past Mount Rainier High School and into Des Moines, where SR 516 ends. SR 509 continues north onto Marine View Drive through the city of Normandy Park and into SeaTac. The highway turns southeast onto Ambaum Boulevard and Des Moines Memorial Drive to a trumpet interchange, the southern terminus of a limited-access freeway section of SR 509. The four-lane freeway travels west of Seattle–Tacoma International Airport through a closed half-diamond interchange with South 176th Street and a partial cloverleaf interchange with South 160th Street before entering the city of Burien. SR 509 passes Highline High School before reaching a partial cloverleaf interchange with Southwest 148th Street and the western terminus of SR 518. The freeway continues north into the West Seattle neighborhood of Seattle and intersects South 128th Street in a diamond interchange and South 112th Street in a half-diamond interchange. SR 509 enters the Industrial District of Seattle and heads through an interchange with Cloverdale Street and Myers Way before ending at a partial cloverleaf interchange with SR 99 south of the First Avenue South Bridge over the Duwamish Waterway. The roadway continues north as the SR 99 divided highway toward downtown Seattle and the Alaskan Way Viaduct. 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 2011, WSDOT calculated that the busiest section of SR 509 was the limited-access highway between Burien and its northern terminus in Seattle, serving 54,000 vehicles, while the least busiest section was within Dash Point and Dash Point State Park, serving 2,600 vehicles. SR 509 between SeaTac and Seattle is designated as part of the National Highway System, which includes roadways important to the national economy, defense, and mobility. The highway from Tacoma to Federal Way and from SeaTac to Seattle are designated as part of WSDOT's Highways of Statewide Significance, which includes highways that connect major communities in the state of Washington. History The present route of SR 509 from Des Moines to Seattle roughly follows a wagon road constructed in the late 1890s by King County along the Puget Sound. The highway between Federal Way and Kent was codified in 1923 as part of State Road 1 and in 1937