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 95
With a length of , I-95 is the longest north–south Interstate and the sixth-longest Interstate Highway overall. I-95 passes through more states than any other Interstate Highway at 15 states (as well as a very brief stretch in the District of Columbia while crossing the Potomac River), followed by I-90, which crosses 13 states. According to the U.S. Census Bureau, only five of the 96 counties (or equivalents) along the route are completely rural, while statistics provided by the I-95 Corridor Coalition suggest that the region served is "over three times more densely populated than the U.S. average and as densely settled as much of Western Europe". According to the Corridor Coalition, I-95 serves 110 million people and facilitates 40 percent of the country’s gross domestic product.