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.
U.S. Route 16
US 16 is also known as Mount Rushmore Road in western South Dakota. The highway enters South Dakota east of Newcastle, Wyoming. It travels near Jewel Cave, the third-longest cave in the world. The highway goes through the city of Custer and shares alignment with US 385. East of Hill City, US 16 splits off US 385. It then becomes a four-lane divided highway, with the two roadways separated by up to a half-mile (0.8 km) in some places, including the old gold-mining town of Rockerville, South Dakota, which is contained entirely between the two roadways. In Rapid City, a truck bypass runs along Catron Boulevard and Elk Vale Road up to Exit 61 on I-90.