When winter weather hits many areas of the country bridges and overpasses can become icy and dangerous. If you’ve ever been in a vehicle during freezing conditions and passed the road sign that said “bridge might be frozen ahead” then you know that it is important to take extra precautions while driving on these structures. But what makes it so that bridges and overpasses freeze before roads? The reason is actually quite simple.

When cold air passes over a bridge it is cooled from above and below. This causes it to lose heat and ice up quickly. On the other hand, adjacent roadways have a surface that traps in heat and it takes longer for them to reach the freezing temperature.

Another factor that makes bridges prone to icing up first is their construction. Most bridges are made of metal and steel which are known to conduct heat very well. This means that any heat that a bridge absorbs quickly radiates back out into the surrounding air, cooling it faster than roadways which are often made of asphalt.

In addition to these factors, bridges also have the additional factor of their location. Many bridges are built over rivers, creeks, and bodies of water which freeze very quickly when it gets cold. These frigid areas under the bridge will cause it to quickly cool and then ice up, again faster than roadways which do not have these cold patches beneath them.