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.

The initial two-lane highway, named the Echo Lake Cutoff, was completed in December 1964 after the opening of a section around Tiger Mountain, which would later be the site of over 170 accidents in the 1980s.

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.

[6][7][12] SR 18 continues northeast along the southeastern city limits of Kent, through an interchange with Southeast 304th Street, which serves Green River College,[13] towards Covington.

[16][17] The SR 18 freeway ends northeast of a partial cloverleaf interchange with Issaquah-Hobart Road at the base of Tiger Mountain,[18] becoming a two-lane highway with at-grade intersections for the remainder of its route.

[19] I-90 provides access to the cities of Issaquah and Seattle to the west and North Bend and Spokane to the east, traveling over the Cascade Mountains through Snoqualmie Pass.

[30] The designation had been the result of lobbying from business leaders in Tacoma, who sought a direct connection to Snoqualmie Pass that would improve access to Eastern Washington.

[2][36] The final 7 miles (11.27 km) of the Echo Lake Cutoff, from an entrance to Tiger Mountain State Forest to I-90, was officially opened on December 1, 1964.

[39] Expansion of SR 18 from a two-lane rural road to a four-lane controlled-access freeway began in 1992 response to six fatalities in over 170 accidents in a ten-year period, giving the highway a reputation of being a "dangerous roadway".