Washington State Route 18

State Route 18
SR 18 highlighted in red
Route information
Maintained by WSDOT
Length	28.41 mi[1] (45.72 km)
Existed	1964[2]–present
Major junctions
West end	SR 99 in Federal Way
Major intersections	SR 161 in Federal Way I-5 in Federal Way SR 167 in Auburn SR 164 in Auburn SR 516 in Covington SR 169 in Maple Valley
East end	I-90 near North Bend
Location
Country	United States
State	Washington
County	King
Highway system
State highways in Washington Interstate US State Scenic Pre-1964 1964 renumbering Former
← SR 17 → SR 19

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.