Seattle drivers will face a new detour and more disruptions as the major project enters its next phase.

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Starting Monday, motorists will contend with yet another traffic shift as Seattle reaches the halfway mark in rebuilding its Mercer Street corridor.

The city is spending $164 million to improve its major east-west connection between Interstate 5 and Seattle Center. The goal is not just to untangle the five-decade “Mercer Mess” of winding road lanes, but to create a tree-lined boulevard next to Amazon, UW Medicine and other businesses in the redeveloping neighborhood.

Today’s eastbound-only Mercer Street will be expanded for two-way traffic by the end of this year, project manager Angela Brady says, while nearby Valley Street will be narrowed into a two-lane arterial with bicycle lanes next year.

Weather permitting, Mercer will close this weekend so workers can mark the detour route.

Eastbound traffic on Mercer will shift Monday a few yards to the left, between Eighth and Ninth avenues north. Drivers will temporarily travel on what will eventually become the westbound lanes. Some 37,900 cars and trucks a day go east on Mercer.

For the rest of 2012, it should be slightly easier to reach Mercer from the north and much harder to reach it from South Lake Union. The traffic disruption follows others around town, including new tolls on the old Highway 520 bridge and lane reductions for Highway 99 reconstruction in Sodo.

The Mercer rebuild is partly funded by a $30 million federal stimulus grant, plus $32 million in fees, land and other contributions by landowners who benefit, chiefly Paul Allen’s Vulcan Inc. When it’s all done, official forecasts predict a quicker trip westbound because drivers can go directly from I-5 to Seattle Center, but eastbound congestion heading to I-5 would be similar to today.

The traffic changeover was supposed to happen last fall, but Brady said work was slowed by rain and unforeseen obstacles in the soil, where century-old utilities are being replaced. Workers even found the buried engine of a Model T.

During this year’s detour, “Mercer’s not going to be too bad,” Brady said walking the site Tuesday, while cars blasted her as they rolled through a puddle. “Just from South Lake Union to Mercer, that’s going to be a challenge.”

Here’s what to look for:

Tearing up Fairview: Fairview Avenue North is being rebuilt near the Mercer junction, so just one northbound lane will be available approaching the Mercer/I-5 interchange, worsening afternoon gridlock. But there will be two lanes southbound, so the state can flush morning commuters off I-5 as they exit toward downtown.

Dead ends: As Mercer is rebuilt, there will be no turns onto it directly from northbound Boren Avenue North, Terry Avenue North, or even Westlake Avenue North.

A new turn: A portion of Westlake that has been closed to traffic will reopen, allowing southbound cars to turn left onto Mercer and reach the freeway.

Streetcars: Trains will run normally through the construction zone.

Pedestrian paths: Sidewalks and makeshift paths will be maintained for people to cross Mercer and reach the lake.

Mercer narrows: The right-hand lane of today’s eastbound Mercer, used for right turns by local traffic, is being permanently removed.

Businesses have been nervous about traffic challenges, in particular the effects of tearing up Fairview Avenue and Mercer Street at the same time, said Jerry Dinndorf, president of the South Lake Union Community Council.

“It’s going to be a mess when they change over,” he said, but added that the city “has been very positive in their response.”

The chamber and Seattle transportation staff are discussing schedule tweaks to reduce lane closures on Fairview, as well as ways to promote carpooling and alternate routes, said Dinndorf. The city’s Brady said she is looking at re-timing traffic lights.

“We all support the Mercer Street improvement. We all think it’s going to be a great benefit to the neighborhood,” Dinndorf said. “The goal is to make the pain less during construction.” The area will gain access that hasn’t existed for 40 or 50 years, he said.

Dinndorf said it will be easier to reach Lake Union Park, and with future changes around nearby Highway 99, new transit routes should become possible on Mercer or nearby east-west streets.

The freshly built lanes include a feature that could cause a new variety of “Mercer Mess” in 2013 — pockets on the roadside for drivers to parallel park. Since there are only three lanes each way, traffic could be hindered by drivers stopping abruptly to park.

Parking spaces there are meant to “activate” the new Mercer Street by aiding access to future retail businesses, officials say.

Mayor Mike McGinn, echoing a common view by new-urbanist thinkers, said a row of parked cars will add a buffer between pedestrians and traffic. First Avenue, Westlake Avenue North and Broadway are good examples in Seattle. However, the need for parking buffers is debatable, as the new Mercer has wide sidewalks, as well as street trees.

“City streets have to support a balance of uses,” said McGinn. If parking creates an obstacle, the city can restrict it at peak hours, he said. “Until you actually see it in operation, it is a little bit of conjecture.”

Freight advocates had urged the city to build a fourth eastbound lane. But the city opted for three lanes each way — among other reasons, because a wider road would be difficult for pedestrians to cross. Brady points out that I-5 can absorb only two lanes of merging traffic (three when Express Lanes point north), making the freeway ramps more of a bottleneck than Mercer itself.

“You’re not going to go quicker if you add a couple lanes here,” she said.

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