The State Highways of Washington in the U.S. state of Washington comprise a network of over of state highways, including all Interstate and U.S. Highways that pass through the state, maintained by the Washington State Department of Transportation (WSDOT). The system spans 8.5% of the state's public road mileage, but carries over half of the traffic. All other public roads in the state are either inside incorporated places (cities or towns) or are maintained by the county.