Many bridges are made of metal and concrete, both of which are good heat conductors. 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. The energy losses from a bridge occur along the top surface and also along its side and bottom.
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. Whether something warms or cools is related to its energy gains and losses. So, the road must cool faster than the roadway.
Water on a road or bridge will freeze once the surface becomes colder enough. 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.
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