State Route 518 (SR 518) is a state highway located entirely within King County in the U.S. state of Washington. The freeway begins in Burien at an interchange with SR 509 and travels east to serve Seattle–Tacoma International Airport and intersect SR 99 in SeaTac. SR 518 continues east into Tukwila and ends at an interchange with Interstate 5 (I-5), as the freeway becomes I-405. The highway was codified in 1937 as the western segment of Secondary State Highway 1L (SSH 1L) and designated as SR 518 during the 1964 highway renumbering. During the 1970s, the roadway was widened into a freeway to serve the airport and serves over 100,000 vehicles per day. Route description SR 518 begins at a partial cloverleaf interchange with SR 509 east of Burien and travels east as a divided freeway, passing north of Highline High School. The freeway intersects Des Moines Memorial Drive and 154th Street north of Seattle–Tacoma International Airport and continues east into SeaTac. SR 518 intersects the Airport Expressway and International Boulevard, signed as SR 99, south of the Tukwila International Boulevard station. The freeway follows the Central Link light rail line into Tukwila and intersects 51st Avenue before an interchange with I-5. At the interchange, located northwest of the Westfield Southcenter Mall, the Central Link turns north along I-5 and SR 518 ends as the roadway continues east as I-405 towards Renton. 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 2011, WSDOT calculated that between 38,000 and 102,000 vehicles per day used the highway, mostly between Seattle–Tacoma International Airport and I-5. SR 518 are designated as part of the National Highway System and as a Highway of Statewide Significance for serving Seattle–Tacoma International Airport. History A highway between Burien and Tukwila has existed since the 1890s and was first codified in 1937 as a segment of SSH 1L, which extended from PSH 5 in Renton west to intersect PSH 1 and US 99 in Tukwila and end at SSH 1K south of Sunnydale, later renamed Burien. The section between PSH 5 in Renton and PSH 1 in Tukwila was removed in 1957 after the construction of the Seattle–Tacoma