Bridges Freeze Before Roads
If you live where it's cold (and we sure do), you frequently see this sign. Hopefully you apply caution especially when the temperatures are approaching freezing (32 degrees).
But just why is this so and what can you do about it?
The first reason is that bridges are suspended in mid-air. Freezing wind strikes the bridge from above and below, and on both sides. Roads on the other hand are only losing heat from their surface. And even while the temperature on the road surface is dropping, the ground under the road retains heat which is enough to prevent icing as temperatures in the atmosphere drop below freezing.
The second reason is that most bridges today are built with steel and concrete. These materials are good heat conductors. This means that any heat that is contained in the materials themselves travels to the surface where the heat is lost through the air flow around it. Roads are mostly made from asphalt, which is a poor conductor of heat which means they retain heat longer which delays freezing on the surface.
So for your safety, slow down when driving on bridges in icy weather. Sometimes ice can be hard to see, especially at night when the temperatures are dropping fast.