SSH 1W was added to the state highway system in 1937. This route originally began at Jct PSH 1 (US 99) near the King-Snohomish county line and headed NW to Edmonds. In 1957, this highway was extended east from Edmonds to Jct PSH 1 (US 99) in Lynnwood. In 1959, SSH 1W was extended further east to Jct SSH 2J at Thrashers Corner. This later extension did not take effect until 1961. State highway markers were not posted on this extension until even later.
The US 99 to Edmonds portion of SSH 1W became part of SR 104 in 1964. The Edmonds to SSH 1J (since 1964, SR 527) portion became SR 524 although at first SR 524 signs were only posted from Edmonds to US 99. In January 1965, I-5 opened from Seattle to Everett. SR 524 signs were then posted between US 99 and I-5. It was not until 1992 that SR 524 signs were finally posted along this route from I-5 to SR 527.
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