I agree with Karen about exercising the dog on leash after it had been trained properly to follow along at a safe slow speed on a very quiet street. If anyone has ever read the book that the Monks of New Skete have written, this is the way they exercised their Shepherds, sometimes from the back of a truck also but with someone holding a leash. We've also used this method and never had a problem.
But to let the dog "chase" the car is sheer stupidity. We took in a rescue GS and the elderly couple who had owned her had let her chase visitors cars when they left so the dog could get exercised. They lived on a private road and the dog always came back. But it did teach the dog to chase cars and when we were talking with one of our neighbours one day the dog spotted a car coming and tore the lead out of my hands and charged the car....yes it did get hit broadside. Luckily it only suffered a fractured tooth but this habit could never be broken.