Meepa's example of the woman in NY is THE example used by my psychology professor to emphasize the "Bystander Effect". Take any introductory psychology class and you'll hear about it. The sad fact is that most people freeze and don't act when something atrocious happens in front of them.
Don't let the Bystander Effect influence you! Call the cops if you ever see or hear anything suspicious like rape, murder, or whatever. It's generally not a good idea to be a hero...