Four eastbound lanes of Mercer Street will squeeze into three lanes of newly poured concrete. Nearby Fairview Avenue North is also being torn up and rebuilt this year.

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Just about everybody who came of age driving through Seattle has fantasized at least once about how to fix the Mercer Mess.

Politicians once considered a so-called Bay Freeway spur to link Interstate 5 directly to Highway 99, but voters killed that idea in 1972. For 40 more years, motorists continued to plod through Mercer Street stoplights to reach I-5. Even the Sonics’ move to Oklahoma City, and the reduced traffic from KeyArena, didn’t reduce the street’s popularity.

Mercer remains a workhorse that carries 39,400 vehicles a day eastbound, including a recent surge of morning commuters out to I-5 and Highway 520.

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But starting Monday, the mission of Mercer changes.

From now on, instead of just moving cars past South Lake Union, the city’s goal is to move people into South Lake Union, one of the fastest-growing neighborhoods in Seattle’s history.

For many drivers, the pangs of change will finally become real when construction detours begin Monday. Four eastbound lanes of Mercer Street will squeeze into three lanes of newly poured concrete. Nearby Fairview Avenue North is also being torn up and rebuilt this year.

The final product will be a six-lane, two-way boulevard, lined by trees, huge sidewalks and parking spaces. The spiffy new route is meant to serve and attract pedestrians, new residents and companies to a hub that already employs thousands at Amazon and biotech firms.

What it won’t do is add significant lane capacity or save time for most people driving through.

Still, business coalitions in the area support the project, noting that it’s not advisable or even possible at this location to build more road lanes that would invite a significantly greater influx of cars.

But some industrial advocates, and quite a few gridlocked drivers, think the idea of spending $164 million and not gaining road capacity is folly.

After the project is done next year, the city’s own transportation study says traffic for eastbound drivers will stay at “Level E or F,” meaning delays of at least 50 seconds per intersection.

Trips west from I-5 to Seattle Center should be a couple minutes faster, and safety should improve for bicycling and walking. Supporters hold out hope for more transit.

“They weren’t going to ‘fix’ the Mercer Mess. That was never a rational outcome,” says Mark Hallenbeck, director of the Washington State Transportation Center. The land is simply too valuable to waste on expanses of additional road lanes, he said.

“Making it a pleasant place to work and live — in that it’s succeeding, but not congestion.”

Growth or roads?

Just a few days ago, Skanska USA announced plans for a 12-story office building on Fairview, between Harrison and Republican streets. Amazon is finishing its final campus building, while UW Medicine expands its research center.

Nearly 400 apartments are under construction, and developers are seeking permits for an additional million square feet of offices.

Paul Allen’s Vulcan, the largest landowner, estimates the area has attracted 15,000 new office and biotech employees, and hundreds in retail and restaurants — just the halfway point toward full build out.

To move these newcomers, government so far has delivered one streetcar line that’s gaining ridership and a two-way conversion of Westlake Avenue North, and is finally building its long-awaited Mercer Corridor Project.

A second, $99 million Mercer West phase from Dexter Avenue North to Elliott Avenue West is supposed to break ground this year, featuring a six-lane, two-way underpass at Aurora Avenue North. It would open by late 2015, about the same time the Highway 99 tunnel would be done.

“We believe the plans will be successful, but obviously the work is not yet complete. The infrastructure in SLU has long been neglected — for decades — and we’re still playing catch-up,” Vulcan spokesman David Postman said in an email.

City Council members have described the boom in South Lake Union as nothing less than a northward expansion of downtown.

Based on the analogy, SLU isn’t up to speed. Downtown would grind to a halt without most employees using transit, carpools, ferries, bikes, walking, or telecommutes instead of driving alone — about 56 percent, says a study for Commute Seattle.

The non-car share for South Lake Union is nowhere near that high, said Brian Kemper, interim traffic engineer for Seattle Department of Transportation.

