Washington State Route 18

State Route 18 (SR 18) is a 28.41-mile-long (45.72 km) state highway in the U.S. state of Washington, serving southeastern King County. The highway travels northeast, primarily as a controlled-access freeway, from an intersection with SR 99 and an interchange with Interstate 5 (I-5) in Federal Way through the cities of Auburn, Kent, Covington, and Maple Valley. SR 18 becomes a two-lane rural highway near Tiger Mountain as it approaches its eastern terminus, an interchange with I-90 near the cities of Snoqualmie and North Bend.

SR 18 was established during the 1964 state highway renumbering as the successor to the Auburn–Federal Way branch of Primary State Highway 5 (PSH 5) and the Auburn–North Bend branch of PSH 2, which were created in 1931 and 1949, respectively. The initial two-lane highway, named the Echo Lake Cutoff, was completed in December 1964 after the opening of a section around Tiger Mountain, which would later be the site of over 170 accidents in the 1980s. SR 18 was gradually widened into a four-lane freeway beginning in Auburn in 1992 and most recently finishing in Federal Way in 2007. The highway around Tiger Mountain and near the I-90 interchange remains a two-lane road, with a funded project planned to re-build the existing interchange with I-90.