Definition: A tendency to avoid taking responsibility for a task or problem because you assume others will do so.
Contrary to logic, the more people who witness an accident, crime, or emergency, the less likely the person is to get help. Rather than falling on one person, responsibility is experienced as diffuse (shared), leaving everyone assuming someone else will take it.
The most famous example of this social phenomenon is the murder of Kitty Genovese. Walking home one night after waitressing, Kitty was raped and then stabbed to death in Queens, New York. Thirty-eight people heard her screaming and/or turned on their lights and leaned out their windows to investigate. She was attacked 3 times in 35 minutes, but it was only after it was too late that someone finally called the police. When reporters investigated, every witness claimed s/he believed someone else would have called for help.
As a result, if you (or a character!) need help, you should pick someone from the crowd and give that person specific instructions: "Call for an ambulance and tell them someone collapsed and needs medical attention."