The A Line began service on October 2, 2010,[3] running from Tukwila to Federal Way, mostly along Pacific Highway South.

From there, riders can transfer to Sound Transit's Link light rail, the RapidRide F Line, or to other King County Metro buses that serve Burien, Tukwila, SeaTac, Georgetown, SoDo, and downtown Seattle.

Its southern terminus is Federal Way Transit Center[4] and connects riders to buses serving Tacoma, Pierce County, and Auburn.

[5] This corridor was previously served by King County Metro route 174[6] which carried an average of 5,570 riders on weekdays during the last month in service.

[7] Since the implementation of RapidRide on the corridor, ridership has grown 81 percent and the A Line served an average of 10,100 riders on weekdays in spring 2015.