The Edmonds–Kingston ferry at the end of State Road 21 has been in operation under various companies since 1923 and was served by the 14-car City of Edmonds.

SR 104 begins at an intersection with US 101, located south of Discovery Bay on the Olympic Peninsula.

[4] The highway serves as the northern terminus of SR 3 east of the bridge and turns northeast towards Port Gamble, located on the Kitsap Peninsula.

[5] The highway continues east from the intersection to Kingston, where it splits into a one-way pair before the designation travels onto the Edmonds–Kingston Ferry.

The roadway travels southeast along Edmonds Way towards the Snohomish–King county line, intersecting SR 524 Spur and 5th Street at a grade-separated interchange.

[16] Every year, WSDOT conducts a series of surveys on its highways in the state to measure traffic volume.

[2][22][23] The highway between Port Gamble and Kingston was established in 1915 as part of State Road 21, which extended south to Bremerton.

[2][22] The highway was widened and relocated onto a limited-access bypass between downtown Edmonds and Aurora Village in 1974, at a cost of $8 million.

[35][36] The bypass, which included an interchange at SR 99, was proposed in the 1960s and required several years of right-of-way acquisition.

The study concluded that an extension would likely be subject to litigation from local residents based on feedback from earlier meetings and would not provide major benefits.

[38][39] The Hood Canal Bridge, officially named after highways director William A. Bugge, sank during the February 13, 1979 windstorm and was replaced by a ferry operating between Lofall and South Point until the west span was re-opened on October 23, 1982.