Seattle’s road-maintenance director, Steve Pratt, hoped to outfox the freeze.

By sprinkling salt solution on the West Seattle Bridge corridor at noontime Monday, the city would make driving safer for thousands of football fans and commuters passing through later Monday afternoon.

Alas, the de-icer itself caused a few cars to slide and crash. Police closed the bridge from around 1 to 3 p.m., in both directions.

“I’m going to put this in the category of ‘no good deed goes unpunished,’ ” Pratt said afterward.

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He said there were three, minor-injury crashes on the curve next to the Nucor Steel mill, where drivers turn downhill to cross the high bridge eastbound. The crashes were reported beginning at 12:37 p.m., according to Real Time 911.

Many years ago, that curve went untreated, and on any frosty day, vapor from steelmaking would condense on the concrete, turn to ice and cause spinouts. Nowadays, such crashes are rare. Seattle has applied salt brine or salt compounds for the past four years, without causing spinouts.

“The only thing I can assume is, perhaps the (road) deck is a little too warm,” Pratt said.

The Seattle Department of Transportation (SDOT) was hearing forecasts of a 1 p.m. cold front, but the air just before noon was 43 degrees and the pavement 38 degrees, he said.

The de-icer used Monday is called FreezGard, an opaque brown liquid containing magnesium chloride. It takes five to 10 minutes to crystallize on the pavement, said Pratt.

Immediately after it’s sprayed, the solution is slightly slippery, “particularly if vehicles are exceeding the posted speed limit,” said SDOT spokesman Rick Sheridan. The posted limit is 40 mph near the steel mill, 45 mph on the bridge.

A 2010 test by Spokane County deputies, after a serious-injury crash, found that liquid de-icer made pavement slightly more slippery — similar to after a rainfall. Usually the de-icer is sprayed on overnight, when there is less traffic. Pratt said the city will think twice before applying it again in the daytime. Another idea is a rolling slowdown, where a vehicle following the de-icing truck delays traffic behind it for several minutes.

But on Monday, Pratt was haunted by Nov. 22, 2010, when freezing rain stranded afternoon traffic on the Alaskan Way Viaduct, and salting crews couldn’t maneuver around the stopped cars to defrost the decks.

Pratt, who was doing other work in West Seattle on Monday, said he drove onto the high bridge from the Delridge Way onramp, trouble free. But police closed the entire high bridge, including the Delridge onramp, as an apparent precaution. Road crews dropped sand to improve traction, before the bridge reopened.

Ice storms often baffle government agencies in the Seattle area, where hills and rapidly changing air currents make conditions fickle.

The city has applied liquid salt compounds since December 2009, after the city’s slow ice response in December 2008 when SDOT avoided salt, and some streets took days to thaw.

Mike Lindblom: 206-515-5631 or mlindblom@seattletimes.com. On Twitter @mikelindblom