Mo. Rev. Stat. § 226.792: U.S. Route 66, original roads and highways to be renamed "Route 66".

All highways and roadways, and portions thereof, within this state which constituted the original United States Route 66, as determined by the Missouri department of transportation, shall be renamed "Route 66" along those portions which constituted the original United States Route 66 and no political subdivision shall designate such portions otherwise.