The Route 66 Drive-in is exactly the kind of mid-century Americana that belongs in Springfield: cars, neon, popcorn, and a film under the Illinois sky. It is a working seasonal cinema rather than a museum piece, which makes it more fun than a static roadside sign.
Go only when a screening fits your evening, and check the season before planning around it. For Route 66 travellers it is an easy way to turn nostalgia into an actual night out instead of another photo stop.