Weather and Its Impact on Urban Freeway Traffic Operations

Adverse weather reduces the capacities and operating speeds on roadways, resulting in congestion and productivity loss. A thorough understanding of the mobility impacts of weather on traffic patterns is necessary in order to estimate speed and capacity reductions. Nearly all traffic engineering guidance and methods used to estimate highway capacity assume clear weather. However, for many northern states, inclement weather conditions occur during a significant portion of the year. This paper describes how the authors quantified the impact of rain, snow, extreme cold, wind, poor visibility, and pavement surface conditions on freeway traffic flow for the metro freeway region around the Twin Cities. The research database includes four years of traffic data from in-pavement system detectors, weather data over the same period from 3 automated surface observing systems (ASOS), and two years of available weather data from 5 road weather information systems (RWIS) sensors at the freeway’s roadside. Our research classifies weather events by their intensities and identifies how changes in weather type and intensities impact the speed, headways, and capacity of roadways. Results indicate that severe rain, snow, and low visibility cause the most significant reductions in capacities and operating speeds. Rain (more than 0.25 inch/hour), snow (more than 0.5 inch/hour), and low visibility (less than 0.25 mile) showed capacity reductions of 10%–17%,19%–27%, and 12% and speed reductions of 4%–7%, 11%–15%, and 10%–12%, respectively. Speed reductions due to heavy rain and snow were found to be significantly lower than those specified by the Highway Capacity Manual (2000)