If headed south on SR 99, SR 509 branches off after you cross the Duwamish Waterway. SR 99 heads down and runs along the east side of Seattle-Tacoma International Airport, and SR 509 heads down the west side. But while SR 99 remains a major route all the way to Tacoma, SR 509 dead-ends at the junction of Des Moines Memorial Drive and S. 188th Way or tapers off into the City of Burien.

After hearings and community input, state transportation planners have decided the four-lane highway should be extended through South 188th Way and then head southwest toward I-5, crossing SR 99 and connecting with the interstate somewhere around South 210th Street. The project is intended to improve regional highway connections and enhance southern access to SeaTac.

At present, travel along I-5 and SR 99 are at best arduous during peak times. Moreover, the state estimates traffic volume will increase another 35% by 2020. As now planned, the project would cut through the Tyee Valley Golf Course and dislocate an estimated 350 homes and numerous businesses. Before the project's tentative ground breaking in 2004, the issue of funding the $411 million project also will need to be resolved.