Hi
That is a hard one to answer. Did a car come close to him on one of your walks? The one thing that I do is that I always move to the side of the road and put my dogs on 'still'. My lab is terrified of most things including cars and lawn mowers and she has gotten used to it as long as she is near me. I walk them beside me and don't let them go in front. The one thing I would suggest is to maintain a positive front for your dog and stay focused on your walk. Let her see that you are the dominant person and she has to follow you at all times. Or as Cesar Millan says, Stay calm and assertive.
I hope this helps you.
Dee Council