The Bystander Effect dictates that groups can generate something called diffusion of responsibility, which allows individuals to absolve themselves of responsibility so long as others remain uninvolved.
The bystander effect is often sited when violence happens in broad daylight or in crowds (or both) and no one helps.
Case in Point: http://www.huffingtonpost.com/2009/10/27/police-people-watched-gan_n_334975.html
The police are there to discourage bad behavior. They rarely thwart violence in action.
Your safety is your responsibility.
Be Safe, be Strong, Be Aware, Be Happy