SR 20 Trunk
SR 20 begins at Jct US 101 at Discovery Bay and zigzags back and forth across Northern Washington via Washington Pass to Jct US 2 in Newport. When first posted in 1964, the western terminus of SR 20 was at Jct SR 536 at Fredonia about 5 miles west of Mount Vernon-Burlington. Several miles of SR 20 in the North Cascades were not completed and open to traffic until September 1972. The eastern terminus was Jct US 97 at Okanogan. Before 1964, the highway from Fredonia to Okanogan had been designated PSH 16.
Although the E-W extent of SR 20 is less than that of US 12 in Washington State, SR 20 is slightly longer than Washington US 12. This makes SR 20, at 436.55 miles, the longest highway in the state.
SR 20 Spur
SR 20 Spur was designated as such in 1973. Before 1973, this route was part of SR 536. SR 20 Spur begins at Jct SR 20 at Sharps Corner about 4 miles SE of Downtown Anacortes and heads northwesterly to the Anacortes ferry terminal. 1994 state legislation added the ferry route from Anacortes via the San Juan Islands to Sidney, British Columbia, Canada to SR 20 Spur. The spur, minus the ferry route, is 7.78 miles long.
SR 20 Business
SR 20 and US 97 bypass downtown Okanogan and Omak. Old US 97 through the two downtowns is designated as SR 215, Business US 97, and Business SR 20.
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