When first posted as such in January 1964, SR 518 began at Jct SR 509 in Burien and headed east to Jct US 99/SR 405 north of Seattle-Tacoma International Airport. Where SR 518 ended, SR 405 began. During the late 1960s and early 1970s, SR 518 was moved to a new freeway that connected Burien with Tukwila. The eastern terminus of SR518/western terminus of I-405 was shifted from Jct US 99 to Jct I-5 after I-5 opened from Downtown Seattle to Midway in January 1967. The western terminus of SR 518 was shifted a few hundred feet east when SR 509 was moved from 1st Ave S to the Burien Freeway in the late 1960s. Today, SR 518 is 3.42 miles long.
Before 1964, SR 518 had been part of SSH 1L.
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