Weather conditions are constantly changing. They can impact drivers, vehicles, roads, and even agencies. This paper explores how weather affects roads, traffic flow, and operationaldecisions.

Weather Impacts on Safety

On average, there are approximately 5,891,00 vehicle crashes per year. More than half of these crashes are weather-related. In fact, weather-related crashes account for approximately 21% of total crashes – nearly 1,235,00 crashes annually. Of these weather-related crashes, about 45% involve wet pavement; another 30% involve slippery surfaces such as ice, snow, or sand. Roughly one out of every five weather-related crashes happens during precipitation.

In addition to being hazardous, weather-related crashes can lead to serious injury or death. For example, in 2017 alone, there were 4,852 deaths and 417,000 injuries related to motor vehicle crashes. An additional 2,945 lives were lost due to weather-related factors, including accidents involving pedestrians, bicyclists, motorcycles, and heavy trucks.

The vast majority of most weather–related crashes happen on wet pavements and during rainfall: 70%, on wet pavement and 46%. During winter conditions, however, the proportion of weather-related crashes that occur on icy pavement is slightly larger: 18%; while 16% of weather-relate crash occur on snowy or slush pavement.

Weather Impacts on Mobility

The National Transportation Safety Board (NTSB) recently published a report about the effects of severe winter storms on vehicle travel. In addition to providing data on the reduction in vehicular mobility during such events, the NTSB also discussed the potential impacts of extreme weather on the operation of traffic signals.

According to the report, capacity reductions can be caused bylane submersion due to floodwater, by lane obstruction due to windblowndebris, and by both. Road closures and access restrictionsdue to hazardous conditions (e.g., large vehicles in high winds)also diminish roadway capacity.

On signalized arterial routes where there are no dedicated lanes,speed reductions can vary from 10 to 25 percent, depending onthe type of precipitation and the length of the storm. Snowfallreductions can range from 30 to 40 percent. Saturatedflow rates can drop by 2 to 21 percent. Start-up delaysincrease by 5 to 50 percent.

Average arterial traffic volumes can decline by15 to 30 percent depending on the severity of the weather event.Travel times can increase by 11 to 51 percent on wetpavement and by 5 to 50 percent on snow-coveredpavement.

Saturation flow rate reductions may rangefrom 2 to 21 percent. These reductions can lead to longer queue lengthsat intersections.

On freeways, light rain can reduce average speedsby 3 to 13 percent. Heavy rain can lower speeds by 3 to16 percent.