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Why are bridges and ramps so slippery during the winter?

Why are so many metro highway ramps still covered in snow?
Why are so many metro highway ramps still covered in snow? 01:53

MINNEAPOLIS -- Snow and chilly weather made for some slick spots on bridges, overpasses, and ramps on Wednesday.

The Minnesota Department of Transportation is in charge of clearing 30,000 lane miles of state highways. It's more than the equatorial circumference of earth.

Just in the metro, there are more than 4,000 lane miles to clear. 

Bridges and overpasses are particularly troublesome because of the air that comes underneath. Ramps have a thiner layer of pavement, which makes them extra slippery because the snow adheres to the ground more quickly.

"If you're coming off a ramp and there's a stop sign there and you're sitting idle, the exhaust pipe drips a bit and those add moisture to the scene and will freeze up if temperatures are right," said Anne Meyer with MnDOT.

It's why some people, like Aspen Cooper, are taking extra precautions. 

"Getting into an accident or spending $10 for an Uber," Cooper said. "I'll take the $10."

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