The SR 99 Alaskan Way Viaduct is being replaced with an approximately two-mile-long double-deck tunnel underneath downtown Seattle. Replacing the viaduct with a tunnel allows the highway to remain open for much of construction, thus minimizing closures and impacts to traffic. Once the tunnel opens, the viaduct will be taken down to clear the way for new public space along Seattle’s downtown waterfront.

Seattle Tunnel Partners was responsible for designing and building the SR 99 tunnel. In addition to boring the tunnel and building the highway within it, STP is responsible for building highway ramps and other connections at the north and south ends of the tunnel. They are also constructing buildings at each tunnel portal to house lighting, ventilation and other systems needed to operate the tunnel.