Recent Press Releases

Crowd turbulence: the physics of crowd disasters

PR10(07)

Tue, 22 May 2007

In January of last year, the annual pilgrimage of Muslims to Mecca in Saudi Arabia, known as the Hajj, was marred by a fatal crush in the crowds gathered at the town of Mina for a ritual symbolic stoning of pillars called the jamarah. Over 300 people were killed. But this year’s Hajj passed off without incident. The difference seems to have been due to a bit of physics, as Dirk Helbing of the Technical University of Dresden in Germany will explain in a plenary talk on 12 April at the Condensed Matter and Materials Physics conference organised by the Institute of Physics.

Helbing was asked by the Hajj organizers in Saudi Arabia to apply his expertise in the physics of crowd movement to planning new routes and safety measures for the Mina pilgrimage. He and his co-workers analysed video footage of the crowd disaster of 2006, which helped them to understand what happens as a crowd reaches a dangerous density. They used these insights to reorganise the pilgrims routes to and from the “jamarat bridge”, the two-tiered structure built to improve access to the jamarah pillars.

In the footage, Helbing and his colleagues saw that the thick crowd suddenly began to display motions reminiscent of turbulent flow in liquids, where entire groups were pushed back, forth and sideways without any control. These erratic movements can cause people to fall and be trampled. Helbing will describe how such hazardous states of motion are signalled in advance by prior changes in the crowd behaviour, making it possible to identify danger spots and introduce crowd-control measures before the ‘turbulence’ sets in. In this way and others, the physics of crowds should make it easier and safer to move around open spaces.

Helbing has previously developed computer models of pedestrian movements, in which individuals move through a throng towards their destination while aiming to avoid collisions, as though there is a repulsive force between them. Such models show that crowds often ‘self-organise’ into efficient patterns, for example by forming counterflowing streams as people move along a corridor in opposite directions. But crowd panic can undermine these efficient solutions and lead to jamming, where the crowd pressure can become dangerously high. This talk will highlight how valuable an understanding of the physics of crowd motion can be.

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