Analysis of factors affecting the frequency of crashes on interstate freeways by vehicle type considering multiple weather variables

Recently, there has been an increasing trend in traffic crash fatalities in the United States, including the State of Arizona. Given this fact, it is imperative to analyze factors that affect crash frequency, particularly those occurring on high-speed freeways. To address this issue, this study utilizes crash data from interstate freeways to explore factors affecting the frequency of crashes with a focus on the impacts of weather (precipitation, wind, etc.). Negative binomial models with random parameters were estimated to examine the effects of several variables on crash frequency. Characteristics of crashes involving different vehicle types were examined through estimation of separate models for passenger vehicle–, freight-, and motorcycle-involved crashes. The study also utilized different weather data sources which have not been considered jointly in previous research (both simulated and observed weather data), providing new insights on the feasibility of using simulated weather data in traffic safety analyses. The analyses of the impacts of weather on passenger vehicle crashes provided inconsistent results between different weather data collection methods; however, results were consistent for the freight-involved analysis. Ultimately, the findings provide important insights that may be useful in planning countermeasures to improve freeway safety and guide further research.