Scientists studying pedestrian motion are bringing crowd-control measures to the Hajj, the annual pilgrimage of millions of Muslims to Mecca. In previous years, hundreds of people have been trampled to death in Mina, a town four miles away from Mecca where pilgrims gather to ritualistically throw stones at three pillars called the jamarat. Last year, Dirk Helbing, a professor of traffic modeling at the Desden University of Technology, and his colleagues were provided video of the 2006 Mina crowds. Based on computer simulation of crowd dynamics, they suggested measures to help alleviate the risk.
As the mêlée thickened, first the throng stopped passing steadily onto the bridge and instead moved in waves, so that individuals would be repeatedly stopping and starting. But then, as the crowd became even denser, it changed to another mode in which clumps of people were jostled in all directions, apparently at random and against their wish to move steadily towards the jamarat.
"Pilgrims were being pushed around," says Helbing. If they stumbled and didn't get back on their feet quickly enough, they were trampled. The movements look like those in a fluid when it becomes turbulent, which hasn't been seen before in human motion....
For the 2007 Hajj, Helbing consulted with the Saudi authorities to plan a new route and schedule that pilgrims would be compelled to follow, rather than meandering at will to the jamarat. "All 1.5 million registered pilgrims got a timetable and a route in order to distribute them uniformly in space and time," says Helbing. In case the more-than-a-million unregistered participants confounded this plan, they also had the capacity to use real-time data from surveillance cameras to alter the schedule, guided by their models of crowd behaviour. That capacity wasn't needed this year, but the scheme is in place for future...
Helbing says that several of the new measures were controversial, with some experts worrying it would make things worse. But the scheme was a success. An important step was to introduce a one-way system, with roads designated only for walkers coming from the stoning back to the camp. "Last year you had to push a lot to get to the camp," says Helbing. "This year you could comfortably follow the stream all the way. Everyone was very happy."
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