SSH 1I was added to the state highway system in 1937. This route began at Jct PSH 1 (US 99) in Everett and headed west via Mukilteo Blvd to Mukilteo, then southerly, ending at Jct PSH 1 (US 99) south of Everett and north of Lynnwood.
In 1965, the southern terminus of SSH 1I was moved from Jct US 99 to Jct I-5. The section of I-5 from the north end of Seattle to the south end of Everett had opened to traffic in January 1965.
In 1967, the freeway corridor linking I-5 in the south end of Everett with the then new Boeing 747 plant became the east-west leg of SSH 1I. This freeway did not open to traffic until around 1968 or 69. Mukilteo Blvd was dropped from the state highway system.
Today, the east west leg of SSH 1I is SR 526. The north-south leg is now part of SR 525.
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