From Mt. Vernon to the Anacortes Ferry Terminal, the modern SR 20 (and SR 20 Spur, west of Sharpes Corner) was designated SR 536, and prior to 1964 as the Anacortes Branch of PSH 1. When the North Cascades Highway was completed, and the SR 20 designation extended Westward, the SR 536 designation was supplanted, and the SR 20 Spur designation created to keep the route to the ferry terminal within the highway system.