chasing cars

Question
our dog has just been hit by a car for the 5th time, we live on a farm and when someone goes passed our gate it its shut he will run out after it, and chase it he has slowly stopped chasing our car and the quad bikes but he still chases others especially our farm owners car (he runs his dogs with him sometimes too) when he drives passed he speeds up we have tried everything we can think of and really need help now as we are going to be moving to a town soon and dont want to lose him.

Answer
You didn't mention your dog's breed, but I don't believe that it will affect my answer. You need to contain your dog with a fence, or an underground fence. The herding instinct is very strong in some breeds and the instinct to protect is strong in most. Car chasing is an almost impossible habit to break. You can try using water balloons (from inside a moving car) or a water gun. This MIGHT work and might not. My best advice is to contain your dog in whatever way is necessary to keep him out of the street. Sorry I can't be of more help
Cindi