I-405 functions as SR 405 in Washington State. When the current state highway numbering system was first posted in January, 1964, most of I-405 had not been completed to Interstate standards. SR 405 shields were posted along substandard sections of I-405. Interstate shields replaced the SR shields as sections were brought up to standard.The last SR 405 signs came down circa 1970-71 when the last multilane divided freeway link was completed from Kirkland to Woodinville.
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