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.
Route description
I-5 southbound in Federal Way, approaching its interchange with SR 18
SR 18 continues northeast along the southeastern city limits of Kent, through an interchange with Southeast 304th Street, which serves Green River College,[13] towards Covington. The freeway intersects SR 516 in a diamond interchange and 256th Street Southeast in a partial cloverleaf interchange before leaving Covington.[14][15] SR 18 intersects Southeast 231st Street in a diamond interchange located north of Maple Valley, providing a connection to SR 169, while the freeway travels on an overpass over SR 169 towards a partial cloverleaf interchange with 244th Avenue.[16][17] The SR 18 freeway ends northeast of a partial cloverleaf interchange with Issaquah-Hobart Road at the base of Tiger Mountain,[18] becoming a two-lane highway with at-grade intersections for the remainder of its route. The highway travels on the east side of Tiger Mountain in the Issaquah Alps and provides access to a trailhead in Tiger Mountain State Forest before it reaches its eastern terminus, a diamond interchange with I-90 located west of North Bend.[19] I-90 provides access to the cities of Issaquah and Seattle to the west and North Bend and Spokane to the east, traveling over the Cascade Mountains through Snoqualmie Pass. The roadway continues past the interchange as Snoqualmie Parkway into the city of Snoqualmie, intersecting SR 202.[6][7][20]
Every year, the Washington State Department of Transportation (WSDOT) conducts a series of surveys on its highways in the state to measure traffic volume. This is expressed in terms of annual average daily traffic (AADT), which is a measure of traffic volume for any average day of the year. In 2012, WSDOT calculated that the busiest section of SR 18 was its interchange with SR 167 in Auburn, serving 97,000 vehicles, while the least busiest section of the highway was its eastern terminus at I-90, serving 19,000 vehicles.[21] SR 18 is designated as part of the National Highway System for its whole length,[22] classifying it as important to the national economy, defense, and mobility.[23] WSDOT designates the entire route of SR 18 as a Highway of Statewide Significance,[24] which includes highways that connect major communities in the state of Washington.[25]
History
I-90 eastbound approaching its interchange with SR 18 near Snoqualmie
The modern corridor that SR 18 follows was added to the state highway system in 1931 as a branch of State Road 5 that ran from the main highway in Auburn to State Road 1 and U.S. Route 99 (US 99) in Federal Way via Peasley Canyon.[26][27] The branch was retained by State Road 5 when it was re-designated as PSH 5 as a new highway code was established in 1937.[28][29] A branch of PSH 2 was designated in 1949, traveling southwest from North Bend, around Tiger Mountain and through Auburn before ending at a junction with PSH 1 and US 99 in Milton.[30] The designation had been the result of lobbying from business leaders in Tacoma, who sought a direct connection to Snoqualmie Pass that would improve access to Eastern Washington.[31]
Construction on the Echo Lake Cutoff Road, along the route of the North Bend–Auburn branch of PSH 2 and the Auburn–Federal Way branch PSH 5, began in 1955. The two-lane highway would cost $9 million (equivalent to $98 million in 2024[32]) and the 32-mile-long (51 km) route was chosen to avoid Tiger Mountain and connect the city of Tacoma to Snoqualmie Pass.[33] The first section, between US 99 and Auburn, was opened on December 19, 1958.[34][35] SR 18 was designated on the corridor during the 1964 state highway renumbering and codified into state law in 1970, replacing the branches of PSH 2 and PSH 5.[2][36] The final 7 miles (11.27 km) of the Echo Lake Cutoff, from an entrance to Tiger Mountain State Forest to I-90, was officially opened on December 1, 1964.[33][37] A new interchange with I-5 in Federal Way was opened to traffic on January 31, 1967,[38] officially completing SR 18.[39]
Expansion of SR 18 from a two-lane rural road to a four-lane controlled-access freeway began in 1992 response to six fatalities in over 170 accidents in a ten-year period, giving the highway a reputation of being a "dangerous roadway".[40][41]WSDOT widened SR 18 to four lanes and added new interchanges between SR 167 and the Green River within Auburn.[42] The Washington State Legislature briefly planned to toll the new SR 18 freeway in January 1995 to pay off loans for re-construction, but the plan was protested by local residents and rejected months later in May.[43][44] Additional work was completed in 1997 from the Green River to Southeast 304th Street in Covington,[45] and in 2007 from Covington through Maple Valley and to Issaquah-Hobart Road.[46][47] The western end of the freeway was extended in Federal Way through Peasley Canyon to I-5 in Federal Way,[48] which was further improved in 2011 with the completion of interchange improvements to the "Federal Way Triangle".[49][50]
WSDOT is planning to widen the remaining section of SR 18, between Issaquah-Hobart Road south of Tiger Mountain to I-90 near Snoqualmie, to four lanes with a median barrier to separate opposing lanes of traffic and prevent head-on collisions. The western section will include a right-in/right-out interchange at the Tiger Mountain summit; $640 million was approved in the 2022 Move Ahead Washington transportation package.[51][52] The I-90 interchange is planned to be rebuilt as part of a separate project scheduled to be completed in 2025;[53] it was originally set to include flyover ramps and a double roundabout, but was later changed to a diverging diamond interchange.[54][55]
Major intersections
The entire highway is in King County. All exits are unnumbered.
^Washington State Legislature (1931). "Chapter 29". Session Laws of the State of Washington (1931 ed.). Olympia, Washington: Washington State Legislature.
^Washington State Legislature (March 17, 1937). "Chapter 190: Establishment of Primary State Highways". Session Laws of the State of Washington (1937 ed.). Olympia, Washington: Washington State Legislature. pp. 935–936. Retrieved April 14, 2013. A primary state highway to be known as Primary State Highway No. 5, or the National Park Highway, is hereby established according to description as follows: Also beginning at Auburn on Primary State Highway No. 5, as herein described, thence in a westerly direction by the most feasible route to a junction with Primary State Highway No. 1.
^Washington State Legislature (March 22, 1949). "Chapter 225: Establishing of Primary and Secondary State Highways". Session Laws of the State of Washington (1949 ed.). Olympia, Washington: Washington State Legislature. p. 773. Retrieved April 14, 2013. A primary state highway to be known as Primary State Highway No. 2, or the Sunset Highway, is hereby established according to description as follows: Also from a junction at a point approximately four miles west of North Bend in a general southwesterly direction by the most direct and feasible route by way of Auburn to a junction with State Road No. 1 in the vicinity of Milton.
^"First Section of Echo Lake Cutoff Opened to Auburn". Washington Highway News. Vol. 8, no. 4. Washington State Department of Highways. February 1959. p. 7. OCLC29654162. Retrieved August 29, 2018 – via Washington State Department of Transportation Library Digital Collections.
^Prahl, C. G. (December 1, 1965). "Identification of State Highways"(PDF). Washington State Highway Commission, Department of Highways. Retrieved April 14, 2013.
SR 99 in Federal Way
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
I-90 near North Bend
