The effects of inclement weather conditions on freeway traffic operations

Adverse weather reduces the capacities and operating speeds on roadways resulting in congestion and additional productivity loss. Without a solid understanding of the mobility impacts of weather on traffic, freeway operators do not have the estimates of reductions in capacities and speeds to predict and simulate the impacts of traffic management strategies, when faced with inclement weather. Practically all traffic engineering guidance and methods used to estimate highway capacity assume clear weather. For major metropolitan areas in snow-belt states, inclement weather conditions occur during a significant portion of the year. This research classified weather variables by intensity and evaluated impacts of these weather categories on freeway capacity and operating speeds. The study area included Twin Cities metropolitan area freeways in Minnesota. The research database included 4 years of traffic data from roughly 4000 loop detectors and weather data over the same period from the five Road Weather Information System (RWIS) and Automated Surface Observing Systems (ASOS) sensors at three airports in close proximity to the freeway system. Results indicated that severe weather conditions caused the most significant reductions in capacities and operating speeds. Heavy rain (\u3e 0.25 inch/hour), heavy snow (\u3e 0.5 inch/hour), cold temperatures (\u3c-20° Celsius), and low visibility (\u3c0.25 mile) showed 10-17, 19-27, 6-10, and 10-12 percent reductions in capacities. Additionally, significant speed reductions of 4-7, 11-15, and 12 percent due to heavy rain, heavy snow and low visibility were obtained. Speed reductions due to heavy rain and snow were found significantly lower than those recommended in the Highway Capacity Manual 2000 (HCM 2000). Additionally, other weather variables presently not included in the HCM 2000, such as cold temperatures (\u3c-20° Celsius) and low visibility (\u3c0.25 mile) were classified by their intensities and investigated to identify their impacts on freeway capacities and operating speeds