Many many years ago some folk who'd recently moved into a house across the back alley held a really really noisy barbecue in their back garden. I was tempted to ask them to turn it down a bit. But then I noticed a number of them were wearing Hells Angels leather waistcoats. The Hells Angels don't like non members wearing their logo, so these were real Hells Angels. I didn't ask them to turn down their music. But if I had, I think they would have done. It became apparent that the last thing that my new neighbours wanted was for police to have an excuse to come into the area. The scary thing was that that included no more petty crime. For years, folk didn't have to worry if they forgot to lock their front door. It was scary that  the Hells Angels had that kind of control of all local crime. Undercover police surveillance of the house was so cack-handed that I could believe that for some reason it was supposed to be obvious to the gangsters. So, I doubt if it was the police presence that was creating a crime free zone. Once the Hells Angel moved out, crime levels went back to normal. Though I think the house was still owned by them because when I told the arsehole  who moved in afterwards that I was going to speak to his landlord about his antisocial behavior he got really really scared.