Every bridge in the state of North Carolina has a sign like this. Why does the bridge ice first?

You may think a bridge is a big heavy thing, but it is only a few feet thick, and on a cold day will be cooled from all directions. Thermal energy is being removed, and unlike the road, there is nothing in contact with it to replace this energy. So it gets cold sooner, and becomes ice-covered when the road is still too warm for water to freeze.

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