SSH 1K was added to the state highway system in 1937. This route began at Jct PSH 1 (US 99) in Seattle and headed southerly to Des Moines, then southeasterly, ending at Jct PSH 1 (US 99) at Midway. In the mid 1950s, when the Alaskan Way Viadict and related roadways were added to PSH 1 (US 99), the northern terminus of SSH 1K was shifted southward to Jct US 99 near the First Ave S Bridge.
Today, SSH 1K from Seattle to Des Moines is part of SR 509. The Des Moines to Midway highway is now part of SR 516.
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