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.
Appalachia
The effort to connect Appalachia with the outside world has required numerous civil engineering feats. Millions of tons of rock were removed to build road segments such as Interstate 40 through the Pigeon River Gorge at the Tennessee-North Carolina state line and U.S. Route 23 in Letcher County, Kentucky. Large tunnels were built through mountain slopes at Cumberland Gap to speed up travel along U.S. Route 25E. The New River Gorge Bridge in West Virginia, completed in 1977, was the longest and is now the fourth-longest single-arch bridge in the world. The Blue Ridge Parkway's Linn Cove Viaduct, the construction of which required the assembly of 153 pre-cast segments up the slopes of Grandfather Mountain, has been designated a historic civil engineering landmark.