Alaskan Way Viaduct Replacement Program – Benefits of Transportation

When the decision was made to replace Seattle’s seismically vulnerable Alaskan Way Viaduct with a deep-bored tunnel, the Washington State Department of Transportation had four guiding principles: Improve public safety, provide efficient movement of people and goods, bolster local, regional, and state economies, and enhance Seattle’s waterfront as a place for people.

WSDOT accomplished all this and more with the new State Route 99 tunnel opened and demolition of the Alaskan Way Viaduct.

The elevated viaduct carried SR 99 through Seattle since the 1950s and was a critical link for passengers and freight.

In 2001, a magnitude 6.8 earthquake shook and damaged the structure. Concrete cracked and columns shifted as much as 5 inches. Repairs kept the highway functioning, but removing the viaduct became a critical public safety issue.

To move SR 99 underneath downtown Seattle, WSDOT chose to push the limits of engineering, building a tunnel with the world’s largest-diameter tunnel boring machine. Northbound and southbound traffic would move along 32-foot roadways stacked inside the tunnel. Public safety guided every design decision.

This $3.3 billion project also promised monumental improvement to Seattle’s waterfront and downtown. Partners and community members all joined with WSDOT for more than 20 years to advise, consult and ultimately decide how this massive project would help create a better city.

In February 2019, the two-mile-long tunnel opened. Drivers now travel underneath downtown Seattle in a tunnel built to withstand powerful earthquakes and equipped with state-of-the-art fire safety and security systems. That same year (2019), WSDOT demolished the towering, noisy viaduct that had long separated Seattle’s downtown from its waterfront. In the viaduct’s footprint, Seattle will soon complete a multi-modal thoroughfare, a promenade, and 40 new acres of space for the public to enjoy.

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