Raindrops speckle a car window in December on 16th Street NW in Washington. (Oliver Contreras for The Washington Post)

If it’s raining, whether a drizzle or a downpour, it’s wise to slow down and drive carefully. It could save your life.

A new study shows precipitation of every intensity substantially increases the risk of deadly motor vehicle crashes. These crashes are among the leading causes of accidental deaths in the United States and claim about 35,000 lives each year.

The study, published in the Bulletin of the American Meteorological Society, found a 34 percent increase in the risk of a fatal crash when precipitation is falling.

Its researchers evaluated the connection between 125,012 fatal crashes and the weather between 2006 to 2011 in the United States By mapping radar data to the time and location of the crashes, researchers could analyze links to the precipitation and its intensity

They found the heavier the precipitation, the greater the risk of a deadly crash. Light precipitation increased the risk of a fatal crash by 27 percent, while the risk more than doubled during heavy precipitation.

Scott Stevens, the study’s lead author, found it “surprising” that even light rain bumped up the risk of fatal crash substantially.

“The signal is still present at light rain rates,” he said in an interview. “I think this might be an unappreciated risk.”

Stevens, a meteorologist at NC State University, said the study also found:

  • Rain increases the risk of fatal crashes the most in the morning, close to rush hour. It did not have much effect at night, probably because of decreased traffic.
  • Rain increases the risk of fatal crashes more in rural areas than urban areas, probably because motorists are driving at higher speeds.
  • The risk of fatal crashes during precipitation is much higher in winter than summer, because snow is generally more hazardous than rain.

Examining the data by region, the researchers found precipitation increases the risk of fatal accidents most in the Northern Rockies and Upper Midwest, probably because a significant portion of the precipitation there falls as snow.

Climate change, the study said, could make the situation worse. Precipitation and precipitation intensity is already increasing in the United States, and these trends are projected to carry forward. “[T]he implication is that ever-increasing exposure to heavy downpours may lead to an associated climb in fatal crashes,” the study concluded.

Considering the risk precipitation already poses to drivers, the study said awareness campaigns, improved messaging in forecasts and even “the expansion of variable speed limits that adapt to road conditions” could make traveling safer.