This confusing road sign warns of a hidden winter weather danger

As millions of Americans begin 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.

More: Black ice, a winter danger 'It happened so fast': Black ice turns roads into invisible skating rinks

That's why many highways sport "Bridge ices before road" and similar signs to caution drivers approaching a bridge or overpass, so they have time to slow down.

Bridges can get icy for several reasons

Actually, bridges and overpasses lose heat in a couple of other interesting ways, according to a helpful tutorial on roadway icing by University of Washington professor of atmospheric science Clifford Mass.

Roadways can stay warmer than the surrounding air temperature because of the amount of heat coming up from the ground below.

Even at night when temperatures fall, that heat conducted from below the road’s surface can reduce the potential for surface icing.

“Bridges ... do not receive heat from the ground below.  Thus, bridges are much more vulnerable to roadway icing at night compared to normal road surfaces,” he wrote.

That’s particularly true early in the winter when the ground is still relatively warm.

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“Temperature changes at the surface take days or weeks to extend more than few inches into ground,” Mass notes.

This article originally appeared on USA TODAY: Why do road signs warn about icy bridges? Confusing signs explained.