State Route 18 (SR 18) is a 28.41-mile-long (45.72 km) state highway in the U.S. state of Washington, serving southeastern King County. The highway travels northeast, primarily as a controlled-access freeway, from an intersection with SR 99 and an interchange with Interstate 5 (I-5) in Federal Way through the cities of Auburn, Kent, Covington, and Maple Valley. SR 18 becomes a two-lane rural highway near Tiger Mountain as it approaches its eastern terminus, an interchange with I-90 near the cities of Snoqualmie and North Bend.

State Route 99 (SR 99), also known as the Pacific Highway, is a state highway in the Seattle metropolitan area, part of the U.S. state of Washington. It runs 49 miles (79 km) from Fife to Everett, passing through the cities of Federal Way, SeaTac, Seattle, Shoreline, and Lynnwood. The route primarily follows arterial streets, including Aurora Avenue, and has several freeway segments, including the tolled SR 99 Tunnel in Downtown Seattle. SR 99 was officially named the William P. Stewart Memorial Highway by the state legislature in 2016, after a campaign to replace an unofficial moniker honoring Confederate president Jefferson Davis.