Unlike roads, bridges and overpasses freeze quickly when temperature drops. They also dry out last.
Bridges cannot keep heat like a road. A road takes advantage of insulation and warmth from the soil below and only loses heat from the surface. A bridge is surrounded by cold air, from both above and below. It cools down much quicker.
When the core of a bridge is made out of steel, the steel acts as an excellent heat conductor and quickly eliminates any differences in temperature, speeding up the process even more.
Whenever the temperature drops, be extra careful when you approach bridges and overpasses.