SR 99 tunnel

Overview of purpose and need

In February 2001, a 6.8 magnitude earthquake struck near Seattle, causing several foundations of the Alaskan Way Viaduct to shift as much as five inches.  Engineers believe the viaduct would have collapsed had the earthquake lasted a few moments longer. During the next decade, state and local agencies studied more than 90 alternatives to replace the aging viaduct which carried SR 99 through Seattle. In 2009, a bored tunnel was chosen. Building a large single-bored tunnel allowed traffic to continue moving above ground as a tunnel was built below. 

How you benefit

  • The Alaskan Way Viaduct was built in the 1950s, long before modern seismic codes were established. Originally planned to carry 60,000 vehicles a day, by 2010 it carried 110,000 vehicles daily. Decades of wear and tear as well as its vulnerability to earthquakes made replacing the viaduct critical to public safety.
  • The SR 99 tunnel is designed to carry the volume of vehicles that previously used the viaduct to bypass downtown Seattle.
  • Burying SR 99 allowed the viaduct to be demolished, opening up Seattle's central waterfront. 
  • The SR 99 tunnel is among the safest tunnels ever built, designed to withstand strong earthquakes and equipped with smart safety systems to keep traffic moving and people safe.


  • February 28, 2001: The Nisqually earthquake struck near Seattle, sinking the viaduct several inches and emphasizing the need to replace the viaduct with a safe alternative.
  • 2009: State and local leaders agreed to replace the waterfront section of the viaduct with a bored tunnel beneath downtown Seattle.
  • 2010: Seattle Tunnel Partners was chosen as the design-build contractor to build the SR 99 tunnel. STP chose Hitachi-Zosen to build the tunnel boring machine for the job – the largest such machine in the world. 
  • July 30, 2013: Tunneling machine “Bertha” began digging the tunnel’s route, beginning near the Seattle stadiums.
  • April 4, 2017: Bertha completed tunneling, breaking into the receiving pit near Seattle’s Space Needle.
  • January 11, 2019: SR 99 closed through downtown Seattle so crews could realign the highway with the new tunnel by completing the tunnel entrances and exits.
  • February 4, 2019: The SR 99 tunnel opened to vehicles.
  • November 9, 2019: Tolling began on the SR 99 tunnel.

Funding and tolling

  • The SR 99 tunnel was one of 30 projects that together comprise the Alaskan Way Viaduct Replacement Program. 
  • The budget for the SR 99 tunnel project is $2,145.6 million, which included the design-build tunnel, roads that provided north and south access to the tunnel, new streets near the tunnel’s north portal, and the South Access 2 project. Funding came from state, federal and local sources, including the Port of Seattle.
  • The 2012 Legislature directed WSDOT to toll the SR 99 tunnel to pay back $200 million in bonds that helped finance construction. Tolls also pay for ongoing tunnel operations and maintenance costs, including tunnel safety systems, similar to other toll roads in Washington
  • The toll rate is determined by time of day, and whether drivers have a Good To Go! pass. Drivers without a Good To Go! account pay an extra $2 per trip. Tolls are higher for vehicles with more than two axles. The toll rates are set by the Washington State Transportation Commission.

Photos and videos



Map highlighting SR 99 tunnel route between South Royal Brougham Way at south and Mercer Street at north