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As millions of Americans look ahead to a new season of winter driving, they may see a sign with a cryptic warning: “Bridges freeze before roads.”
Or a sign might say, “Ice on the bridge in front of the road,” or “Beware of ice on the bridge.”
Drivers probably know that signs ask them to be careful on bridges in icy conditions, but they may not know why. The answer is a simple fact about energy. When it’s cold, bridges cool faster than roadways because they’re more exposed to the elements than land roads.
Bridges have air flowing both at the top and bottom, while roads have soil underneath to regulate temperature.
Bridges and overpasses are therefore particularly dangerous. The roads are clear of snow, ice, and black ice, which can lead drivers to believe that there is nothing to fear. However, when crossing a bridge or overpass, the road surface can suddenly change from stable to slick.
That’s why many highways have “icy bridges ahead of the road” or similar signs posted to warn drivers approaching bridges or overpasses, giving them time to slow down.
Bridges can freeze for several reasons
Bridges and overpasses actually lose heat in several other interesting ways, according to an informative tutorial on frozen roads by Clifford Mass, a professor of atmospheric science at the University of Washington.
Roads can remain hotter than the surrounding air temperature due to the amount of heat that rises from the ground below.
Even at night, when temperatures have dropped, heat is conducted from beneath the road surface, reducing the chance of the road surface freezing.
“Bridges…receive no heat from the ground below. Therefore, bridges are much more vulnerable to night-time road freezing than normal road surfaces,” he wrote.
This is especially true at the beginning of winter when the ground is still relatively warm.
“It takes days to weeks for surface temperature changes to propagate more than a few inches underground,” Mas said.
