‘Bridge ices before road.’ But why? We asked PennDOT

HARRISBURG, Pa. (WHTM) — “Bridge ices before road,” as anyone who has seen countless signs to that effect — just before countless bridges, big and small alike — knows.

But why?

The answer might be somewhat obvious for large bridges spanning wide waterways, sometimes at heights above surrounding roadways.

“But there are bridges pretty much everywhere you go,” said Fritzi Schreffler, a PennDOT spokesperson. “If you see another road going underneath you, you’re on a bridge.”

So why do all bridges — even barely noticeable ones that don’t pass over water and aren’t any higher than the rest of the roadway on either side of them — tend to freeze before the rest of the roadway?

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“It’s because there’s nothing under it. There’s no ground attached to it,” Schreffler said. “Whatever warmth the ground might have is not going to affect that piece.”

Another way to think about it?

“Sometimes the birds are swooping around bridges, and they go up and around them all the time,” Schreffler said. “Well, the wind and the cold air are doing exactly the same thing. It is just wrapping itself around that, and it just allows it to cool that area down so much more quickly than anywhere else.

“Which” — Schreffler continued — “is why you will see maybe the roads are pretty good where you’re traveling, and suddenly you hit one of the bridges going across the Susquehanna, and you see cars are sliding everywhere. It’s because the conditions have changed immediately.”

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