Police very very rarely prevent crimes from happening. They are there to serve the wealthy and protect property. That's their job. We're there to pay their salaries. They are very good at pursuing evil doers after-the-fact however, but as we all know there's never one there when you need one. The only instance I can recall where the police prevented a crime happened many years ago in Montreal when the holdup squad spotted a gang of bank robbers in their car, surrounded them and shot them to pieces.
Best thing is avoid places where the low-life scum live and play, and don't go out at night without at least one large dog or two CFL players.