Why do bridges freeze before roads?

CHICAGO (WLS) -- You hear us say it all the time: watch those bridges and overpasses for slick spots. But why do bridges and overpasses freeze first?

Most roads lose heat evenly at night. But unlike normal roads - which have ground to help insulate them and keep them warmer - cold air can reach above and below the bridges, cooling the concrete on both sides.

This results in quicker heat loss and a faster freeze than normal roads.

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