Have you ever seen this sign on a road? Ever wondered why the bridge ices up first? There are actually two reasons.
The first reason is that the underside of the bridge is exposed to the elements. Cold air gets all around the bridge structure, making any water on the surface freeze. The road surface is protected by the soil underneath. Eventually, the road becomes cold enough for the wet surface to freeze.
The second reason is that bridges and roads are made from different materials. Bridges are made mostly of steel and concrete. Both of these materials conduct heat well. If you heat them, they get hot fast. If you cool them, they get cold fast. So, once the air temperature drops, the bridge starts losing heat, leading to relatively quick formation of ice. The road is made of asphalt, which holds heat in. Therefore, it takes a long time for the road to become cold enought to freeze.