Identifying traditional and nontraditional predictors of crash injury severity on major urban roadways.

OBJECTIVE: This study identifies and compares the factors that contribute to injury severity on urban freeways and arterials and recommends potential countermeasures to enhance the safety of both facilities. The study makes use of an extensive data set from the State of Florida in the United States. To obtain a more complete picture, this study explores both traditional and nontraditional severity predictors. Some traditional predictors include traffic volume, speed limit, and road surface condition. The nontraditional predictors are comprised of those rarely explored in previous severity studies, including crash distance to the nearest ramp location, detailed vehicle types, and lighting and weather conditions. METHODS: The analysis was conducted using the ordered and binary probit models, which are well suited for the inherently ordered property of injury severity. RESULTS: An important finding is the significance of the distance of crash to the nearest ramp junction/access point, for which the increase in the distance yielded a severity increase at both facilities. Other significant factors included traffic volume, speed limit, at-fault driver's age, road surface condition, alcohol and drug involvement, and left and right shoulder widths. In comparing both facilities, sport utility vehicles (SUVs) and pickup trucks showed a fatality/severity increase on freeways and a decrease on arterials. Furthermore, the detailed list of variables such as crash time provided pertinent severity trend information that showed that, compared to the other periods, the afternoon peak period had the highest reduction in fatality/severity. CONCLUSIONS: Both probit models succeeded in identifying significant severity predictors for each facility. The binary probit model outperformed the ordered probit model based on the higher elasticities (marginal effects) for the fatality/severity probability change, as well as the goodness of fit. As such, this study provides the guidelines for assessing the impact of important roadway and traffic characteristics on crash injury severity along freeways and arterials.