When the current state highway numbering system was first posted in January 1964, SR 401 started at Jct US 101 at Megler and headed NNE to Jct US 830 at Naselle, where SR 401 ended. Before 1966, US 101 crossed the Columbia River via a ferry from Astoria, Oregon to Megler, Washington. When the US 101 bridge across the Columbia was completed in 1966, the southern terminus of SR 401 was moved about a mile SW of Megler to Point Elice, the north end of the bridge. US 830 in the Naselle area became SR 4 in 1968. Today, SR 401 is 12.13 miles long.
Before 1964, SR 401 had been SSH 12B.
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