List of Interstate Highways in Washington
Interstate 90 (I-90) is the longest primary Interstate Highway in Washington, which connects Seattle to Spokane and the Idaho border, measuring , while I-82, connecting the Oregon border city of Umatilla to Elensburg via Yakima, is the shortest at . The longest auxiliary Interstate Highway in Washington is I-405, a bypass of Seattle through the Eastside, at , and the shortest is I-705, a spur into downtown Tacoma, at . One route, I-605, has been proposed over the years to form another Eastside bypass, however there are no plans to construct this fifth auxiliary route.
After passage of the Federal Aid Highway Act of 1956, which formed the Interstate Highway System, the American Association of State Highway Officials had approved I-5 and I-90 across Washington on August 17, 1957. I-82 was approved on October 17, 1957, two months after I-5 and I-90, as part of a addition to the Interstate Highway System.
Washington State Route 20
Although US 12 has a larger east–west extent, SR 20 is the longest highway in Washington at , only longer than US 12. The highway has been called "The Most Beautiful Mountain Highway in the State of Washington."
SR 20 begins at a wye junction with US 101 at the southwest end of Discovery Bay on the Olympic Peninsula, just north of the terminus of SR 104. The highway travels northerly through the rural Quimper Peninsula and follows the east shore of Discovery Bay before reaching a junction with SR 19 near Jefferson County International Airport. SR 20 turns north onto the highway and passes Fort Townsend State Park on the southern outskirts of Port Townsend near Glen Cove. Within Port Townsend, the highway travels east through suburban areas as Sims Way and towards the historic downtown as Water Street. To the southwest of Port Townsend's historic district, the route turns southeast into the city ferry terminal. SR 20 continues onto the Port Townsend–Coupeville ferry which travels northeast across the Admiralty Inlet to the Keystone terminal on Whidbey Island. The ferry, operated by Washington State Ferries, takes approximately 30 minutes between terminals and runs year-round.