In 1926, the Pacific Highway Auto Trail was redesignated US 99. State Road 1 was also designated along the auto trail's path in Washington at this same time. In 1933, SR 1 became Primary State Highway 1 and was signed along its entire route. PSH 1 later spawned branch highways and secondary state highways (SSH) in many areas in Washington, including Seattle. The historical branches in Seattle go as follows:
- SSH 1J (now SR 513)
- SSH 1K (now SR 509)
- PSH 1-EP (Evergeen Point Branch, now SR 520)
- PSH 1-WM (West Marginal Branch, now part of SR 99)
In 1957, pieces of US 99/PSH 1 became part of the new I-5. Other new sections of PSH 1/US 99/I-5 were also under construction at this point. Futher changes occured in 1964, when Washington adopted a new state highway system. PSH 1 was now joined by SR 99 (the state's new designation for US 99) and SR 5 (the state's new designation for I-5). The I-5 portions of the highway now carried the designations I-5, US 99, PSH 1, SR 5 and SR 99. The non-I-5 portions carried the same highways minus the I-5 designation.
In 1967, US 99 was completely de-commissioned in Washington state and was now relegated to being the present day SR 99 from Fife to Everett. Due to this, PSH 1 no longer followed SR 99. However, unlike US 99, PSH 1 continued to exist in Washington State along I-5's entire length. Finally in 1970, Washington eliminated the old state highway system and caused PSH 1 to fade into oblivion with its former running mate.
Today, SR 99, I-5, US 12 and many other highways follow portions of the now-defunct Highway 1.