Dogs hear what we do not. They hear someone at the road or approaching long before we do. So, realize, that he is not barking at nothing. He is doing his job, alerting you. However, living in a noisy neighborhood makes it difficult because all of those alerts are not the kind you want to hear. Calm him down, hold him close and tell him it's okay - it's just the neighbors. You will probably have to do that for a while until he can decipher those noises from someone coming to the door.
Owner of dogs for many years.