First of all, in my experience if individuals in groups see emergency situation, they react slower than they see it by themselves or they just ignore the dangerous situation. When I was shopping in Manhattan, I saw a middle age guy shocked by the heat and fell down. However, many walkers saw him but didn't help him, and just walked away from this man. Although this man got help later on, at that time many witnesses didn't help him immediately. In this case, the crowd on the street behaves like a group, and ignores the man. The most significant idea in this passage is " Often observers think nothing needs to be done because some else has already taken care of it, and the more observers there are , the less likely any one person is to call for help" In other words, people often think it isn't my business, someone will take care of it. Which indicate individuals are more likely less reactive in the groups.
I have experienced "Diffusion of responsibility" or "Social loafing" when I read a news about subway murder ignorance news, if there was a person gave victim a hand, he could get out from the rail way, and he could save his life. However, according to the news picture, no one helped him when he was asking for help under the platform. Based on this example, I think people in groups have less responsible for accidents, and this irresponsible behavior can put victims into terrible situation.
I think it is happen because in more crowd city human being's mentality is different than normal city. When I was living in California, I experienced people try to help each other, unlike crowd New York city that dwellers try to step back when they see situations. once my car ran out battery in a California shopping mall parking lot, I could easily get help from other shoppers to charge my car. However, in the heavy traffic New York City, I could call the car service for getting help.
In conclusion, it is true that when many observers see an accident, they probably step back and wait until hero come out. In my opinion, this is a sad fact, because everyone may have unexpected emergency situation in their lives.