 state route&nbsp;518 (sr&nbsp;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&nbsp;509 and travels east to serve seattleâtacoma international airport and intersect sr&nbsp;99 in seatac. sr&nbsp;518 continues east into tukwila and ends at an interchange with interstate&nbsp;5 (i-5), as the freeway travels becomes i-405. the highway was codified in 1937 as the western segment of secondary state highway&nbsp;1l (ssh&nbsp;1l) and designated as sr&nbsp;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&nbsp;518 begins at a partial cloverleaf interchange with sr&nbsp;509 east of burien and travels east as a divided freeway, passing 
State_highways_in_Washington_(state) Transportation_in_King_County,_Washington