There's no such thing as US 210, and if there were, Kansas City is probably too far south for such a designation. The sign should be for Missouri 210, of course. This photo, taken October 2003, is on Interstate 435 northbound at the Front Street exit, just before the bridge over the Missouri River. Missouri 210 runs parallel to the river on the north side of the waterway.
Remarks made by other participants in the Usenet newsgroup misc.transport.road indicate that this error has been around since at least 2000, and perhaps a few years earlier.