A 1947 document exists showing a proposed extension of U.S. Route 410 into Seattle. In Seattle, U.S. Route 410 was slated to follow U.S. Route 10's original allignment up Rainier Avenue north to the now deconstructed Lakeway Expressway and would have ended there at its parent U.S. Route 10 (which was the Lakeway Expressway and Rainier Avenue between Lake Way and Dearborn Street). Had U.S. Route 410 ever been re-routed to Seattle, its existance would no longer be a violation of the AASHO guidelines.
Despite this document, no evidence exists showing that U.S. Route 410's western terminus was ever re-routed to Seattle. Instead, U.S. Route 410 stayed on its exact same route until it was replaced by U.S. Route 12 in the late 1960's. Today, there are still businesses and roadsigns along the old routing which display U.S. Route 410's old number.
U.S. Routes 99, 10 and 410 were the only three U.S. Highways that existed or were planned to exist within Seattle's city limits.