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.
Interstate 5 in Oregon
Interstate 5 is the second-longest freeway in Oregon, at , and is the only Interstate to traverse the state from north to south. The highway connects several of the state's largest metropolitan areas, which lie in the Rogue and Willamette valleys, and passes through counties with approximately 81 percent of Oregon's population. As a component of the Interstate Highway System, I-5 is also designated as an important highway under the National Highway System. It is officially designated under Oregon's named route system as the Pacific Highway No. 1, a name shared with Oregon Route 99 (OR 99) and its split routes north of Junction City. OR 99 runs concurrent to I-5 through most of southern Oregon, splitting from the freeway to serve city centers and use other alternate routes, while OR 99W and OR 99E serve corridors on opposite sides of the Willamette River. The state legislature also designated I-5 as the Purple Heart Trail and Korean War Veterans Memorial Highway in 2015.