Understanding crowd behavior in disasters

approach would be much more complicated than a physical one. “Now we experience a big surprise: The cognitive model is substantially simpler!”

With the new model, researchers can simulate the behavior of individuals as well as the movements of crowds. The simulations demonstrate that pedestrian flows self-organize. When pedestrians walk in opposite directions, such as in a heavily frequented railway underpass, separate lanes of uniform walking direction form — both in the model and in reality. This minimizes mutual obstructions, and makes the flow of pedestrians more efficient. It may be seen as a result of collective intelligence.

If a crowd quake strikes

The model also shows what can happen if the pedestrian density exceeds a critical value. If people have to go shoulder to shoulder, stop-and-go waves may arise. If the density increases even further, the coordinated pattern breaks down completely, giving rise to a turbulent motion which can eventually lead to fatal crowd disasters, in which people are trampled to death.

Helbing has observed that, before such a disaster, the crowd undulates, and people trapped in the crowd are pushed back and forth and, in fact, into all possible directions. The forces are transmitted from one person to another and add up, sometimes over distances of more than ten meters, and potentially with fatal consequences. “Force chains are forming, which can push people suddenly into unpredictable directions, almost like in an earthquake. ” In such a “crowd quake”, one can easily fall to the ground, and the disaster takes its course.

Real-time video analysis could help prevent disasters

In Duisburg , Germany, at the Love Parade in the summer of 2010, twenty-one people died in a crowd disaster, and more than 500 were injured. The new model can help to elucidate the cause of the accident and plan prevention measures for future mass events. At first, it was assumed that the disaster had happened because people fell from a narrow staircase onto the crowd. A closer analysis, however, reveals that the crowd began to undulate before the panic broke out. “A crowd quake was the cause of the disaster.” Crowd management at major events needs special tools to assess such situations. For example, a real-time analysis of surveillance videos can help to save lives, as it facilitates to recognize earlier where problems are building up.

Crisis management by the FuturICT Flagship Project

The project illustrates what is planned on a much larger scale by the FuturICT EU project, led by Helbing. Pedestrian crowds are a perfect example of a social system, in which social interactions can lead to unintended consequences. Even though normal pedestrians try to avoid harming others, crowd disasters have occurred again and again. How they happen has long been a mystery, but thanks to a better understanding of social interactions, the underlying mechanisms have been identified, and early warning signs as well. Moreover, one can take counter measures and prevent crises, as has been successfully shown by the re-organization of the Hajj, the annual Muslim pilgrimage to Mecca.

“From this perspective, the pedestrian crowd project fits the framework of FuturICT perfectly, which aims to develop new risk management tools. “Indeed, one of the goals of the project is to prevent disasters through the use of new computer technologies and by learning to better understand the complexity of social systems,” says Helbing.International workshop on crisesAn international workshop on Coping with Crises in Complex Socio-Economic Systems will be held in Zurich, 20-25 June 2011. Social systems typically feature crises, that is, unstable and dangerous situations that are characterized by abrupt and large-scale changes. Such disruptions are very hard to predict with any precision and even harder to control. Indeed, crises often convey an impression that key decision makers have lost control and that events unfold in an unstoppable and even catastrophic way. Examples include environmental crises, the collapse of transportation systems, as well as financial and social crises such as poverty, social conflicts, or wars. These and other issues will be addressed during the meeting.

— Read more in Mehdi Moussaïd et al., “How simple rules determine pedestrian behavior and crowd disasters,” Proceedings of the National Academy of Sciences (18 April 2011) (doi: 10.1073/pnas.1016507108)PNAS