JohnQ
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All of us who live in or travel through regions where the temperatures dip below freezing have noticed that bridges freeze before the rest of the road. We also get the warnings every year from the weather services and highway departments to watch for ice on bridges but they never tell us why only on bridges. The answer to why ice forms on bridges before the rest of the road is no mystery. There are actually several reasons and all are based on pure science.
The first reason why bridges freeze first is because bridges are open to the cold air from all sides. Temperature is transferred to roadways and bridges primarily through the air. The regular road is only exposed to the air from the top. Also, the ground is a great source of insulation. Warmth is held deep in the ground and that warmth transfers to the underside of the road. It takes a long time for the cold air and colder surrounding ground and water to get the road to a freezing temperature. Since the bridge is directly exposed to the air from the top and bottom, it freezes first.
The second reason why bridges freeze faster than the road is because of what bridges are composed of. Bridges are usually supported by metal beams. Even those bridges made of concrete are reinforced by steel rebar in the center of the concrete. Metal transfers temperature faster than soil and rock. Comparing two bridges in the same conditions, bridges made primarily of metal will freeze over before bridges made of stone.
Bridges are the site of many accidents in the winter so it is important to remind yourself that it may be icy whenever it is very cold outside. Always assume bridges have ice on them even if you can’t see it. It could be covered by snow or it could be only a thin layer, barely visible. Never accelerate on a bridge. Maintain a safe and steady speed. Also, never suddenly brake. A slow, prolonged braking should be used if you ever need to stop on a bridge.
 Posted 5402 day ago
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