During a day when the temperature gets below freezing, one of the most dangerous things to do is drive on a bridge. This is because a bridge freezes before any other part of the road, but why is that the case?
For a normal road, the rate of heat loss is also normal because the road has the ground to help insulate it. The soil just below the surface is warm enough to prevent the road from losing heat too fast.
Bridges, on the other hand, are more susceptible to cold air. When the cold air flows around, above and below bridges, the bridges begin to freeze. Unlike normal roads, there is nothing surrounding the bridge to help insulate it.
With cold air flowing near the bridge and nothing to help insulate it, the bridge loses heat more quickly compared to the road. That is why you typically see a sign saying “bridge ices before road.” The next time you approach a bridge on a day when the temperature is below 32 degrees, remember to take it easy.