SR 18 begins as South 348th Street at a signalized intersection with SR 99, named the Pacific Highway, in the city of Federal Way. The highway travels due east through an intersection with the Enchanted Parkway, which carries SR 161 southwards towards Wild Waves Theme Park and the city of Puyallup, to a hybrid cloverleaf-stack interchange with I-5, providing access to Seattle and Vancouver, British Columbia to the north, and Tacoma and Portland, Oregon to the south. SR 18 becomes a full four-lane freeway as it descends into Peasley Canyon east of a diamond interchange with Weyerhaeuser Way, located south of the Weyerhaeuser headquarters.