Why do bridges freeze before roads?

While driving on Ohio highways, you have probably often seen the warning signs advising you that bridges freeze before roads do. You do well to take these warnings seriously, as catastrophic accidents may result from overdriving conditions on an icy bridge, but perhaps you have wondered as you drive along why bridges freeze more quickly than roads.

According to Forbes, the materials used in bridge construction, such as concrete and steel, are typically good conductors of heat. The better a material is at conducting heat, the more quickly its temperature will lower under cooling atmospheric conditions. Air, soil and paving materials such as asphalt are poor heat conductors. A material that is not good at conducting heat is a good insulator, meaning that it is better at holding heat in. 

Bridges cool more rapidly than roads in part because they use materials that conduct heat well in their construction, and when the atmospheric temperature drops below freezing, ice is more likely to form on the bridge. Under the same conditions, however, ice is less likely to form on the road connected to the bridge, in part because the road gains heat from the ground, i.e., the soil, underneath its surface. In other words, the ground serves as an insulator, preventing the road surface from losing heat as quickly as the bridge. 

The measure of how well or how poorly a material conducts heat is thermal conductivity. The higher the thermal conductivity, measured in units of watts per kilometer, the easier the material conducts heat. Asphalt's thermal conductivity is 0.75 W/km, soil's conductivity is 0.2 W/km, while a metal such as copper has a thermal conductivity of more than 400 W/km.

The information in this article is not intended as legal advice but provided for educational purposes only.

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