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.
Idaho State Highway 200
ID-200 is the westernmost portion of a chain of Highway 200s which extends east through Montana, North Dakota, and Minnesota. At only in length, Idaho's Highway 200 is the shortest in the chain while Montana's Highway 200 is the longest. There is another highway called SR 20 in Washington that would complete the chain of Hwy 200s, but ID-200 is no longer directly connected to WA-20. (The concurrency with US 2 from Newport/Oldtown to Ponderay was removed in the early 2010s, however the legacy of the concurrency continues with the mile markers from Ponderay to Montana.)