Pressure waves that travel through tightly-packed crowds on the verge of panic could warn of impending disasters, such as the stampede on Saudi Arabia鈥檚 Jamarat Bridge during the Hajj of 2006, researchers say.
The team studied footage of the tragedy and found crowds can experience sudden changes like shock waves, turbulence, and even 鈥渃rowd quakes鈥� when built-up tension is suddenly released. They think CCTV analysis could spot and warn of dangerous tensions before such an event.
of the Swiss Federal Institute of Technology, Zurich, analysed the Jamarat Bridge footage along with Anders Johansson of Dresden University of Technology, Germany, and HE Habib Al-Abideen from Saudi Arabia鈥檚 Central Directorate for Holy Areas Development.
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Compression waves
The researchers used software to simplify the video and represent members of the crowd as moving patches of colour. They measured features such as the density, speed, and 鈥減ressure鈥� of the crowd.
As might be expected, the stampede occurred because too many people were funnelled into too small an area. But the team鈥檚 analysis uncovered new features that might give forewarning of similar disasters.
Previous research suggested crowds move in smooth flows like a fluid, without sharp changes in direction. But, in this study, once the density of the crowd reached more than seven people per square metre this principle broke down.
Sharp compression waves moved through the crowd, shifting people back and forth 鈥� like grain or sand when driven through a funnel, says Helbing. During this period, which lasted 20 minutes, each person was alternately moving or stationary, and the waves of movement travelled through the crowd every 45 seconds.
Turbulent movement
At even higher densities, the crowd鈥檚 movement became turbulent. Each person was jostled in random directions. One shove could affect people up to 12 metres away.
鈥淭he forces add up and are transmitted by the bodies,鈥� says Helbing. 鈥淚t鈥檚 dangerous because it鈥檚 difficult to know which direction it鈥檚 coming from, so it鈥檚 hard to keep balance.鈥� Once someone falls, they may get trampled. This is when a stampede occurs, with the disaster quickly escalating out of control.
He says the build up of forces from many different directions is very similar to a different kind of disaster, with a sudden release of tension that spreads very quickly. 鈥淭he situation is as uncontrollable as an earthquake,鈥� he said.
He hopes that in the future, organisers of events could use his software to analyse live CCTV footage and direct emergency services to areas where the tension is building up. 鈥淚 think it would be advisable to apply the software to rock concerts, soccer games, or any big events,鈥� Helbing said. Although preventing over-crowding in the first place is the best solution, he adds.
, a consultant on crowd behaviour from Crowd Dynamics, described the maths as 鈥渆xcellent鈥�, but was concerned about the implications of this research. 鈥淚t perpetuates the myth that panic is to blame for the disaster. People only panic when they鈥檝e exhausted all logical methods of escape, at which point it鈥檚 already past the safety limit.鈥�
was presented at the Fifth International Conference on Nonlinear Mechanics.
