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

Seattle Tunnel Partners is 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.