I would agree that you should get dog obedience help to break the habit. When I was young we had a dog that never would stop chasing cars. We tried all sorts of methods, none of which worked. Not even getting hit with a glancing blow from a car stopped him (three days later he was back at it again). We lived rurally and didn't keep him tied up (this was 1964, way before leash ordinances were common). Chasing cars did him in. We did the poor dog no favors by letting him run free and not breaking him of the habit.