How much empathy do you feel towards other people? If you saw someone fall off their bike in traffic, would you stop and help - or just walk away? Many people would give in to apathy, go about their business and just do nothing. So it was a big surprise when about 100 bystanders got together recently to move a bus in east London to help a cyclist trapped under it. A video of the incident went viral on social media.

According to Zoheb, a driver who stopped his car to take part in the rescue, about five people gathered to move the bus. He says: "There was no chance we could do it, it was more an invitation for other people to help, really."

The initiative paid off. Diners from nearby restaurants joined in. There wasn't much coordination but it didn't take long to develop a collective understanding of what the objective was.

Spontaneous collaboration among strangers doesn't happen often. People in a crowd are not sure what to do - they don't have a plan. It's one of the reasons bystanders often do nothing, according to Dr Mark Levine, professor of social psychology at Exeter University in Britain. "The presence of others can inhibit you from helping", he says.

The key to positive group behaviour and intervention, Levine explains, "is building a sense of shared identity". Action has to be decided quickly, Levine says. "The longer you leave it, the harder it is to make a decision. If you don't immediately act then you kind of think 'Well, actually I probably couldn't have done anything anyway'."

But the people who took the initiative like Zoheb might make a difference. The cyclist ended up in hospital and the images of the collective effort might inspire others to more acts of solidarity.