From Science (April 20, 2007):
The annual Pilgrimage, or haj, to Mecca in Saudi Arabia offers one of the world's greatest challenges in crowd control. The millions in attendance create a volatile environment in which pilgrims have been trampled to death performing a ceremony on the Jamarat Bridge in Mina, where they hurl stones at pillars representing the devil. In January 2006, more than 360 people were killed in a stampede near the bridge. Haj officials have since instituted new safety rules and enlarged the bridge; they have also sought advice from experts in traffic and crowd flow.
One of these experts is Dirk Helbing, a physicist at the Technical University of Dresden in Germany, who was asked to suggest safe routes for crowd movements. Last week, he reported on this work at a physics conference at the University of Leicester, U.K. He and co-workers analyzed videotapes of the 2006 disaster, observing how thick crowds of people, like high-density flows of fluids, can turn "turbulent," causing groups to move erratically. When this happens, people fall and get trampled. Helbing described how it is possible to identify changes in crowd behavior in advance of the turbulence and thus pinpoint danger spots.
So next time you are in a crowd, you can ask people to be less "turbulent."
The changes have apparently been effective: At the latest haj, from 29 December to 1 January, there were no major incident.