Why do bridges ice before the road?
People in cold climates are used to signs warning that “bridge freezes before road.” Water on surface will freeze once the surface becomes cold enough, but why does the road cool faster?
Warming and cooling result from the net energy flow: gains minus losses. As you face an evening bonfire, your front warms because energy gain outweighs energy loss. For opposite reasons, your back cools, because it loses more energy to the cold night air than it gains.
The fundamental reason why a bridge freezes first is that it loses energy from the top, sides and bottom, while the roadway loses energy mainly from the top. With more surface area to exchange energy with the atmosphere, the bridge cools down to air temperature more quickly.
Also, steel bridges are excellent heat conductors, and so when cold air contacts the steel, rapid heat transfer to the air produces even faster cooling.
Although a roadway also loses heat to the cold air above, it gains energy from the ground, which slows the cooling process. This energy gain is another reason why a road cools and ices more slowly than a bridge.