A $2-billion deep-bore tunnel along Seattle’s downtown waterfront appears to have overcome what may be its final obstacle to construction, garnering the approval of nearly 60% of voters in a referendum on whether the city should give the notice to proceed with the project.

The tunnel, scheduled for completion in 2015, is the centerpiece of a $3.1-billion Washington State Dept. of Transportation program to replace the seismically suspect double-decked Alaskan Way Viaduct. That thoroughfare carries approximately 110,000 vehicles each day along one of Seattle’s primary north-south arteries. Damage from the 2001 Nisqually Earthquake sparked a decade-long, often-controversial effort by WSDOT to find a suitable strategy for replacing the Viaduct and the aging seawall that supports its backfilled foundation soil.

In 2009, the 1.7-mile, 57-ft dia tunnel emerged as WSDOT’s top replacement alternative compared with constructing a new above-ground structure, and doing away with an express route entirely in favor of a beefed-up street system. State and local officials assembled a funding plan that draws on federal funds, state gas tax money, a contribution from the Port of Seattle, and toll revenue from the completed tunnel. That funding also covers upgrades to the Viaduct’s less-controversial southern section, already under way.

Earlier this year, WSDOT awarded the tunnel’s design-build contract to Seattle Tunnel Partners, a joint venture of Dragados USA and Tutor Perini Corp. Other team members include Frank Coluccio Construction, Mowat Construction, HNTB, and Intecsa.

But despite a planned September groundbreaking, tunnel opponents refused to give up. Seattle Mayor Mike McGinn, a longtime tunnel skeptic, regularly battled with the pro-tunnel city council, contending that downtown property owners would be on the hook for potential cost overruns arising from the uncertainties of the waterfront’s soils. Opponents also worried that the tolls would divert traffic to already clogged city streets.

Though the city council dismissed those concerns, McGinn backed the effort to put the question to Seattle voters—did they support the city’s already negotiated agreements on utility relocations, street use, design review, and liability, allowing the project to begin on schedule?

While the city council could have kept the project moving in the face of a “no” vote by passing a new ordinance, that outcome would have almost certainly led to a new round of efforts to slow or block the project. 

In conceding the outcome of the vote, which was conducted entirely by mail, McGinn stated, “I worked to give the public a direct vote on the tunnel. The public said move ahead with the tunnel, and that’s what we’re going to do.”