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Why rain is more dangerous than snow

(Photo by Astrid Riecken For The Washington Post)

As the Northeast braces for Storm Jonas, residents of the nation’s Capital prepare for the worst. Literally, the worst:

On Wednesday evening, two days before the blizzard was supposed to strike, an inch of snow dusted the Washington metro area. Thirty-minute commutes stretched into six hours. D.C. Mayor Muriel Bowser apologized for under-preparing and


On Friday, up to 30 more inches of snow is expected to frost the city. Public transportation, including the Metro and buses, close this evening and won’t run again until at least Monday. Grocery stores look thoroughly ravaged. (Reporter’s personal observation: THERE IS NOT A SINGLE EGG FOR MILES.)

This Google map shows the DC region can’t handle an inch of snow

And while the storm is a clear, imminent threat, national data offers an almost comforting message to nervous drivers: Your chances of crashing in snowy weather are lower than you think.

Of the roughly 5,760,000 vehicle crashes in the United States each year, nearly a quarter occur in adverse weather, according to the U.S. Department of Transportation. An estimated 6,000 people are killed and 445,000 are injured in weather-related crashes each year, nine years of data show. The vast majority happen, however, on rainy -- not snowy -- days: 73 percent of crashes happen on wet pavement and 46 percent during rainfall. Only 17 percent happen during snow or sleet and 13 percent on icy pavement.

Of course, the data partly reflect that it simply rains more often than it snows. Some states practically never see a flake.

But even in some places known for persistently snowy winters, rain was associated with more fatal accidents, according to a recent analysis of federal data by the Auto Insurance Center. That's true in the Washington area and even in Alaska, where December to March can bring between 20 and 80 inches of powder -- rain still proved to be deadlier.

Roads in Sarah Palin's homeland might be safer in snow because the state is well-equipped with salt and plows. But if you’re forced to drive this weekend, try to relax and remember an elementary lesson from science.

“Cold snow is not that slippery,” said Cliff Mass, professor of atmospheric sciences at the University of Washington. “Friction depends really on how wet something is. It’s actually not that dangerous sometimes.”

Therefore, tires tend to have better traction in a snow flurry than on rain-slicked pavement. The trouble starts in wet conditions when drivers turn or brake quickly.

Mass said the first rain in Seattle, where he lives, is “an accident land.”

Melting snow, though, can deliver the same chaos.

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