Why do bridges freeze before road surfaces?

(WHTM) – While driving, you may see signs that indicate that a bridge will freeze before a road does in winter weather conditions. But did you ever wonder why that is?

There are two reasons for this, according to How Stuff Works.

When the wind or cold air hits a road surface, it only loses heat at the surface. But when the same air hits a bridge, the cold air hits the top, bottom, and sides of the structure.

While roads can stay relatively warmer due to air that is trapped underneath the surface, bridges have no way to trap heat, since they are exposed on all sides.

The second reason has to do with the materials roads and bridges are made of.

Roads are usually made with asphalt, which is a poor conductor of heat, meaning that heat does not pass through it easily. Because of this, road surfaces can trap heat from the warmer ground below. Most bridges and bridge surfaces are made of concrete and steel, which can allow heat to pass through, also causing the bridges to freeze.

If you find yourself driving on an icy bridge, avoid braking, changing lanes or accelerating on the bridge, since any sudden moves can cause your vehicle to slip and slide.