Have you ever noticed that the roads seem to freeze more quickly when you are crossing a bridge or overpass? If so, you have a good reason to take extra caution.

This happens on clear nights with little wind, when the ground loses its infrared heat to space. The road surface can then rapidly cool and form ice during the night. Adding to the danger is fog which can form over the roadway and obscure the condition of the surface of the road. The resulting surface of icy, untreated pavement can lead to serious vehicle accidents.

You may also notice that certain areas of the road tend to be icy first and remain icy longer. This is especially true of sections that are shaded by trees, hills, or other objects. These areas are colder and retain their moisture longer, which makes them more prone to the formation of ice.

Bridges and overpasses tend to freeze more quickly than roads because of their material composition. A bridge is a structure that has open air beneath it, which means that it loses its heat to the surrounding air on two sides. Roads, on the other hand, have a layer of soil which helps to trap in heat, and as a result, they only lose their temperature to freezing or below much slower than a bridge.

Roadways are also typically made of materials such as steel and concrete, which conduct heat very well, and it takes more time for their temperature to drop to freezing or below. Bridges, on the other hand, are often made of concrete or asphalt cement which are poor conductors of heat.