To replace the aging Alaskan Way Viaduct along State Route 99, which cut through downtown Seattle, the Washington State Department of Transportation (WSDOT) constructed a new tunnel to run under the downtown corridor. The $3.3 billion, 2.8-mile tunnel will collect toll revenue on the estimated peak travel flow of 7,000 vehicles per hour. The impetus for the SR 99 tunnel was to replace a seismically at-risk elevated highway, reconnect downtown Seattle with its waterfront, and provide a faster and more direct route to drivers that need to get through the downtown area. The viaduct will be replaced with new parks, pedestrian and bike paths, and waterfront access. The innovative double-deck tunnel is built to withstand a 9.0 seismic event, improves accident management, and incorporates smart technology to monitor the tunnel’s conditions and manage traffic.