Gateway Arch Replica
Architect Eero Saarinen designed the Gateway Arch, signature landmark for St. Louis, completed in 1965 (and opened in 1967). It's 630 ft. tall and 630 ft. wide. Visitors ride a tram capsule up a leg and gaze down upon the city and the Mississippi River.
But if you don't intend to take the ride, and only view the Arch from a distance, that's where the one in Vandalia comes in handy. Its replica "baby arch" is 1/10th the height of the original -- but who can tell? The big motel sign might require some explaining, but it saves you an hour or more driving to the real one.
The "baby arch" tribute seems to date back to the 1960s, thought the motel brand can change overnight.