Compared to a roadway, a bridge has more surface area to exchange energy with the atmosphere, and thus will cool down to the air temperature — and freeze — quicker. (Photo credit: Beaver Dam Daily Citizen archives)
Living in a cold climate, we are used to seeing signs that say “bridge freezes before road.”
The fundamental reason is that a bridge hangs above the ground, while the roadway rests on the ground. Water on a road or bridge will freeze once the surface becomes cold enough. So, the bridge must cool faster than the roadway.
Whether something warms or cools is related to its energy gains and losses. So, as you stand facing an evening bonfire, your front warms because it gains more energy than it loses, while your back cools as it loses more energy to the cooler night air than it gains.
The energy losses from a bridge occur along the top surface and also along its side and bottom. Compared to a roadway, a bridge has more surface area to exchange energy with the atmosphere, and thus will cool down to the air temperature quicker.
Many bridges are made of metal and concrete, both of which are good heat conductors. Thus, when cold air comes in contact with the bridge surfaces, heat is quickly transferred from the bridge to the colder air, cooling the bridge and its surfaces.
A roadway also loses heat from its surface to the cold air above. However, the road surface also gains energy from the ground. So, while the roadway will cool down, it does not cool as fast because of the energy gains it gets from the warmer ground below. Because of those extra energy gains, the roadway cools more slowly and doesn’t form ice as quickly as the bridge.
