Big Al: the keys and the highway are a metaphor for personal freedom.

Hootenanny: "Billed Out" = Entered in a railway book as a passenger

Billed out and bound to (or for) is a general freight & passenger expression, works for an old Trailways or Greyhound bus too.

I always took it as the indigent version of the Midnight Special. If not picked up by family, or paying their own way, released inmates were bused to the State or County line by the prison.

They had to walk... or run, from there on.