One of the busiest freeway ramps in south King County will close for more than three weeks on Monday as crews continue work to fix a major chokepoint in Federal Way.

Starting at 5 a.m. Monday, crews working for the Washington State Department of Transportation (WSDOT) will completely close the ramp from westbound State Route 18 to northbound Interstate 5 until Thursday, Sept. 1. Nearly 10,000 vehicles use this ramp each weekday.

“Many of the alternate routes are already heavily traveled, so we need drivers to plan ahead,” said WSDOT Engineering Manager Messay Shiferaw. “We expect congestion to be the worst during the first few days as drivers get used to the closure.”

A 2.5-mile detour will direct westbound SR 18 drivers onto northbound SR 99 to South 320th Street. Drivers coming from Auburn can use northbound SR 167 and westbound SR 516 to reach I-5. WSDOT is working with the cities of Federal Way and Auburn to time traffic signals on alternate routes and keep traffic moving.

During the closure, crews will demolish the existing westbound SR 18 to northbound I-5 ramp, move and reopen it in a new location less than 100 yards away. When it reopens on Sept. 1, the entrance will be located farther east on SR 18. This will make room for a future flyover ramp between westbound SR 18 and southbound I-5.

Starting Sept. 1, westbound vehicles coming from Weyerhaeuser Way will have access to only northbound I-5. The Weyerhaeuser Way to westbound SR 18 on-ramp closed May 31.

The work is part of a $112 million project to ease congestion and improve safety at the I-5/SR 18/SR 161 interchange. Two new flyover ramps will eliminate lane weaving that can lead to congestion and collisions.

Later this fall crews will install large steel bridge girders to support the new flyover ramps above I-5. That work will require occasional overnight closures of one direction of the freeway.

The new flyover ramps will open in mid-2012 and the project is expected to be complete in late 2012.