BlsdMama: GSD owner/lover here. I agree with the sign and box suggestions. I also encourage the purchase of an electronic collar (Here, we refer to it as the "Cats Are Not Snacks Collar.") It is a very valuable tool. I would do so out of fear for my furry friend being harmed while trying to catch a car. :(
(And for all of you who will be terribly concerned about my vicious dog taking out his thwarted cat "aggression" on humans, the only time he's actually caught cats [twice], he did not harm them. They were slobbered on and REALLY cranky, but not harmed. Much like the gopher he caught in the backyard; the poor GSD was outraged when the gopher bit him. Then the little ghetto dog we rescued went over, killed the gopher, and tried to eat it. Hmm, maybe he's dangerous?)