“It’s something we need to build toward,” he said. The city under mayors Paul Schell, Greg Nickels and now Mike McGinn has promoted a South Lake Union growth strategy, but the growth is coming faster than local, regional and federal money.

Peter Philips, president of the pro-industry Seattle Marine Business Coalition, faults city leaders for tearing up multiple corridors without a coherent strategy to keep traffic moving.

Sodo work on Highway 99 began before a key First Avenue South ramp to upper Spokane Street was ready. Highway 520 may bottleneck at Foster Island for years, until the state finds money to build the Seattle landings for the new six-lane bridge.

The east side of the Mercer project is under way before funding is secured to build the part west of Dexter.

“We are experiencing the new normal. We are embarking on a whole lot of congestion, and development projects, at the same time,” Philips said.

What would success look like?

About 80,000 cars, trucks, transit riders, cyclists and pedestrians now pass through the area daily, the city says. Travel there would grow, without adding many cars.

Experts say volumes east on Mercer toward I-5 would not change much from the current 39,400 trips per weekday. They could not, because the freeway entrance is saturated.

“Free flow of vehicles is sort of an oxymoron there,” said Councilmember Richard Conlin, back in 2008. “We’re making a commitment to make this a neighborhood. Neighborhoods don’t have freeways running through them.”

Adjoining streets would be more crowded, as people travel to work or homes south of Lake Union. The future direct route west on Mercer would be simpler than winding west nowadays, along bumpy Valley Street.

“This will be an enormous help to us, to have Mercer straightened out,” said Ann Farrington, creative director for the Museum of History & Industry, which is moving to the refurbished Lake Union Park this year.

Seattle DOT’s Kemper offers a bullish vision of the future road grid, when streets that dead-end at Aurora Avenue North are extended east as planned, by 2016, when that part of Aurora is converted into the Highway 99 tunnel:

Thomas Street would remain a calm lane suitable for pedestrians, cyclists and the streetcar.

Harrison Street would carry buses directly from the Space Needle to the middle of office campuses, maybe continuing up to Capitol Hill. Long commute routes, say Queen Anne to Overlake, could run on Mercer, with stops alongside its extra-wide sidewalks.

Republican Street and Mercer would cater to drivers, deliveries and taxis.

State experience suggests even though I-5 capacity is maxed out, at 300,000 trips a day, over the years more people utilize it by taking transit or carpools. A similar evolution is possible at Mercer, said Mark Bandy, a traffic engineer for the state Department of Transportation.

But many pieces of this puzzle remain scattered, to say the least.

King County Metro Transit is asking state government for new revenue sources to add bus trips or avoid service cuts. Planners are trying to solve problems such as the No. 8 bus that is often trapped on gridlocked Denny Way late afternoons.

Sound Transit’s nearby light-rail hub at Westlake Center offers south connections but won’t connect to Northgate, Lynnwood and Bellevue until the early 2020s.

McGinn last fall was unable to get an $80 car-tab fee onto the ballot that might have stretched the South Lake Union streetcar to Ballard — but Seattle did win planning grants, and a spokesman said McGinn still aspires to add transit links, if money can be raised.

To truly ease congestion on Mercer, single-occupant trips need to be reduced, using aggressive measures, says Hallenbeck. He suggests event garages around Seattle Center might serve as park-and-rides (forbidden under current city policy) for buses going crosstown or crosslake. Or take people to South Lake Union using more private bus services, akin to Microsoft Connector.

For now, the Seattle area’s car and bus users, who have considerable experience adapting in the 2010s, face another immediate challenge through the work zone.

And first thing Monday morning, said Kemper, the city will deploy an engineer to witness the traffic conditions at the corner of Fairview and Republican.

Staff reporter Eric Pryne contributed to this report. Mike Lindblom: 206-515-5631 or mlindblom@seattletimes.com. On Twitter @mikelindblom.