US 166, running concurrently with US 400, is a very short route in Missouri, about a mile or so, before it ends at Exit 1 of I-44 in Newton County. Both Kansas and Oklahoma are close to this spot.
Note how Missouri used to deal with three digits on a standard-size shield: the numbers for 166 are crowded in, which is doable because 1 is a narrow-width character, while the numbers for 400 are in a smaller font than that usually used for Missouri highway signs. In 2001, Missouri started using wider markers for three-digit US and numbered state highways, as well as for two-letter supplemental routes.
The route enters Missouri from Kansas about a mile from this location.
Photo courtesy of Mark Stauter