The severity of the holiday weekend’s congestion through the Interstate 5 corridor in Skagit County will depend on how many travelers heed the warnings to avoid it.

“We’re hopeful that people will adjust their plans, but this is one of the biggest travel weekends that we have,” said Washington State Department of Transportation spokesman Mike Allende.

By Friday afternoon, drivers were reporting delays of an hour or more due to the Thursday evening collapse of a Skagit River bridge that normally carries about 70,000 vehicles a day.

Through the day Friday, traffic grew heavier on the detour routes,
and the delays could increase over the weekend, Allende said.

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“It’s going to be slow going out there. … We’re advising people to stay home if they can, or leave as early as possible,” Allende said.

At the same time, there are some people who likely aren’t going to listen: participants, for example, in Bellingham’s annual 93-mile “Ski to Sea” relay Sunday. It will attract as many as 4,000 participants.

Rather than sending all traffic on a single detour route, the WSDOT offered several alternatives.

Northbound traffic, for example, is being rerouted off I-5 at Fir Island Road (Exit 221) south of Mount Vernon. One route going west at that point re-connects with I-5 at Burlington, but drivers headed to Canada are advised to go east and connect with Highway 9.

The detours, however, still have to cross the Skagit River. Most routes send thousands of cars across narrow bridges never intended for that level of traffic.

While acknowledging the concerns, WSDOT spokeswoman Kris Olsen said, “We would not put traffic on a bridge we thought was unsafe.”

She said the department was continuing to tweak the detour routes, and it had done things like synchronize the stop lights through Mount Vernon to move cars along.

By midmorning Friday, traffic was already thick going northbound along nearby South Burlington Boulevard, and by noon, the southbound route was also slowed.

In an effort to ease the crush, Amtrak Cascades announced it would add another round-trip train between Seattle and Bellingham in coming weeks. The railroad, working with the state Department of Transportation, BNSF Railway and Sound Transit, plans to add a morning departure from Seattle.

Amtrak Cascades already provides four trips a day over the Skagit River Rail Bridge, which is fully functional. Amtrak Cascades Thruway Buses add 10 more trips through the area.