Snow-covered and icy roads can lead to treacherous and potentially deadly driving conditions when winter weather strikes across the U.S., but bridges and overpasses can be dangerous even if precipitation isn’t falling from the sky.
If you've ever driven on bridges and overpasses in areas where winter weather occurs, you may have seen the bright yellow road signs warning drivers that it will become frozen before the road does.
Here's why that happens.
Why do bridges and overpasses freeze before roads?
There are a few reasons why bridges and overpasses will freeze before a road does.
One of those reasons is that the entire bridge structure or overpass is exposed to the elements, whereas only a road’s surface is exposed to freezing temperatures.
The animation above shows that cold air flows across the surface and encompasses the bridge, according to the National Weather Service.
As it does, both the top of the bridge and under it will be exposed to frigid temperatures, leading to a frozen surface much faster than a road surface.
Another reason why a bridge or overpass will freeze before a road is because of its construction materials, according to the Indiana Department of Transportation.
Most bridges are built using metal, which is a great heat conductor. Heat within the bridge will quickly rise to the surface and freeze when it meets the frigid air above and below the structure.
So, even if the bridge's surface looks clear, there could be slick spots on the road, making it extremely dangerous.
On the other hand, roads are usually made with concrete and asphalt, which don’t conduct heat well.
So, warmth beneath the surface of the road will stay there longer, and it won’t freeze as quickly as a bridge or overpass.