[3][4] The ferries depart from Colman Dock and travel northwest across Elliott Bay and Puget Sound to Winslow, part of the city of Bainbridge Island.

The highway continues northwest to end at a partial cloverleaf interchange with the SR 3 freeway north of the city.

[16] SSH 21A originally began at the Port Blakely ferry dock when it was codified in 1937,[17] but was moved north to Winslow in 1949.

[18] The highway connected Bainbridge Island to the Kitsap Peninsula via a car ferry that began in the 1920s and was replaced by the 1,229-foot-long (375 m) Agate Pass Bridge after it opened on October 2, 1950 with a 35-cent toll.

[19] The bridge, which cost $1.4 million, had its tolls removed in October 1951 after it was paid for by a bond issue passed by the Washington State Legislature.

The Eagle Harbor Transportation Co. operated various steamers on the route, including the Bainbridge and Chippewa,[27][28] until WSF was created in 1951 to manage most ferries in the Puget Sound.