Monday, November 28, 2011

Black Friday Crowds: How Physics Can Prevent Tramplings

In the wake of Black Friday's shopping frenzy, several media outlets have reported on shoppers being trampled during the rush for bargains. While this problem rears its head every year, physicists have been researching this area for years. And they have some suggestions for preventing crowd disasters.


A crowd gathers outside a Boise store for Black Friday deals. Image courtesy Robert Barney via Flickr.


To model crowd interactions, physicists often turn to computer simulations of particles. In particular, researchers pored over video evidence of a stampede during a crowded religious pilgrimage that killed over 300 people several years ago. Their research culminated in a highly-cited paper that motivated several changes to the pilgrimage, making it much safer the next year.

The team measured crowd pressure—defined as the density of the crowd multiplied by how much people's speed varies from the average speed of the crowd. Once crowd pressure reaches a certain limit, frenzies often ensue and tragic events can occur.

In light of this research, organizers implemented several changes for the subsequent year's pilgrimage, leading to a safer event. For instance, the amount of pilgrims allowed in the area was kept constant, and the flow capacity of the area was improved through elongation and a more elliptical shape.

Other researchers have looked at the problem of panicking crowds trying to squeeze into small openings—a particular danger for door-busting customers. So what can stores do to minimize injuries? Counterintuitively, placing columns along an entrance or exit can help. By forcing a crowd into a more ordered shape, people can get through the area more quickly and safely. Also, people who mix an individual exit strategy with herd mentality seem to fare the best in panic situations.

While the bargain-seeking crowds may have been daunting this year, physics research has revealed some improvements we can make to stay safe. If, on the other hand, you want to avoid the crowds altogether, you can probably find some good deals today: "Cyber Monday."

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