The Downtown Seattle Transit Tunnel is part of the "Third Avenue Transit Spine", the busiest transit corridor in Seattle, serving a combined average of 54,000 weekday riders with bus stops on the surface. The tunnel has a theoretical capacity of 40 trains per hour per direction with a minimum of 90-second headways, carrying 22,000 passengers per hour per direction, but is only able to carry 12,000 per hour per direction in 4-car light rail vehicles with current systems in place. Prior to the start of Link light rail service, the DSTT could serve up to 145 buses during the afternoon rush hour. , the DSTT carries 52,600 daily riders, of which 10,000 are on light rail.