When the current state highway numbering system was first posted in 1964, there was no SR 123. When US 12 was extended from Lewiston, Idaho to Aberdeen, Washington in late 1967, SR 143 starting a few miles east of Packwood and heading north to Cayuse Pass was renumbered SR 123. The SR 143 designation was later moved to a different highway. Today, SR 123 is 16.36 miles long.
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