You may have seen the “Bridge Ices Before Road” signs around the tri-state area on all kinds of roads, from interstates to smaller streets. There’s an important reason for these signs.

Bridges and overpasses are known to freeze during the winter, but why do they sometimes freeze faster than the roads? Both are outside in the same weather conditions, yet they are not always the same temperature. How is that possible?

Bridges are at a higher elevation, allowing air to flow above and below them. This means the colder air can cool the bridge at the same time.

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On top of that, bridges are usually made of steel and concrete. These materials conduct heat very well. If heat becomes trapped in the bridge, it is quickly transferred to the surface and loses heat.

Meanwhile, most roads are made of asphalt, which does not conduct heat very well. The heat trapped within and below a road will likely stay there longer, increasing the time it takes a road to freeze over. The ground and asphalt together help keep road temperatures more stable.

Bottomline, as cold air surrounds the surface of a bridge, it loses heat. Ice rapidly forms as the temperature drops to the freezing point. Major impacts include slick spots and black ice, notorious for causing accidents.