i have a 9month old border collie puppy. and he used to do exactly the same! with perseverance it will fix itself almost.
we have tried:
a down. and stay on the pavement when a car passes. and i know its hard work if 4 or 5 cars pass one after the other. but it was my step one. and once he stays and doesn't move or get up to chase the car, i reward him for doing so.
Miller Used to stalk [herding gene] and then lunge forward.
so eventually what actually fixed it was a tug on the lead everytime. if he even slightly showed interest in the car. and a firm NO! did the job. and then walking in a relaxed mode he was also rewarded.
i got alot of advice also, such as take your dog to a car park. aka [tesco or asda?] where there are cars moving all the time. but its also a secure spot and start there. walking around. teaching your dog that a car is nothing to fear and its not a target.
my dog still watches for cars, but there is no afterthough.
but this is on a lead. i would never allow him to walk off lead on the pavement beside a road. because i have no idea how he would react. i hope this has helped or given any advice to you.