Have you ever seen that yellow sign before driving over a bridge that says, “Bridge Ices Before Road” and wondered why? A major reason is that cold air surrounds the bridge from above and below, which means that heat within the bridge deck is lost from all sides. Roadways, however, are protected by the ground below which helps trap in heat.
Another reason is that bridges are often constructed with concrete and roads are typically paved with asphalt. Asphalt pavement is much less dense than concrete which means that the asphalt absorbs heat much better than concrete because the heat can penetrate deeper into the pavement. Additionally, asphalt pavement will retain the heat better than concrete and lose trapped heat more slowly than concrete as the ambient temperatures fall.