Washington State Route 518

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 travels 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.