Why Do Bridges Freeze Before Roads?

I often bike past a bridge in my neighborhood that has the “Bridge freezes before road” sign, but I have never felt any temperature difference between the road and the bridge. Is it a frivolous sign?

Bridges do freeze before roads. Bridges are exposed to cold air on all sides whereas roads are exposed to it only above the surface. In cold weather, bridges lose heat from both above and below the surface whereas roads lose heat only from above the surface.  

Bridges are built with steel and concrete, which are good conductors of heat whereas roads are built with asphalt, an excellent insulator. The steel and concrete aggravates heat loss whereas asphalt helps trap the heat radiated from the soil below.

Earth stores heat during warm periods. Soil temperatures can take several hours to cool below freezing even though air temperature has dropped below freezing. This helps keep roads warmer for a longer time than bridges.