Where 400 Is Smaller Than 54

US 54 and smaller US 400 shield in Wichita, Kansas
Photo courtesy of David Backlin

Why is the US 400 shield so much smaller than the US 54 shield on this sign gantry? It's on Interstate 135 in Wichita, Kansas. David Backlin speculates that the cause may be the fact that the US 54 designation for Kellogg Avenue came first, with US 400 being designated later.