SR 181 could be called the incredible shrinking highway. When the current state highway numbering system was first posted in January 1964, SR 181 began at Jct SR 18 on the west side of Auburn and headed north on the West Valley Highway (called Washington Ave near downtown Kent), passed thru the west side of Kent, became Interurban Ave as it crossed SR(now I-)405 in Tukwila, cotinued along Interurban Ave and East Marginal Way to Pacific Hwy (at the time US 99), and then headed east on the Boeing Access Rd to Jct SR 900/Empire (now Martin Luther King Jr) Way. The Pacific Hwy S to SR 900 portion of SR 181 was dropped from the state highway system in 1970. In 1971, the north end of SR 181 was shortened further to Jct SR 599 near I-5. In 1979, the I-405 to SR 599 part of SR 181 was dropped from the state highway system. Finally, effective April 1, 1992, SR 181 from jct SR 18 to Jct SR 516 in Kent was dropped from the state highway system. The remainding route of SR 181, from Jct SR 516 in Kent to Jct I-405 in Tukwila is 6.05 miles long.
Before 1964, SR 181 from Auburn to the Boeing Access Road had been SSH 2M and along the Boeing Access Road, the Duwamish Connection branch of PSH 1
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