SSH 1G was added to the state highway system in 1937. This route began at Jct PSH 1 (US 99) in Mount Vernon and headed east, ending at Jct SSH 1A a few miles east of Mount Vernon. In the mid 1950s, SSH 1G was extended west a few blocks when US 99 was moved to the Mount Vernon freeway, now I-5. SSH 1G is now SR 538.
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