A new model of crowd behavior uses simple visual rules.
What’s the News: When crowds go wrong, they go really wrong
—more than 300 people died in a stampede in Cambodia
last year during a festival, and hundreds more have been crushed to death in periodic disasters near the Muslim holy city of Mecca
. A major flaw of computational models describing how people behave in crowds is that they are often too simplistic or too specific to a situation to explain both normal and disastrous behavior. A new model
manages to recreate both types of behavior, working from two basic visual rules: (1) each person will move in the least crowded direction in their line of sight, and (2) they will adjust their speed to maintain a safe distance from visible obstacles. “This work is an extremely important step in pulling together our fragmented understanding,” says behavioral biologist Iain Couzin, who was not involved in the study (via ScienceNOW
). “We’re now approaching a sort of unified understanding of human behavior in crowds.” How the Heck:
The two simple rules, in combination with some basic physical laws, were enough for the model to recreate normal crowd behavior, such as people forming lanes as they walk down hallways, along with more unusual behavior.
As more people crowd into a tight space, the model develops stop-and-go traffic, and with even more crowding, the phenomenon known as turbulence appeared. This chaotic movement has been at the root of various crowd disasters; capturing its development in a model, especially with such a simple set of rules, is an important achievement.
The fact that these rules are visual---in the model, what people see is key to what they do---is key. Physical laws have dominated computational models of crowds so far, and shifting the focus to the senses humans actually use in such situations could make more complex and accurate modeling possible.
What’s the Context:
Urban planners use models of crowd behavior to design safe public spaces, where dangerous bottlenecks and other features that can trigger panics are avoided.
Many models treat people like particles, meaning that they obey simple physical rules like avoiding others and taking the most direct route to a destination. This is usually sufficient for most purposes, but it doesn’t explain unusual behaviors that crop up in extreme situations.
The best models incorporate a fair bit of freedom on the part of individuals. Those developed by Paul M. Torrens at University of Arizona, for instance, include body language and subtle human behaviors such as a tendency to veer right to avoid obstacles.
Reference: M. Moussaid, D. Helbing, G. Theraulaz. How simple rules determine pedestrian behavior and crowd disasters. Proceedings of the National Academy of Sciences, 2011; doi:10.1073/pnas.1016507108
Image credit: PNAS