My husband killed a JR when it bolted - the owner was obviously distraught but not half as distraught as he would have been if my husband had instictively swerved and killed more than the JR which would have been the cause in the first place.
My Mum and Dad had 2 and i was brought up with them just in case it appears to be an anti JR thread.
They reigned them in -they coudnt be trusted with other dogs (or humans -scars to prove it) even but luckily Mum and Dad have a huge garden so they had free reign there.
You could try a spray training collar (google it) - the dog wears it, you have a remote, when he chases cars, you press a button, the collar sprays water on the dog (making quite a noise), and distracts the dog.
Might seem cruel, but could save the dog's life - if it continues to chase vehicles, it will almost certainly be killed.
My 10 month old is as bad, he chases anything that moves, doesn't quite understand that the sun doors have to be open for him to chase something outside though! :-/ i'd say it's just a phase, as soon as yours realises that the cars are faster, he will get bored of it. :-)