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PORTLAND -- Freezing temperatures forced the closure of the top deck of Portland's tallest bridge, the Fremont Bridge, for about 45 minutes Thursday morning.

Oregon Department of Transportation officials then admitted, the ice on the bridge was a surprise even to them.

Officials said they were patrolling the city and at 5 a.m. there was no moisture on the road, and then by 6 a.m. traffic cameras and police were seeing cars spinning out of their lanes, and narrowly missing each other.

That's when ODOT completely closed the top deck, shutting down all the on-ramps. De-icer trucks went back and forth over the bridge, spraying magnesium chloride that prevents moisture from sticking. to the asphalt. The 45-minute closure snarled traffic with a backup that lasted through the morning commute.

Background: Icy ramps, bridges snarl Thursday commute

Any bridge surface is going to freeze first because there's no land undernearth it, which holds warmth. There's air underneath it, ODOTspokesman Don Hamilton explained. Bridges freeze first and that's what we saw this morning. Moisture came in and froze on that bridge deck pretty quickly.

Frost covered many streets and cars Wednesday morning, and it had people wondering why the agency didn't use de-icer overnight to prevent the spin-outs.

The forecast didn't have any moisture in it and we were checking through the night, said Hamilton. We don't randomly spray de-icer. It's a real waste of taxpayer money to do that unless we have some good reason to do so.

As for Wednesday night, Hamilton said bridges are ODOT's number one priority and crews will be driving them all night, checking conditions. They will not spray preventative de-icer unless they see moisture forming.

The Fremont Bridge is the tallest in Portland, and officials say it will freeze before other bridges. Trucks also sprayed de-icer on the St. John's bridge Thursday morning, although there were no crashes or spin-offs reported.

Related: Freezing morning temps through the weekend

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