If you see someone sprinting out of the convenience store at top speed, and later the police ask you if you saw anything suspicious, you might tell them this person ran past you like a bat out of hell.
Bats are famously fast when they're flying around — often too fast for you to follow them with your eyes. Nobody has actually seen a bat escaping from hell, but everybody's guess is that such a bat's flight would be extra fast. That's what's behind the expression like a bat out of hell. You can go like a bat out of hell on foot, in a car, on a bike, or any other way.