An Investigation into the Impact of Rainfall on Freeway Traffic Flow

Transportation agencies are seeking to integrate weather data into traffic operations in order to improve system efficiency. In order to do so, it is essential that transportation professionals have a solid understanding of the impact of various weather conditions on traffic flow. The purpose of this research effort was to investigate the impact of rainfall, at varying levels of intensity, on freeway capacity and operating speeds. The findings of this study, derived from traffic and weather data collected in the Hampton Roads region of Virginia, add to the guidance currently available to professionals in the Highway Capacity Manual. The findings are summarized below. • Light rain (intensity of 0.01 0.25 inches/hour) decreases freeway capacity by 4-10% • Heavy rain (intensity of 0.25 inches/hour or greater) decreases freeway capacity by 2530%. • The presence of rain, regardless of intensity, results in approximately a 5.0-6.5% average decrease in operating speeds. These findings indicate that the impact of rain is more significant than currently reported in the Highway Capacity Manual, and therefore indicate the need to carefully examine freeway operations strategies during rainfall events. Smith, Byrne, Copperman, Hennessy, and Goodall 1