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 2
By the time the U.S. Highway system was finalized in late 1926, one relatively minor change had been made to US 2; it was swapped with US 1 between Bangor and Houlton, Maine, placing US 2 along the entire portion of Route 15 east of St. Johnsbury. Several other major parts of the auto trail received numbers, most notably US 30 from Portland, Oregon east to Pendleton, US 195 in eastern Washington, and US 23 in Michigan's Lower Peninsula. In the mid-1930s, much of New York's portion of the road became US 104, and the part southeast of Littleton, New Hampshire to Portland, Maine became US 302, but by far the longest piece was that followed by US 2 between St. Ignace and Bonners Ferry. In 1946, US 2 was extended west of its original western terminus in Bonners Ferry in Idaho to Everett in Washington via Spokane along what was then Alternate US 10.