Mehdi Moussaid Homepage
Collective Dynamics in human Crowds
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In collaboration with Guy Theraulaz and Dirk Helbing, we have developed a simple cognitive model that describes how pedestrians move and how the surprising self-organization of human flows comes about. |
With the increasing size and frequency of mass events, the study of crowd disasters and the simulation of pedestrian flows have become important research areas. Yet, even successful modeling approaches such as those inspired by Newtonian force models are still not fully consistent with empirical observations and are sometimes hard to calibrate. Here, a novel cognitive science approach is proposed, which is based on behavioral heuristics. We suggest that, guided by visual information, namely the distance of obstructions in candidate lines of sight, pedestrians apply two simple cognitive procedures to adapt their walking speeds and directions. While simpler than previous approaches, this model predicts individual trajectories and collective patterns of motion in good quantitative agreement with a large variety of empirical and experimental data. This includes the emergence of self-organization phenomena, such as the spontaneous formation of unidirectional lanes or stop-and-go waves. Moreover, the combination of pedestrian heuristics with body collisions generates crowd turbulence at extreme densities—a phenomenon that has been observed during recent crowd disasters. By proposing an integrated treatment of simultaneous interactions between multiple individuals, our approach overcomes limitations of current physics-inspired pair interaction models. Understanding crowd dynamics through cognitive heuristics is therefore not only crucial for a better preparation of safe mass events. It also clears the way for a more realistic modeling of collective social behaviors, in particular of human crowds and biological swarms. Furthermore, our behavioral heuristics may serve to improve the navigation of autonomous robots.
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The Model
Today's crowd simulation softwares are often based on analogies with physical systems, such as Newtonnian repulsive forces between pedestrians. In contrast, we have elaborated a new cognitive approach assuming that pedestrians try to minimize the coverage of their vision field, while adjusting the walking speed to keep a safety distance from other people. It is based on the concept of heuristics – decisions that people make without much thinking about their behavior. In order to depict realistically crowd motion in situations of overcrowding, these naviguation rules are combined with the physical forces that occur during unintentionnal body contacts.
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Computer Simulations
Numerical simulations of this model show that these two rules are sufficient to generate a
large variety of collective behaviours, such as the
spontaneous separation of opposite flows of pedestrians in bidirectional traffic (as shown in the movie below).
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Crowd disasters
Above a threshold crowd density (around 3 people per square meter), the model foretold a transition from smooth flows to stop-and-go waves. As the density rose further, as happens around congestion zones such as bottlenecks, the movement lost all coordination, displacing pedestrians in random directions. This prediction agrees well with recent observations of crowd disasters that happened to be recorded by a surveillance camera during a pilgrimage at the Mecca. |
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Crowd collective behaviours are density dependant. The model predicts transitions from smooth to stop-and-go waves and crowd turbulence with increasing density. The density threshold vary with various parameters such as the body size of pedestrians. The turbuent flows happen around 5ped/m2 aroung environmental perturbations such as bottlenecks or turning street. |
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The model could help urban planners to design better exit routes for evacuation of large crowds from buildings and to adapt the environment for a safe planning of mass events. For example, it allows for the identification of zones where the occurrence of crowd accidents is likely. |
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Example of simulations of a large crowd moving through a bottleneck, or facing a |
Reference:
M. Moussaïd, D. Helbing, and G. Theraulaz
How simple rules determine pedestrian behavior and crowd disasters
Proc. Nat. Acad. Sci. , 2011 (Online Article)
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