Winter weather: more dangerous than drunk driving?
The Center for Transportation Studies Seventeenth Annual Research Conference, held May 24–25 in St. Paul, featured many presentations of interest to local agencies. This is one of three presentations summarized in this issue of the Exchange.
Laws against speeding, drunk driving, and other irresponsible behavior on the road are designed to protect motorists and pedestrians. But extreme weather is another danger travelers face, and oftentimes the risks it poses are just as high—and in some cases, higher.
To quantify the risks of driving in extreme winter weather, Tom Maze, Iowa State University professor and director of the Center for Weather Impacts on Mobility and Safety (C-WIMS), and research engineer Zachary Hans conducted a study using 10 years’ worth of crash data from the Iowa Department of Transportation. Maze presented findings from their project summary.
The researchers found that in both rural and urban areas, traffic crash rates increase during inclement weather, but the severity of those crashes tends to be higher in rural areas than in urban ones. This is because during snowstorms, traffic in urban areas slows due to congestion, so crashes tend to occur at lower speeds than they do during clear conditions. But in rural areas, where traffic is less congested, drivers can select their own speed, allowing some drivers to travel at speeds too high for the extreme conditions. This results in higher average crash severity during winter weather in rural areas.
They also found that crashes involving winter weather in both urban and rural areas tend to be more severe on higher design standard facilities, like interstates and freeways, compared with lower speed roadways. Although the reasons for this are unknown, they speculate that higher design standard facilities allow drivers more opportunity to drive at speeds unsafe for the conditions.
Severe winter storms increase the risk of being involved in a crash by as much as 25 times—much higher than the increased risks created by behaviors that state governments already have placed sanctions against, like speeding or drunk driving. “Highway agencies might wish to better manage and restrict the use of highways during times of extreme weather,” Maze argued, given the higher crash risks they documented.
Another interesting question Maze posed was whether people need to “re-learn” how to drive in winter weather every year. The study showed that the first snowstorm of the year tends to have higher crash rates and higher rates of crash severity than other snowstorms during the remainder of the winter. The research also found the winter weather-involved crash rates declined over the entire season; this demonstrates that as drivers are exposed to more snowstorms, they become more expert at driving in winter weather conditions. This same trend occurred in data sets from every year the researchers studied, leading them to conclude that drivers do, in fact, seem to forget their lessons about winter driving each year after the snow melts.
Maze concluded that relative winter weather crash frequency and severity is dependent on three factors: the location (urban or rural), the type of roadway, and driver experience with winter weather. Maze’s findings were presented to the Iowa DOT to help officials make adjustments to snowplow routing and schedules and winter maintenance application, as well as to address the winter weather crash risks this research uncovered.
—Marni Ginther, LTAP intern