| Interstate Highways in Washington | |
|---|---|
_Metric.svg)   Highway markers for Interstate 5, Interstate 90, and Interstate 182 | |
 Map of highways in Washington with the Interstates highlighted in red | |
| System information | |
| Length | 764.26 mi[1] (1,229.96 km) |
| Formed | June 29, 1956 |
| Highway names | |
| Interstates | Interstate nn (I‑nn) |
| Business Loops: | Business Loop Interstate nn (BL I‑nn) |
| Business Spurs: | Business Spur Interstate nn (BS I‑nn) |
| System links | |
The Interstate Highways in Washington are segments of the national Interstate Highway System that lie within the U.S. state of Washington. The system comprises 764 miles (1,230 km) on seven routes that are owned and maintained by the Washington State Department of Transportation (WSDOT); the design standards and numbering across the national system are managed by the Federal Highway Administration (FHWA) and the American Association of State Highway and Transportation Officials (AASHTO).
Washington has three primary Interstates and four auxiliary routes; the seven routes serve most of the state's major cities. The longest of these is Interstate 90 (I-90), which is 298 miles (480 km) long and connects the state's two largest cities, Seattle and Spokane. I-5 is the only Interstate to span the state from south to north, traveling from the Oregon state line to the Canadian border. The system also includes several business routes that are not maintained or managed by WSDOT, but rather by local governments.
The general plan and federal funding for the Interstate Highway System were approved by the U.S. Congress in the Federal-Aid Highway Act of 1956. The federal plan incorporated elements of the Washington state government's plan for limited access highways, including an urban tollway that was later cancelled. The original allocation of two primary routes and two auxiliary routes was later expanded in the 1960s and 1970s with the addition of I-82 and two more auxiliary routes. The last segment of the Interstate Highway System in Washington, a section of I-90 between Seattle and Bellevue, was completed in 1993. An eighth route, popularly named I-605, has been proposed in several forms since the 1960s but was never submitted for formal approval.