Mercer Street will carry two-way traffic for the first time since 1968, but much more work remains to be done in the neighborhood.

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The “Mercer Mess” as you’ve experienced it for 44 years will come to an end Monday, when Seattle converts the busy eastbound corridor to a six-lane, two-way boulevard.

Drivers leaving Interstate 5 will continue straight toward Seattle Center, instead of turning right and snaking along Valley Street.

At the same time, sidewalks as wide as 20 feet will help pedestrians and bicyclists reach the growing campuses of Amazon and UW Medicine.

Automobile congestion eastbound toward I-5 isn’t expected to improve much compared with delays on the former eastbound-only street, but westbound drivers leaving I-5 should save a couple of minutes, the city’s traffic studies predict.

Even as Mercer’s new lanes are done, pavement and utility construction on Fairview Avenue North will intensify, making it harder to reach I-5 after work. Fairview shrinks to one lane northbound, from Harrison Street to Mercer, for several weeks. To compensate for this choke point, traffic police will help drivers navigate side streets — and two crosswalks will close so drivers can make an easier right turn to Mercer, en route to I-5 or the Eastlake neighborhood.

Transit bus routes 70, 71, 72 and 73 will move out to Eastlake Avenue East instead of Fairview.

The transition happens at 5 a.m. Monday, after a weekend construction closure.

The $164 million project continues into 2013, when Valley Street will be rebuilt as a two-lane arterial plus bikeways. The greener street will complement Lake Union Park, where the Museum of History & Industry reopens in December in the former Naval Reserve Building.

Another $98 million “Mercer West” project will stretch the new two-way boulevard under Aurora Avenue North, reaching the Lower Queen Anne area.

This stretch of Mercer Street has pointed eastbound only since November 1968, shortly after the state opened new freeway ramps to and from I-5. A mess immediately followed — so the city tried to relieve the pressure by letting eastbound cars take the entirety of Mercer Street, while diverting westbound cars to Valley Street.

A photo of the earlier two-way Mercer shows a traffic cop swarmed by weaving cars, as The Seattle Times’ Dick Moody mentions the possible solution of a crosstown Bay Freeway, never built. But his Oct. 10, 1968 Troubleshooter column presciently warned: “There is, however, no scheme announced for correcting soon the sorting-out pattern observed here.”

Until 2012.

Mike Lindblom: 206-515-5631 or On Twitter @mikelindblom.