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.
Special routes of U.S. Route 95
The route now known as US 95 Spur was previously known as US 630 and US 30N. US 630 was an east–west United States highway. It was decommissioned in or around 1931. With a length of around , it was the shortest US route ever signed. It was concurrent with US 30N for its entire length. An earlier route numbered US-630 was proposed from Echo, Utah (northeast of Coalville) to Ogden, Utah, but was never signed in the field. U.S. Route 30N was a split route off of US-30 and was coterminous with US 630 at its west end in Oregon. East of Weiser, it was concurrent with what is now US-95 to Fruitland, Idaho, where it reconnected with US-30.