When first posted as such in January 1964, SR 505 began at I-5 Exit 60 and headed east to Toutle, crossed the Cowlitz River on the 1920 vintage old US 99 bridge, then headed easterly and southerly to Jct SR 504 a few miles NE of Toutle. Several changes were made to the state highway system effective April 1, 1992. Former SR 603 from Winlock to I-5 Exit 63 was redesignated as part of SR 505. Former SR 505 from I-5 Exit 60 to Toledo was dropped from the state highway system. Former county roads from I-5 Exit 63 to Toledo were added to SR 505. A new Cowlitz River bridge was built as part of SR 505 at Toledo. Today, SR 505 is 19.29 miles long.
Before 1964, SR 505 had been SSH 1Q.
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