The Alaskan Way Viaduct is a defunct elevated freeway in Seattle, Washington, United States, that carried a section of State Route 99 (SR 99). The double-decked freeway ran north–south along the city's waterfront for 2.2 miles (3.5 km), east of Alaskan Way and Elliott Bay, and traveled between the West Seattle Freeway in SoDo and the Battery Street Tunnel in Belltown.
The viaduct was built in three phases from 1949 through 1959, with the first section opening on April 4, 1953. It was the smaller of the two major north–south traffic corridors through Seattle (the other being Interstate 5), carrying up to 91,000 vehicles per day in 2016. The viaduct ran above Alaskan Way, a surface street, from S. Nevada Street in the south to the entrance of Belltown's Battery Street Tunnel in the north, following previously existing railroad lines.
The viaduct had long been viewed as a barrier between downtown and the city's waterfront, with proposals to replace it as early as the 1960s. Questions of the structure's seismic vulnerability were raised after several earthquakes damaged similar freeways in other cities, including some with the same design as the viaduct. During the 2001 Nisqually earthquake, the Alaskan Way Viaduct suffered minor damage but later inspections found it to be vulnerable to total collapse in the event of another major earthquake, necessitating its replacement.