Why Do Bridges Freeze Before Roads?

Most people who live in cold regions have seen signs that say "bridge freezes before road," but do you know why bridges freeze first?

There are actually several reasons why bridges freeze before roads. The first is that a bridge is exposed from below and above, unlike a roadway. Roads don't freeze over as quickly because they can take advantage of insulation and warmth from the soil below. Bridges, on the other hand, are beset with cold temperatures from all sides, which can cause them to ice over quickly. Bridges also freeze rapidly because many of them are located over cold spots, like frozen rivers or deep ravines.

The third reason that bridges freeze before roads is their construction. The materials used in the core of a bridge are excellent heat conductors. As a result, they draw heat out from the inside of the bridge, transferring it to the surface where it is quickly lost when ambient temperatures are very cold. Bridges often quickly reach the general air temperature, so when the thermometer reads 32 degrees Fahrenheit, the bridge will freeze.

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Door County, WI (1985)
Up to 30 in. of snow over the past 2 days in parts of Door County, WI (NW of Green Bay). Lake squalls.

Johnstown, PA (1986)
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Boston, MA (1861)
46 degrees on this date, -14 degrees on the 8th, and 60 degrees on the 11th.

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