13:09 07 September 2007
NewScientist.com news service
David Robson

'Crowd quakes' could be predicted by CCTV analysis

   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's 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 "crowd quakes" when built-up tension is suddenly released. They think CCTV analysis could spot and warn of dangerous tensions before such an event.

Dirk Helbing 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's Central Directorate for Holy Areas Development.
Compression wavesThe 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 "pressure" of the crowd.

As might be expected, the stampede occurred because too many people were funnelled into too small an area. But the team's 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 – 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. ...