The tunnel was designed to help relieve bus traffic on downtown Seattle streets and, at some future date, to accommodate light rail trains. It required the use of dual-mode buses to reduce downtown pollution while accommodating buses on suburban routes.
The tunnel is served by five stations: Convention Center, Westlake Center, University Street, Pioneer Square, and International District. Each station features its own distinctive architectural style and ensemble of public artworks. The tunnel was designed and built under the authority of Metro Transit, which became part of King County government in 1994.
Construction of the tunnel began on March 6, 1987, and was completed on schedule three and a half years later. Metro held a Grand Opening on September 14, 1990, and regular bus service began the following day. Just over 15 years later, on September 24, 2005, the tunnel was closed for upgrading and retrofitting so that it could be used by light rail as well as buses.
The retrofit was part of the Sound Transit light rail project approved by voters in 1996. Although rails were included when the tunnel was first built, they weren't insulated adequately to prevent stray electric current from trains from corroding nearby utility lines. As correcting this required removal of the original rails, Sound Transit also took the opportunity to lower the tunnel roadway to allow level train boarding. (As of 2006, the schedule called for the tunnel to re-open in 2007 and for light rail to start operating in 2009).