Adverse weather degrades the capacities and operating speeds on roadways, resulting in congestion and productivity loss. Without a solid understanding of the mobility impacts of weather on traffic patterns, freeway operators do not have the estimates of speed and capacity reductions to predict and simulate the impacts of traffic management strategies. 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, and various pavement surface conditions on freeway traffic flow for the metro freeway region around the Twin Cities. The research database includes four years of detector occupancy information from roughly 4,000 detectors, weather data over the same period from 3 automated surface observing systems (ASOS) at nearby airports, and two years of pavement surface condition data from 5 road weather information systems (RWIS) sensors in close proximity to the freeway system. Our research classifies the rain and snow events by their intensity levels and identifies how changes in precipitation intensity impacts the speed, headways, and capacity of roadways. Results indicate that severe rain and snow cause the most significant reductions in capacities and operating speeds. Heavy rains (more than 0.25 inch/hour) and heavy snow (more than 0.5 inch/hour) showed capacity reductions of 10%-17% and 19%-27% and speed reductions of 4% 7% and 11%-15%, respectively. Speed reductions due to heavy rain and snow were found to be significantly lower than those specified by the Highway Capacity Manual (2000).
Samenvatting