News Service

11-15-06

Every winter is a crash course in driving across snow and ice

AMES, Iowa -- Whenever the first snowstorms of the winter blow into the state, Iowa drivers start crashing. But as winter wears on, drivers adjust and the crash rate drops sharply, according to an Iowa State University study of 10 years of crash data.

"As drivers are exposed to more snowstorms they become more expert at driving in winter weather conditions," says a report written by researchers at Iowa State University's Center for Transportation Research and Education. "In other words, drivers tend to relearn how to drive in winter weather conditions every winter and forget what they learned over the summer."

A research team led by Tom Maze, director of the Center for Weather Impacts on Mobility and Safety at Iowa State's Center for Transportation Research and Education, studied crashes on Iowa's primary roads from the winter of 1995/1996 to the winter of 2004/2005. They developed a winter weather crash rate by dividing the number of winter weather crashes by a measure they developed to record drivers' exposure to snow. The resulting numbers show the crash/snowfall ratio peaking in November then dropping sharply in December. The rate makes smaller drops in January and February and steeper drops into March and April.

The researchers -- Maze, Zach Hans, a research engineer for the Center for Transportation Research and Education and former Iowa State graduate students Manish Agarwal and Garrett Burchett -- also reported these findings in their paper, "Weather Impacts on Traffic Safety and Operations":

• A study of traffic on Interstate 35 in northern Iowa showed that drivers stayed home in snowy weather -- especially when winds picked up and visibility dropped. On snowy days with low winds and good visibility, traffic dropped 20 percent. On snowy days with high winds and poor visibility, traffic dropped 80 percent.
• It's the professionals who drive through snowstorms. The study found that commercial vehicles were a much higher percentage of total traffic during snowstorms. Commercial traffic increased by as much as 70 percent compared to traffic counts on clear days.
• Because they're more likely to be out in bad weather, commercial drivers are more likely to be in winter accidents. On Iowa's rural interstates between 1996 and 2005, trucks and trailers were involved in 12 percent of the accidents when roads were snow or ice covered. Trucks and trailers were involved in 6 percent of all crashes on those roads.
• Winter weather crashes on Iowa's interstates happen most frequently in urban areas such as Des Moines, Cedar Rapids and Iowa City.
• The most severe winter weather crashes on Iowa's interstates happen in rural areas.
• Forty five percent of all accidents on Iowa's rural interstates happen during winter weather.
• Thirty six percent of all accidents on Iowa's urban interstates happen during winter weather.

So the scientific studies back up what mothers, the Iowa State Patrol and weather broadcasters say when the forecast calls for snow. Avoid driving in winter weather when you can -- especially early in the season. And if you have to drive on snow and ice, be careful out there.

"Through experience drivers have generally understood that driving in the snow increases their risk of involvement in a crash," said the researchers' report, "but only recently has it been shown that driving during snowfalls of even moderate intensity increases crash risk even more than illegal risky behaviors like driving while intoxicated or speeding."

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Quick look

Now is the time to drive extra carefully when the forecast calls for snow. A paper by researchers at Iowa State University's Center for Transportation Research and Education reports that the rate of winter weather crashes on Iowa's primary roads is highest in November. The rate plummets as drivers relearn winter driving skills.