Secret of Annoying Crowds Revealed

Science NOW
by Dave Mosher on April 7, 2010

Push, shout, or politely excuse yourself all you want, but those slowpokes in your way just won't budge. A new study shows a long-neglected reason why: Up to 70% of people in crowds socially glue themselves into groups of two or more, slowing down traffic. What's worse, as crowds gets denser, groups bend into anti-aerodynamic shapes that exacerbate the problem. The study may be a boon to urban planners.

Crowd physicists already understand the effects of bottlenecked entrances, dueling streams of pedestrian traffic, and even "turbulence" in shoulder-to-shoulder mobs. In the past 15 years, this work has led to decent mathematical models that architects, city planners, and pretty much anyone dealing with crowds can use to make their spaces safer and more flock-friendly.

Trouble is, the simulations treat people as independent particles—ignoring our love of sticking in groups and blabbing with friends. Small groups of pedestrians change everything, says Mehdi Moussaid, the study's leader and a behavioral scientist at the University of Toulouse in France. "We have to rebuild our knowledge about crowds."

Moussaid and colleagues videotaped two types of posses in Bordeaux, France, from above: the weekday rush and the weekend stroll. Weekday crowds turned out to be about half individuals and half groups (of two or more), whereas weekend footage showed that 70% of people were in groups. Groups of more than four were rare, leading the researchers to conclude that big groups split into smaller ones. In total, the researchers tracked and measured the intricate movements of more than 4500 French pedestrians—data they crunched to build more accurate simulations of crowds.

The researchers found that socializing groups slowed crowd traffic down by about 17%, compared with models in which pedestrian groups didn't interact. They also reveal today in PLoS ONE, that groups of three or more flex into V and U shapes as crowds get denser, with central group members falling back relative to flanking members. This adds insult to injury for pedestrian traffic that is already gummed up, Moussaid says, but it allows the chitchat to continue. "We're not so different from sheep when it comes to crowding. What sets us apart is social interaction," he says. "Walking backwards is not exactly practical, so we form V and U shapes at the cost of speed."

"I'm in discussion with planners from all over the world, ... and the realistic simulation of [group] effects is one of the hottest topics for application," says Tobias Kretz, a software engineer at PTV AG, a company in Karlsruhe, Germany, that consults planners on traffic mobility logistics. Kretz uses a program called VISSIM to model crowd traffic for his clients, and he says Moussaid's work is precisely what he's been waiting for. "We are definitely planning to include the model in ... VISSIM's simulation of pedestrians and make it globally available for traffic-planning projects."

Applications for improving pedestrian traffic on sidewalks, train platforms, malls, and other public spaces aside, Moussaid says he noticed something else during the work: Renegades who rush around lollygagging pedestrians only make things worse. "You're contributing to chaos," he says. "Crowds are self-organized systems, so when you don't cooperate, the system breaks and you slow everyone down."