Mobility and Safety Impacts of Winter Storm Events in a Freeway Environment

This paper describes how data from several Iowa information management systems were used to analyze the mobility and safety impacts of winter storm events. Roadway and weather data were acquired from the roadway weather information system (RWIS), hourly traffic volumes from automatic traffic recorders (ATRs), and crash information from the accident location and analysis system (ALAS). Daily snowfalls were acquired from state and national agencies. Storm and non-storm data for seven interstate roadway segments were considered. Only winter storm events with a duration of four or more hours and a snowfall of 0.51 cm/hr (0.20 in./hr) or more were evaluated. Analysis of the data revealed the impacts of winter weather on freeway traffic. Winter storm events decrease traffic volumes, but the impact is highly variable. The average winter storm volume reduction was approximately 29%, but ranged from approximately 16 to 47%. A positive relationship was found between percent volume reduction, total snowfall, and the square of maximum gust wind speed. Crash rates also significantly increase during winter storm events, possibly the result of a large decrease in traffic volumes and higher crash reporting rates during winter weather. After controlling for exposure, an increase in snowfall intensity and snowstorm duration also increased winter storm event crash frequency. The results of this research can help determine the potential impacts of winter weather, support the eventual development of a dynamic winter weather driveability level of service system, and assist with planning preventive and emergency operations.