SSH 1L was added to the state highway system in 1937. This route began at Jct PSH 5 (now SR 167) in the south end of Renton and headed west, ending at Jct SSH 1K in Burien. In 1957, the Renton to PSH 1 (US 99/I-5) section of SSH 1L was transferred to the Renton Branch of PSH 1 (I-405). The remainder of SSH 1L from I-5 to Burien is now SR 518 .
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