As millions of Americans look toward another season of winter driving, they may see a sign with a cryptic warning: "Bridge Freezes before Road."
Or perhaps the sign reads "Bridge Ices Before Road" or even "Watch for Ice on Bridge."
Drivers probably know the sign asks them to use caution on bridges in freezing conditions, but they may not know why. The answer is a simple fact of energy: When it's cold, bridges get colder faster than a roadway because they are more exposed to the elements than a road on land.
Bridges have air flowing over both their top and bottom, while roads have the earth below helping to regulate their temperature.
That makes bridges and overpasses especially dangerous. A road may be clear and free of snow, ice or black ice, lulling drivers into thinking they have nothing to fear. But when they cross a bridge or overpass, the surface can suddenly turn from steady to slick.
