Phoebe can pretty much ignore traffic unless something really noisy goes by. I talk to her and tell her what a good girl she is for not reacting to the cars, and if she blows it she has to sit until she calms down. no treat. All I can recommend is that you walk her with a halter rather than a collar. You never know what they are going to do and a harness gives you better control.
I had stouter bigger shelties in the past and just used a regular collar. But now I have smaller girls and my Vet and breeder suggested harnesses because a collapsed trachea can happen easily on a sheltie. I also think a harness makes them a bit more secure feeling.