You may hear meteorologists in the winter time to stay aware of icy bridges or overpasses, or even road signs that say "Bridges Ice Before Roads"... It may have you wondering why bridges ice up so much faster.
First off, we need to know that soil/ground stores heat much very efficiently. Because of that, even though air temperatures surrounding roads drop below freezing, because they have contact with the relatively warm soil, the cooling of road surface temperatures becomes hindered.
The difference is stark for bridge road surfaces though-- since they not only have cold air temperatures above them, but cold air flowing BELOW them as well-- this allows the road surface of the bridge to cool MUCH faster.
Since bridges can come up quickly, you may not have time to slow down in time when you transfer from the warm road surface over soil to the cooler road surface on the bridge.
It's always important, especially when air temperatures are at and below freezing when precipitation is falling, to slow down. Roads may just be wet, but elevated surfaces can become dangerous very quickly.