Urban areas have been established in each metropolitan area under federal transportation requirements (Title 23, Section 103, United States Code).
The FHWA’s Surface Transportation Program (STP) assistance program requires a small portion of the STP funds be spent outside the federal-aid urbanized areas and federal-aid urban area. The urban/rural boundaries of the map are also used to maintain the federal functional classification system for highways and arterials.
Known as the “federal-aid urbanized” and “federal-aid urban” areas, the map for King, Kitsap, Pierce, and Snohomish counties is designated by the Regional Council in cooperation with the Washington State Department of Transportation and the Federal Highway Administration (FHWA). The results from the 2010 federal census were used to update the map.
Boundaries for Federal-Aid Urbanized Areas and Federal-Aid Urban Areas [pdf]