A second downtown transit tunnel? Transit enthusiasts at Seattle Subway say it could speed bus trips downtown from Ballard, Interbay, Aurora Avenue, West Seattle, Burien and White Center.

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As the politicians who lead Sound Transit prepare for a 2016 expansion vote, they’re facing a difficult choice within Seattle.

Chances are, proposed tax increases could pay for a Ballard-downtown light-rail route, a Ballard-University District light-rail route or laying tracks from the Chinatown International District to the tip of West Seattle.

But not necessarily all three.

So the transit enthusiasts at Seattle Subway offer a possible solution: Spread the benefits by digging a second downtown transit tunnel with multiple portals. Going underground would speed travel times by maybe 10 minutes for buses arriving from Ballard, Interbay, Aurora Avenue, West Seattle, Burien and White Center.

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Those buses carry close to 60,000 riders, and use is growing. Buses from Renton and Rainier Beach would be a natural fit, too, and there might be room for south and east express lines.

“You’re going to have true bus rapid transit through almost all of downtown,” said Jonathan Hopkins, a member of Seattle Subway, who along with colleague Keith Kyle unveiled their concept in an interview last week.

All this, of course, presupposes the Legislature allows new local taxes to support up to $15 billion in projects in a Sound Transit 3 (ST3) vote, and that voters in urban Snohomish, King and Pierce counties approve those. Last week’s Senate transportation proposal would restrain the package to $11 billion. Sound Transit is currently designing and building a three-pronged, 50-mile light-rail network after winning ballot measures in 1996 and 2008.

A “Westside Transit Tunnel” would run alongside the existing transit tunnel and absorb buses that would otherwise creep along Third Avenue and nearby streets.

Sound Transit light-rail traffic will crowd buses out of the first tunnel by 2021, when a rail extension to Northgate opens. Whenever the Highway 99 tunnel replaces the Alaskan Way Viaduct, buses that traveled the viaduct will switch to surface roadways.

Supporters of a second transit tunnel would have viaduct buses proceed into the Sodo Busway near Fourth Avenue South, then directly under downtown.

The concept hasn’t been widely debated yet.

Kevin Desmond, general manager at King County Metro Transit, has occasionally confided he wouldn’t mind a second tunnel to meet demand.

Sound Transit briefly looked at it during a broad review of corridor options last year, said spokesman Geoff Patrick.

Seattle Subway says it has briefed Seattle City Councilmember Mike O’Brien, a Sound Transit board member; Chris Arkills, transportation adviser to King County Executive Dow Constantine, who chairs the transit board; and Andrew Glass-Hastings, adviser to Mayor Ed Murray.

The tunnel is thought to need only $1 billion, Kyle said, leaving open the chance of building Ballard-UW rail using another $2 billion or so in ST3 funds.

By contrast, conceptual numbers for a long, mostly tunneled West Seattle rail spur alone have hovered around $4 billion, to reach White Center. Seattle Subway suggests a new downtown tunnel first, with West Seattle rail in some future phase.

Arkills, who commutes by bus from West Seattle, said he appreciates the creativity of the second transit-tunnel idea.

But he’s skeptical a second bus tunnel would improve much on the existing Highway 99 bus lane, as well as future waterfront bus lanes Constantine expects to secure between Sodo and Columbia Street, post-viaduct. And the proposal makes buses vulnerable to a choke point at the Fourth Avenue South loop ramp, if they drop from upper Spokane Street to the Sodo busway.

Politically, people are clamoring for rail, Arkills said.

“The executive (Constantine) believes that getting to Ballard and West Seattle is critical to the needs of Seattle for both mobility and successful passage of an ST3 package,” he said.

Sound Transit staffers have been instructed to look for ways to build Ballard and West Seattle extensions cheaper or shorter, he said.

It’s still early.

Patrick said the transit board is expected to hold hearings in June, followed by a possible vote in July on which ideas deserve technical study and which will be tossed aside.