
Hi Andi,
I have a friend currently experiencing this problem (different breed, but don't think that's necessarily relevant). With her dog she only ever sees traffic on country lanes as she is not near any main roads heading for their twice daily constitutionals :) I suggested finding a wide, relatively busy road with good broad pavements on each side and taking her dog daily to sit or walk by literally constant traffic (always oncoming - never from behind). It took only about a week to see a big change in her dog's attitude. I reckon this works for her dog because a) there are so many cars to get over agitated about he exhausted himself within 5/10 minutes on his first outing and b) continual habituation with the problem - but wide roads and pavements meant he was well enough back from the traffic - meant he had no negative experiences.
You don't say how old your dog is but again this may have a bearing on how well (quickly) he will respond. Of course please remember that it is also essential to praise him every time he behaves acceptably :P
Sincerely hope this helps, and that others may offer you some additional and alternative suggestions as obviously it can't be much fun for either of you.
Very best wishes, Teri :)