accident severity distributions on highways. It reveals how traffic, weather, and road features impact injury levels, improving safety project prioritization.
Area of Science:
- Transportation Engineering
- Traffic Safety
- Statistical Modeling
Background:
- Transportation agencies often prioritize highway safety using accident frequencies.
- Accident severity analysis is typically limited to fatalities, neglecting the full spectrum of injuries (property damage, possible injury, injury).
- A comprehensive assessment of safety improvement benefits requires understanding the complete accident severity distribution.
Purpose of the Study:
- To demonstrate a modeling approach for understanding highway accident injury-severity distributions.
- To investigate the influence of traffic, highway, and weather characteristics on these distributions.
- To enhance the methodology for prioritizing highway safety improvements.
Main Methods:
- Utilized a mixed (random parameters) logit model.
- Incorporated random parameters to account for unobserved roadway characteristics, environmental factors, and driver behavior.
- Analyzed highway injury data from Washington State.
Main Results:
- Volume-related variables (e.g., average daily traffic per lane, truck traffic, interchanges per mile) and weather effects (e.g., snowfall) are best modeled as random parameters.
- Roadway characteristics (e.g., horizontal curves, grade breaks per mile, pavement friction) are best modeled as fixed parameters.
- The mixed logit model effectively captures variations in injury-severity distributions.
Conclusions:
- The mixed logit model offers a promising methodological tool for highway safety programming.
- Accurate prioritization of safety improvements can be achieved by considering the full accident severity distribution.
- Understanding the impact of various factors on injury severity is crucial for effective safety interventions.
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