
I have a rescue border collie who spent most of the first year of his life on a chain on a farm and he too is a car chaser and had never really met anything but farm traffic. I started off taking him to a supermarket car park and parking right at the back of it with the tailgate open and getting him to watch me and rewarding every time he glanced at me rather than the cars that were passing by. I chose that car park as the cars move slower in there than they do on a road. I then progressed to finding a spot where he could see a busy road in the distance but he was not near to the traffic - within his own comfort zone - and did the same and gradually worked our way nearer to traffic over a period of time.
I am now able to walk him fairly close to traffic - having a grass verge beyond the pavement and next to the road is now within his comfort zone. I try to find roads where we can walk on the side that faces oncoming traffic as traffic approaching from behind and whizzing past is more of a temptation and is self rewarding as in his eyes he has chased it off. I also have started to walk him around a large industrial estate in the dark as there is very little traffic and the headlights seemed to add to his discomfort as does wet weather when the tires make a swishing noise.
It has been a long, slow process and has to be done in tiny, tiny steps but a year on and he is so much better than he was before but we still have a way to go. Interestingly he isn't the slightest bit bothered by trains that rush past - some of them are the highspeed trains that really move but he shows no interest in those at all and some do pass us very close when we are waiting to cross the tracks on one of our walks where the footpath has a pedestrian gate.