Get set for a honkin' big I-5 mess

From dawn to dusk.

That's how long commuters should expect traffic backups on northbound Interstate 5 this summer while the state Department of Transportation (DOT) closes at least two lanes — all day — for nearly three straight weeks in August.

The extensive lane closures for construction will drastically slow traffic on I-5 and trigger congestion throughout the area.

The DOT expects backups on Interstate 405, the Interstate 90 bridge, Highway 99 and surface streets south of downtown Seattle.

In terms of drivers affected, this is the largest-scale DOT highway closure ever in Seattle, said Jamie Holter, a DOT spokeswoman.

About 130,000 drivers daily use the elevated 1.13-mile stretch of I-5 where construction will take place Aug. 10-29, said Laura Penning, another spokeswoman for the DOT.

The agency hopes half will use different routes during those days, she said. If they don't, the backup could reach Tacoma, the agency said.

The DOT strongly encourages commuters to start planning alternate routes now. The agency will provide maps of those routes and updates on traffic conditions on its Web site to help commuters avoid I-5.

But even if they do, the DOT expects congestion between 4 a.m. and 11 p.m.

DOT officials said the complexity and necessity of the project called for extended closures.

Starting Aug. 10, two to three lanes at a time will be closed all day between the West Seattle Bridge and the I-5/I-90 interchange while crews replace metal joints in the 40-year-old roadway. DOT officials said the joints are outdated and may pop up in the middle of the highway. Crews will also repave the roadway.

"We went through a billion different scenarios," Holter said. "When you take out an expansion joint, a whole series of things happen right after another."

Weekend or night closures were ruled out, Holter said, because replacing the expansion joints is such a complex, time-consuming job.

Along with the I-5 northbound closures, some on-ramps in the vicinity will be closed:

• The Spokane Street on-ramp will close for two weekends.

• The Columbian Way on-ramp will close for the duration of the project.

• One lane of northbound Airport Way South will be for transit and trucks only.

• Northbound I-5 exits to Fourth Avenue South, I-90 and James, Madison and Dearborn streets will be closed periodically.

Other on-ramps also will be closed and northbound I-5 will be reduced to one lane at night.

Planning for this $15 million project began years ago, and Holter said this August provided the window needed to get it done.

"We can't delay this project because of a series of things," Holter said. "Continuous dry weather — we never get dry weather in the spring. The city of Seattle has a lot of projects as well. If we're going to send everybody to the city streets, Seattle can't be paving."

Holter added that the DOT will soon deal with construction on the Alaskan Way Viaduct and will need I-5 to take additional traffic during that construction.

Moreover, the DOT tried to avoid interfering with large public events, but couldn't avoid all of them. There will be six Mariners games during construction.

Manuel Valdes: mvaldes@seattletimes.com or 206-748-5874

An on-ramp that will be closed during I-5 work was incorrectly identifed as the "Columbia on-ramp." It should have been identified as the "Columbian Way on-ramp."