Symbolism, Imagery, Allegory
You know, if we were crazed killers on the run from the law we would not choose to drive in a "big, black, battered hearse-like automobile." (70)
Because nothing symbolizes imminent death like a hearse. And yet, that's the kind of car that Misfit & Co. chose to roll around Georgia in: a car that looks like it should be carrying a coffin to a funeral. It's excellent imagery for O'Connor's story, but super-dumb if you're trying to keep a low profile IRL.
(If we were in the Misfit's shoes, we'd choose something non-threatening, like a VW Bug or a pink Fiat.)