When the current state highway numbering system was first posted in January 1964, SR 12 extended from Jct US 97 at Maryhill to Jct US 410 in Kennewick. The highway from I-5 Exit 68 near Mary's Corner via White Pass to Jct US 410 near Naches became SR 14. US 830 from Vancouver to Maryhill was implicitly an extension of SR 12 with 120 series SRs branching off from US 830. In 1967, US 12 was extended west from Lewiston, Idaho to Aberdeen, Washington. This extension replaced the US 410 designation from Lewiston to the Naches area and from Elma to Aberdeen. Instead of following US 410 over Chinook Pass, closed in winter, US 12 was routed across the Cascade Mountains via White Pass, lower in elevation and opened year round. The SR 14 designation was switched to the route from Maryhill to Kennewick. 120 series highways branching off from US 830 and SR 12 became 140 series routes and 140 series 120.
Before 1964 the highway from Maryhill to Kennewick was the Maryhill-Kennewick branch of PSH 8.
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