"The impacts of weather on the nation's road system greatly affect safety, mobility, and productivity. Weather affects roadway safety through in-creased crash risk, as well as expo-sure to weather-related hazards. On average 7,130 fatalities and 629,000 injuries occur every year during ad-verse weather conditions. Weather impacts roadway mobility by increasing travel time delay, reducing traffic volume throughput and speeds, increasing speed variance (a measure of speed uniformity), and decreasing roadway capacity (maximum rate at which vehicles can travel). Further, weather events influence productivity by disrupting access to road networks, and increasing road operating and maintenance costs."
Publication no. FHWA-JPO-13-036; Federal Highway Administration Joint Program Office 13-036
Bureau of Transportation Statistics: http://www.rita.dot.gov/bts/