If I was developing a game like this, I'd probably allow the starting enemy types to get ran over, but then would introduce zombies that would damage you if you tried to do that, e.g: fat zombies, giant mutated zombies, zombie cows, etc. It would make sense from both the gameplay and logic standpoint(i.e. they damage your bus cause they're big and heavy).
No idea how I'd implement the scavenging part, it needs some more brainstorming.