County. SR 18 continues northeast along the southeastern city limits of Kent, through an interchange with Southeast 304th Street, which serves Green River College, towards Covington. The freeway intersects SR 516 in a diamond interchange and 256th Street Southeast in a partial cloverleaf interchange before leaving Covington. 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. The SR 18 freeway ends northeast of a partial cloverleaf interchange with Issaquah-Hobart Road at the base of Tiger Mountain, 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. 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. 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 average annual 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. SR 18 is designated as part of the National Highway System for its whole length, classifying it as important to the national economy, defense, and mobility. WSDOT designates the entire route of SR 18 as a Highway of Statewide Significance, which includes highways that connect major communities in the state of Washington. History 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. The branch was retained by State