Why do bridges freeze before roads?

[bluemoonKY]

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?

 

[Greg Bernhardt]

Why do bridges freeze before roads?

There is less contact with the Earth and the Earth is usually warmer than the air which the bridge is exposed to on the top and below.

 

[John Park]

They usually freeze at night by radiating heat into the sky. Roads are in contact with the ground, which has a large heat capacity, so heat can be conducted up from the Earth to make up some of the heat lost. A bridge, on the other hand, is mostly surrounded by air which has a low heat capacity and is a good insulator; so it has no good way to recover the heat it loses by radiation.

 

[rootone]

Last year I slipped and fell down, ( not badly hurt but needed to breathe). because of ice on a small footbridge over a local river.
There was no ice generally around, but temperature was just a degree or two above freezing

 

[sophiecentaur]

but temperature was just a degree or two above freezing

You would need to have measured the temperature in various places - right on the ground, up in the air and even under the bridge. It can vary a lot and it must have been zero or below, where the water actually froze.