As the winter driving season kicks off for millions across the U.S., many will come across a sign that cryptically states: “Bridge Freezes before Road.”

(Some variations of the sign might say “Bridge Ices Before Road” or “Watch for Ice on Bridge.”)

While drivers likely understand the sign signals to be careful on bridges during cold weather, the underlying reason might not be as clear. It boils down to a basic principle of physics: Bridges cool down more rapidly than regular roads in cold conditions because they are more exposed to the weather.

Bridges have cold air flowing over them on both the top and bottom, unlike roads which are insulated by the ground beneath them.

This exposure makes bridges and overpasses particularly treacherous. While a main road might appear to be clear, that can change quickly upon entering a bridge or overpass, where the surface could unexpectedly become icy.

This is why many roads have warning signs like “Bridge ices before road” to alert drivers as they approach a bridge, providing them a chance to reduce speed.

Bridges can freeze for a number of reasons

In fact, bridges and overpasses lose heat in several interesting ways, according to atmospheric science professor Clifford Mass from the University of Washington, who provides insights on roadway icing.

Roads tend to maintain higher temperatures than the surrounding air due to heat rising from the ground underneath them.

Even during the night, when temperatures drop, the warmth from the ground can help prevent the surface of the road from icing over.

“Bridges don’t receive heat from the ground beneath them. Consequently, they are particularly susceptible to becoming icy at night compared to regular road surfaces,” he explains.

This vulnerability is especially pronounced early in the winter, when the ground tends to still be warm.

“Temperature variations at the surface can take days or weeks to penetrate more than a few inches into the ground,” Mass adds.