The DSTT was used only by buses from its opening in 1990 until 2005, and shared by buses and light rail from 2009 until 2019. Bus routes from King County Metro and Sound Transit Express left the tunnel north via Interstate 5, south via the SODO Busway, or east via Interstate 90. It is owned by King County Metro and shared with Sound Transit through a joint-operating agreement signed in 2002. The Downtown Seattle Transit Tunnel was one of two tunnels in the United States shared by buses and trains, the other being the Mount Washington Transit Tunnel in Pittsburgh, and was the only one in the United States with shared stations.