Are you reacting to his reaction when he sees a car? The best thing to do is ignore it all and keep walking and looking straight ahead.
You could also sit down in your yard or a place where cars will be going by and try to distract him before they drive by. I'm a big believer in massaging - I would probably have him lay in the grass and start massaging before the car gets close so he's calm, then continue to talk softly and massage as it passes. After a few times, his mind might automatically go into a calm mode when he hears or sees cars coming.
I definitely would ignore any "frightened" behavior though; paying attention to unwanted behavior only reinforces it.