I have yet to have any nightmares (knock on wood), but I can give you a few pointers that will help you to mitigate the risks:
1. Try to work with someone. I'm guilty of trying to shoot rig stuff by myself. Though, I'm trying to avoid it. I'll always look for someone to help me. I'll have the other guy sit behind the wheel, while I manage the rig.
2. Support the rig with your hand: This will help to counter the bounce and minimize blur, as well as prevent damage to the car, in case the thing takes a dump on the hood.
3. Find a piece of flat styrofoam sheet, or you can use a cloth, and wrap the pole around the area where if it were to fall, it would come in contact with the car.
4. Beware of your surrounding. Make sure the car does not roll in an area where the poll may get caught in something, or may harm someone or someone's property.
That should be it. For the most part, if you use your head and think twice before you do something, you should be ok.
Good luck 