All across America there are signs on bridges that warn motorists that bridges freeze before the roads. What the signs mean is that the road that is a bridge freezes before the parts of the road that are not bridges. I believe that the reason that bridges freeze before roads involves evaporation, but I don't know how evaporation causes bridges to freeze before roads. If rainwater (or any type of water) directly below a bridge evaporates, I would expect the rain vapor to travel straight up into the atmosphere. I don't see any reason why rain vapor directly under a bridge would evaporate and then travel sideways to clear the bridge and then move the opposite direction sideways to become directly over the bridge and then condense back to liquid water and then fall on the surface of the bridge and freeze Why do bridges freeze before roads?