The D Line began service on September 29, 2012,[3] running between Carkeek Park in Crown Hill, Ballard, Interbay and Uptown and downtown Seattle.

[6] Since the implementation of RapidRide on the corridor, ridership has grown 53 percent and the D Line served an average of 11,700 riders on weekdays in spring 2015.

[2] The city of Seattle made major improvements to the RapidRide C and D lines after their opening with funds generated by Proposition 1 (which increased sales tax by 0.1 percent and imposes a $60 annual car-tab fee).

[8][9] In March 2016, the city split the previously interlined RapidRide C and D lines apart after the successful passage of additional funding for the two routes.

[7] C Line buses now continue north through parts of Belltown and onto Westlake Avenue in the South Lake Union neighborhood,[10] terminating on Valley Street near the Fred Hutchinson Cancer Research Center campus.