605 is a number tentatively used to identify a proposed outer loop around the Seattle area. Whether this would be labeled as a state route or an Interstate route has not yet been determined. The idea of building 605 dates back to the 1960s. At one time the highway was to have run to the west of Lake Sammamish. Later plans called for a route east of the lake. There are no active plans to build this highway but the idea keeps coming up from time to time.
No legislative route number for 605 has ever existed in the Revised Code of Washington. The number PSH 19 was a tentative number used in some documents, however.
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