Transportation experts are also working to find out whether the spectacular disintegration of the heavily used span over the Skagit River, 60 miles north of Seattle, was a fluke or a sign of a bigger problem.
A trucker was hauling a load of drilling equipment Thursday evening when his load bumped against the steel framework over an Interstate 5 bridge. He looked in his rearview mirror and saw the span collapse into the water behind him.
Two vehicles fell into the chilly river. The three people in those vehicles were not seriously hurt.
As authorities try to find a temporary span, motorists will have to use alternate routes during the holiday and for weeks to come.
"You cannot overstate the importance of this corridor to Washington state," Gov. Jay Inslee said. Traffic on I-5 and surrounding roads was backed up for miles, a situation the governor said would continue indefinitely.
Officials were looking for a temporary, pre-fabricated bridge to replace the 160-foot section that failed, Inslee said Friday. That option could be in place in weeks. Otherwise, it could be months before a replacement can be built, the governor said.
The collapse came on the north end of the four-lane bridge near Mount Vernon, about 60 miles north of Seattle.
"He looked in the mirrors and it just dropped out of sight," Cynthia Scott, the wife of truck driver William Scott, said from the couple's home near Spruce Grove, Alberta. "He was just horrified."
The truck driver works for Mullen Trucking in Alberta, the Washington State Patrol said. The tractor-trailer was hauling a housing for drilling equipment southbound when the top right front corner of the load struck several of the bridge's trusses, the patrol said.
Scott, 41, remained at the scene and cooperated with investigators. He voluntarily gave a blood sample for an alcohol test and was not arrested.
The truck made it off the bridge, but the other two vehicles went into the water about 25 feet below.
Bryce Kenning, of Mount Vernon, said the bridge seemed to explode in front of him. The 20-year-old slammed his brakes, but there was nothing he could do.
"It was like time was frozen -- like a roller coaster where you're not attached to the tracks," Kenning said in a phone interview.
Ed Scherbinski, vice president of Mullen Trucking, said in an interview with The Associated Press that state officials had approved of the company's plan to drive the oversize load along I-5 to Vancouver, Wash.
Mike Allende, a state Department of Transportation spokesman, confirmed the truck had a permit.
"We're still trying to figure out why it hit the bridge," Allende said.
State officials approved the trucking company to carry a load as high as 15 feet, 9 inches, according to the permit released by the state. However, the southbound vertical clearance on the Skagit River bridge is as little as 14 feet, 5 inches, state records show. That lowest clearance is outside of the bridge's vehicle traveling lanes, Transportation Department communications director Lars Erickson said Friday. The bridge's curved overhead girders are higher in the center of the bridge.
The bridge has a maximum clearance of about 17 feet.
The permit specifically describes the route the truck would take, though it includes a qualification that the state "Does Not Guarantee Height Clearance."
It's not rare for trucks to strike bridges in Washington state. The state DOT said there were 21 bridge-strikes involving trucks last year, 24 in 2011 and 14 in 2010.
Officials performed a special inspection six months ago on the bridge that collapsed because there were indications it had been struck by a different vehicle.
A report released Friday said the checkup was done due to "impact damage," and inspectors identified tears, deformations and gouges on the northbound side of the bridge.
In that Nov. 29, 2012, impact, an overheight truck struck a metal overhead truss on the bridge, DOT spokeswoman Broch Bender said. An inspection crew determined the bridge to be safe, with only minor repairs required. She said those minor repairs were added to an existing list of bridge maintenance items to be completed at a future date.
There are no signs leading up to the Skagit River bridge to warn about its clearance height. State Transportation Secretary Lynn Peterson said that under federal and state standards, the clearance is tall enough to not require signage.
Inslee said it will cost $15 million to repair the bridge. The federal government has promised $1 million.
The bridge is used by an average of 71,000 vehicles a day.
A Federal Highway Administration database lists the bridge as "functionally obsolete" -- a category meaning that the design is outdated. But it was not classified as structurally deficient.
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