Construction on the Echo Lake Cutoff Road, along the route of the North Bend–Auburn branch of PSH 2 and the Auburn–Federal Way branch PSH 5, began in 1955. The two-lane highway would cost $9 million (equivalent to $ million in ) and the route was chosen to avoid Tiger Mountain and connect the city of Tacoma to Snoqualmie Pass. The first section, between US 99 and Auburn, was opened on December 19, 1958. SR 18 was designated on the corridor during the 1964 state highway renumbering and codified into state law in 1970, replacing the branches of PSH 2 and PSH 5. The final of the Echo Lake Cutoff, from an entrance to Tiger Mountain State Forest to I-90, was officially opened on December 1, 1964. A new interchange with I-5 in Federal Way was opened to traffic on January 31, 1967, officially completing SR 18.