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 40
Interstate 40 (I-40) is a major east-west Interstate Highway running through the south-central portion of the United States generally north of I-10 and I-20 but south of I-70. The western end is at I-15 in Barstow, California; its eastern end is at a concurrency of U.S. Route 117 (US 117) and North Carolina Highway 132 in Wilmington, North Carolina. It is the third-longest Interstate Highway in the United States, behind I-80 and I-90. Much of the western part of I-40, from Oklahoma City to Barstow parallels or overlays the historic US 66, east of Oklahoma City the route generally parallels US 64 and US 70. I-40 runs through many major cities including Albuquerque, New Mexico; Amarillo, Texas; Oklahoma City, Oklahoma; Little Rock, Arkansas; Memphis, Tennessee; Nashville, Tennessee; Knoxville, Tennessee; Greensboro, North Carolina; and Raleigh, North Carolina.