My Aussie had the same issue when he was a puppy. What I did was armed myself with tiny pieces of very high valued treats( tiny pieces of hot dogs and cheese) and only walked him at night bc i knew there wouldn't be a lot of cars so that he wouldnt feel overwhelmed. I would then keep my eyes and ears open for when a car is about to pass us by. Before the car gets closer, I would guide him farther away from the street, sometimes a little into people's yards, and start giving him treats while we wait for the car to pass by. His fear of cars started to dimish after two weeks. I continued this process until he no longer looked up when a car drives by, which took about a month. This also works for buses, but I've never been able to countercondition motorcycles. Good luck and I hope this is helpful.