Why bridges ice before roads

You may have seen the signs that tell you a bridge will ice before roadways, but why is that?

Posted: Nov. 26, 2018 4:00 PM
Updated: Nov. 26, 2018 6:03 PM

TERRE HAUTE, Ind. (WTHI) - We've all seen the signs that bridges ice before roadways, but why?

What makes the big difference between the two?

First, we need to talk about the difference between how each holds heat.

The roadways on either side of the bridge tend to hold heat better because it takes the heat longer to leave the ground beneath the road.

Since there's usually nothing below a bridge, cold air can move freely over, around, and under the bridge.

In situations where we have winter weather moving through, the cold air moves past the bridge quickly, making it cool down much quicker than the road leading up to the bridge.

The road still cools down, but at a much slower rate than the bridge itself.

Bridges are also usually made of concrete or metal, which conducts cold very well.

The colder the surface of the bridge, the easier it will hold ice and become slippery.

Pair all of these factors together, and you'll almost always see ice on the bridge before you do on the road.

Remember, slow down at bridges in the winter, and if you start to slide, pump your brakes, don't hold them.

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