The highway system of the U.S. state of Washington is a network of roads owned and maintained by several jurisdictions: the state of Washington through the Washington State Department of Transportation (WSDOT) and its counties, cities, towns and communities. The most prominent of these roads are part of three different numbered highway systems, two of which are designated at the federal level, and one at the state level. There are other roads in Washington, including city and county maintained roads, and Forest Highways, none of which are maintained by WSDOT. Several ferry routes, such as the Vashon Island – Seattle passenger-only ferry route – Washington State Route 339, have been codified into state law as a state highway, despite not being located on land.
Interstate Highways and U.S. Routes are assigned at the national level by the American Association of State Highway and Transportation Officials in cooperation with the Federal Highway Administration. Interstate Highways are numbered in a grid – even-numbered routes for east-west routes (but the lowest numbers are along Mexico and the Gulf of Mexico), and odd-numbered routes are north-south routes (with the lowest numbers along the Pacific Ocean). U.S. Routes are also numbered in a grid – even numbered for east-west routes (with the lowest numbers along Canada) and odd numbered for north-south routes (with the lowest numbers along the Atlantic Ocean). There are seven Interstate highways in Washington, and one oft-proposed, ranging from Interstate 5 to Interstate 705. There are eight current U.S. Routes in the state including U.S. Route 101 and U.S. Route 730.
Washington State Routes are assigned by WSDOT. The shield used for state routes is a simplified version of the head of George Washington. Washington state highways are numbered similarly to Interstate and U.S. Routes, where one and two-digit State Routes are considered primary routes, and three-digit routes are considered auxiliary routes, connecting back to one or two-digit routes. Interstate and U.S. Routes are assigned internal State Route numbers to allow this relationship to happen. For example, State Route 531 is considered an axillary route of Interstate 5. Some former U.S. Routes, such as U.S. Route 99 have been converted to state highways after they were decommissioned at the federal level, retaining the same number.
The above three systems make up over 7,000 miles (11,000 km) of the public road system in the state, however that figure is only about eight percent of all roads in Washington (or a total of about 87,500 miles (140,800 km) of roads).