I work with dog-dog aggression a lot. I get lots of practice in the congested city of Chicago, and I use techniques that work. As with any training topic, there are many competing strategies out there, but I urge you to think about the psychology of anxiety and aggression.
On Friday I met a client for the first session and met his wonderful 1.5 year old Labrador Retriever named Riley. Our session ended after some fantastic leash work, placement cues and some work on Stay. As I was leaving, my client opened the door for me just as a delivery man was opening the door on the ground level. My client immediately saw the worst. The door was open, the gate was open and Riley ran through both. We immediately raced after him and, thankfully, Riley did not make it to the very busy street one block away.