Whether Weather Matters to Traffic Demand, Traffic Safety, and Traffic Operations and Flow

Weather affects many aspects of transportation, but three dimensions of weather impact on highway traffic are predominant and measureable. Inclement weather affects traffic demand, traffic safety, and traffic flow relationships. Understanding these relationships will help highway agencies select better management strategies and create more efficient operating policies. For example, it was found that severe winter storms bring a higher risk of being involved in a crash by as much as 25 times–-much higher than the increased risk brought by behaviors that state governments already have placed sanctions against, such as speeding or drunk driving. Given the heightened risk of drivers’ involvement in a crash, highway agencies might wish to manage better and restrict use of highways during times of extreme weather, to reduce safety costs and costs associated with rescuing stranded and injured motorists in the worst weather conditions. However, the first step in managing the transportation systems to minimize the weather impact is to quantify its impact on traffic. This paper reviews the literature and recent research work conducted by the Center for Transportation Research and Education on the impact of weather on traffic demand, traffic safety, and traffic flow relationships. Included are new estimates of capacity and speed reduction due to rain, snow, fog, cold, and wind by weather intensity levels (e.g., snowfall rate per hour).