When you are driving on a road, you can often see signs that say “Roads freeze more quickly when they are salted.” You also may have noticed that bridges and overpasses seem to be the first areas to become icy when temperatures drop. Why is this?
Bridges or any elevated roadway freeze more quickly than surface roads because they are surrounded by cold air on all sides. The air that is surrounding the bridge has a much higher freezing temperature than a roadway, so the bridge will lose heat much more quickly and will begin to ice over as soon as the surrounding air reaches the freezing point.
Another reason that bridges and overpasses ice more quickly is because they are usually made of steel or concrete, both of which conduct heat very well. This means that any heat a bridge traps will be quickly lost to the surrounding air, while a roadway will be able to retain some of the warmth it has absorbed from the ground below it.
If a road does have any salt on it, that will also help to prevent the surface from icing because the salt lowers the freezing temperature of water. However, it will still freeze at some point because water molecules organize themselves into crystal structures when they are below the freezing point.