The question of how to get buses through clogged Mercer Street and into their new I-5 interchange remains a mystery. Agencies have four years to figure that out.

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After years of waiting, transportation executives have agreed to create direct transit lines between booming South Lake Union and the Eastside, by building a fifth reversible Interstate 5 express lane between Highway 520 and Mercer Street, for buses only.

The $70 million project, scheduled to open in 2023, was revealed by the Washington State Department of Transportation (WSDOT) on Wednesday as an update to its $4.6 billion, two-decade rebuild of the Highway 520 corridor. 

The state’s animated diagram of the new lane can be found at this link.

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In addition, a roughly $400 million project begins next March to construct a bigger Montlake interchange, a parklike lid over Highway 520 and new eastbound lanes connecting Montlake to the six-lane floating bridge that opened two years ago.

This phase of the project requires piledriving in the mucky Lake Washington bed, lane shifts on the highway and closure of the Montlake freeway bus station.

To make the bus connections to South Lake Union work, WSDOT will construct a reversible, direct-access ramp at north Capitol Hill from Highway 520 onto I-5 — six years sooner than the state previously intended. From there, contractors will add the bus lane and new retaining wall on the west side of the express lanes.

Though several mainline freeway decks sit on columns, the express lanes themselves rest primarily on the ground, where a bus lane can be built. Then, at Mercer Street, a single reversible South Lake Union bus lane would be centered within the hillside thicket of northbound and southbound ramps where Mercer joins I-5 next to Fairview Avenue North.

Travel on this new lane should be far quicker than today’s trips, where 520 buses can only use the mainline and must weave into a jammed Stewart Street exit south of Mercer.

The question of how to get buses through clogged Mercer Street and into their new I-5 bus entrance remains a mystery. Agencies have four years to figure that out. King County Metro Transit or Sound Transit would reshuffle their budgets to supply the direct Eastside-South Lake Union buses.

As for stopping at Montlake, a new lid station designed to open in 2023 would serve routes that go north toward the University of Washington. It’s possible to re-enter Highway 520 from the lid station, but only by weaving across general lanes.

A joint letter of understanding was signed this month by WSDOT Secretary Roger Millar, Sound Transit CEO Peter Rogoff, Metro General Manager Rob Gannon and Seattle Department of Transportation (SDOT) interim Director Linea Laird.

“They see a demand there. They see South Lake Union as an independent market from downtown, an area that’s underserved,” said Ron Paananen, senior director at HDR, the project’s engineering consultant.

A 2023 opening date is some six years sooner than WSDOT earlier anticipated, because the state chose to finish the busway first, and then build the adjacent Portage Bay Bridge segment of Highway 520 by 2029. At that point, the new I-5 bus lane will be opened to carpools, spokesman Steve Peer said.

WSDOT’s busway design is a significant change from its first concept in 2010, which would have sent the 520 buses merging into one of the existing express lanes. That would cause a bottleneck at north Capitol Hill and possibly stranded buses in general traffic.

This new design still serves only traditional suburb-to-city commutes and won’t reduce congestion on Mercer Street.

A decade ago, Mayor Greg Nickels and SDOT promised to create transit service along Mercer Street, as one argument for the federal government to award $30 million in TIGER grants to help rebuild Mercer. That idea was soon forgotten.

That plan had many obstacles, chiefly that in I-5’s current layout any South Lake Union express buses would have to weave from Mercer’s left-side onramp into the right-side 520 exit going east. And the rebuilt Mercer is already gridlocked, and the city says 80,000 vehicles use the I-5 interchange each day.

Transit planners have toyed with the idea of stopping Highway 520 buses at Sound Transit’s UW light-rail station at Husky Stadium, where riders would have to descend 90 feet to catch a train downtown. But many people prefer a one-seat ride downtown, something the fifth I-5 express lane would support.

Voters approved light-rail stations in South Lake Union, but those won’t be ready for 17 years and people going there from the Eastside will have to change trains at least once.