When the current state highway numbering system was first posted in January 1964, the highway heading north from US 2 via Sacheen Lake towards Usk was signed as SR 311. The highway following the Pend Oreille Valley downstream from Newport to the Canadian Border was SR 31.
SR 20 was extended east to Newport in 1973 after the North Cascades Highway had been completed the previous September. The SR 20 designation replaced the SR 31 designation from Tiger to Newport. SR 311 was renumbered SR 211.
Before 1964, SR 311 had been SSH 6B.
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