SEATTLE – In just about three weeks, Northbound I-5 through Seattle could become a traffic nightmare.

As part of one of the biggest road repair projects in state history, northbound I-5 will be down to two lanes between Spokane Street and the I-90 interchange as contractors hunt for flaws in the freeway and replace deteriorating expansion joints.

The project is slated for August 10-29.

Unless half the normal commuters take the bus, find another route or simply go on vacation, traffic could back up to Tacoma.

"So we need one in two drivers to do something different. It's not enough to say, 'Most everybody else is going to do it and I'll be OK.' We need you too," said Meghan Stopich, a spokesperson for the state Dept. of Transportation.

During the project, maintenance inspectors will be hunting for bad concrete.

By dropping simple hardware store chains on the road, Bob Ferguson can hear that hollow sound that gives away a bad spot in the pavement.

"It works very well, and this is how it's done," said Ferguson, an inspector.

Everytime a concrete flaw is found, it will be marked with paint and then chipped out and replaced after construction gets underway.

Workers will spend much of the time replacing deteriorating expansion joints.

A similar traffic mess is expected next year on I-90 near Mercer Island, when state road crews will replace five worn-out expansion joints.

That will likely mean several westbound lane closures backing up traffic, but planners aren't sure what dates this will take place.