Navigating the Science of Crowds: From Emergence to Evacuation Routes
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When people come together in a crowd, physical and emotional connections define their movement, state of mind and will to act. Understanding crowds can help us manage the panic caused by a terrorist attack; a science of crowds is vital to managing many emergencies, especially when density becomes dangerously high. Panic or chaos in a crowd can kill or injure hundreds, as happens when there is a stampede. Fundamental science and public safety demand that we develop a complete science of crowds using a range of disciplines. Today, work by social psychologists shows that crowds are influenced by the personalities of individual members; thus, crowds can embody altruistic and helpful behaviour as well as the opposite. And now we can extend crowd science further by incorporating quantitative analysis using classical and statistical physics, computational science and the theory of complex systems – the study of groups of interacting entities.
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One relevant concept from complexity theory is ‘emergence’, which occurs when the interactions among the entities produce group behaviour that could not have been predicted from the properties of any individual element. For instance, randomly moving H2O molecules in liquid water suddenly link up at zero degrees Celsius to make solid ice; starlings in flight quickly form themselves into an ordered flock. Emergent behaviour can be predicted if the interaction among the entities is known, as shown in 2014 by researchers at the University of Minnesota who determined how two people in motion interact and, from that, how a crowd moves. The researchers first considered an idea from physics, theorising that, like electrons, pedestrians avoid collision by repelling each other as they get closer. But video databases showed instead that when people see that they are about to collide, they change their paths. From this, the researchers derived an equation for what amounts to a universal force of repulsion between two people, based on time until collision, not distance.
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The formula successfully reproduced the emergent real-world features of a crowd, such as forming a semi-circular configuration while waiting to trickle through a narrow passage, or extemporaneously developing independent lanes as its members walk toward different exits. This makes it possible to simulate crowd behaviour to design evacuation routes, for instance.To be useful in emergencies, crowd analysis must also account for emotional contagion. In fact, a study conducted by the researchers at the KN Toosi University of Technology in Iran illustrated the relevance of crowd behaviour as a multidisciplinary field of science. They employed ‘fear’ for this purpose— they studied how spreading fear can change emergent behaviour. They created a computer version of a public space populated with hundreds of simulated adults and children, and security guards who directed people to the exits. Assuming that the participants were responding to a dangerous event, the simulation escalated them to greater levels of fear and panicked, random movement when they failed to find an exit. Running the simulation, the researchers found that between 18% and 99% could escape, depending on the combination of participants. The greatest number of escapes did not occur with the smallest or largest numbers of people or security agents but at intermediate values. This shows that the emotional state of a crowd can carry its dynamics into a complicated nonlinear stage.
