Why do they put up signs that say Bridge may be. Why not on roads also?

Question

BRIDGE MAY BE ICY

Answer 1

It's because bridges and overpasses are above the ground, therefore air is allowed to pass under them. If the air above and below a bridge is cold, then it will keep the bridge cold. Roads are directly on the earth, and there is no air flow beneath them, so they are warmed by the ground.