Road 5 when it was re-designated as PSH 5 as a new highway code was established in 1937. A branch of PSH 2 was designated in 1949, traveling southwest from North Bend, around Tiger Mountain and through Auburn before ending at a junction with PSH 1 and US 99 in Milton. 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. 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. 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". WSDOT widened SR 18 to four lanes and added new interchanges between SR 167 and the Green River within Auburn. The Washington State Legislature briefly planned to toll the new SR 18 freeway in January 1995 to pay off loans for re-construction, but the plan was protested by local residents and rejected months later in May. Additional work was completed in 1997 from the Green River to Southeast 304th Street in Covington, and in 2007 from Covington through Maple Valley and to Issaquah-Hobart Road. The western end of the freeway was extended in Federal Way through Peasley Canyon to I-5 in Federal Way, which was further improved in 2011 with the completion of interchange improvements to the "Federal Way Triangle". WSDOT is planning to widen the remaining section of SR 18, between Issaquah-Hobart Road south of