Mob stampedes have killed thousands of people in recent years, but they are usually explained in terms of psychology. Now, European scientists say they can predict and prevent crowd panic via computer simulations using the laws of physics.
The new computer model relies on distances, sizes and velocities instead of emotional states but produces results similar to actual panics, the researchers said in Thursday's issue of the journal Nature.
"We think it works particularly well in panic situations because people don't think about what they should do," said Dirk Helbing, a professor at the Institute for Economics and Traffic at Dresden University of Technology.
The computer models reflect the conditions of a room from which people are trying to escape but cannot use or see exits. The virtual victims appear as particles that reflect an average person's speed, size and desired distance from others.
Under normal circumstances, a crowd exits a theater or stadium in an orderly and coordinated fashion because everyone is moving at a leisurely pace and at an adequate distance from one another, the researchers said.
But when the speed of the individuals increases in a panic, they bump into each other, creating friction and violating personal space. As a result, almost everybody moves less quickly.
Eventually, solid arch-shaped barriers of people clump around the exits and even fewer people can pass to safety. Victims collapse and are trampled, creating further obstacles for the others.
"People want to leave faster, but the result is that they are leaving slower and then the tragedy begins," Helbing said. "The question is what can you do about that?"
One solution is to build a partial barrier in front of the exit, the researchers said. It could absorb pressure from the crowd that can become strong enough to crush a person, break a brick wall or bend steel.
"It turned out that we had no injuries," Helbing said of the approach.
As the world becomes more crowded, such simulations will grow more important for architects, event planners and police.
This year, eight people were killed in July at a rock festival in Copenhagen, and a dozen died the same month after a World Cup match in Zimbabwe. Ten years ago, more than 1,400 pilgrims were killed inside a tunnel leading to Mecca.
Helbing and colleagues Illes Farkas and Tamas Vicsek found many causes for panic. In some cases, a crisis like a fire or a poorly planned obstacle can drive the hysteria.
The researchers also used their simulation to devise strategies for surviving crowd panic. In the case of a burning theater where smoke obscures the exits, just acting alone or totally following others can be deadly, they said. The solution is to do a little of both.
"It requires some individualism to explore the environment, to find the possible solutions and then, if there is someone who found the solution, it is good for the others to follow," Helbing said.
The new computer models are especially useful because they consider individual actions within the group, David J. Low, a civil engineer at Heriot-Watt University in Scotland, said in an accompanying commentary.
Most modern buildings are designed assuming crowds flow through the exits like fluid through a pipe, he said.
"This traditional approach assumes that the crowd is made up of identical, unthinking elements," he said. "A fluid particle cannot experience fear or pain, cannot have a preferred motion, cannot make decisions and cannot stumble and fall."