Image: G. Keith Still

In July this year, 12 people died when a nervous crowd rushed to flee the Harare National Sports Stadium during a World Cup tie. On May 31 last year, 54 people died as commuters pushed to get out of the rain at a train station in Belarus. And in 1990, as many as 1,426 people died in a stampede through a tunnel in Mecca. These are but a few of the many crowd disasters that have occured during this past decade alone. Although the panic behavior that caused these deaths is not uncommon, a good explanation is.

In today's issue of Nature, however, Dirk Helbing of the Dresden University of Tehnology, Tamas Vicsek and Ills J. Farkas of Hungary's Etvs Lornd University offer some new insight. Knowing that people in crowds tend to want to move in a particular direction and at a certain speed, they created a computer model, based on self-driven many-particle systems, to simulate the motions of a group trying to exit a room by a single door.

What they discovered was that pandemonium breaks loose when the average desired velocity of the crowd reaches a critical level--a point at which the need for speed becomes greater than the need to avoid collisions. They also ran simulations in which they assumed the crowd could not see the exits. Here, too, individuals struggled with a balancing act, pitting their tendency to follow the herd against their desire to find an escape route; the model showed that a mix of both worked best. Now the researchers are calling for additional data and video footage, hoping to refine the model into something that provides practical ways to engineer against crowd disasters.