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The Danger of the Madding Crowd: When Groups “Stampede”

September 27, 2015

It is said that there is safety in numbers, but there is danger as well. Just this week 700 people died when pilgrims taking part in the Hajj were overwhelmed by the heat and massive overcrowding. Such disasters are a worldwide phenomenon because the primary ingredients for such tragedies—large numbers of people who gather in spaces that are too confining—are present across the world. In the U.S., for example, a gathering of concertgoers waiting to gain admission to a concert by the rock band The Who in Cincinnati was transformed from a group of expectant fans to a dangerous crowd when the venue doors first opened. The back of the group moved faster than the front, and the flow jammed near the clogged doors. People were literally swept off their feet by the surge and some slipped to the concrete floor. Those around them tried to pull them back to their feet, but the overcrowded mass of people pushed on toward the open doors. As the rear of the crowd continued to push forward the crowd swept past those who had fallen, and they were trampled underfoot. Eleven people were killed.

What causes a group to stampede?  The principles that describe how fluids and gasses flow through constrained spaces apply equally well to the movement of large groups. Most flow systems are stable and predictable, but when they are overloaded—too much fluid is pumped through too small a pipe—the system fails. Similarly, when too many people attempt to move through a constrained space, bottlenecks form along the group’s path at points where its pathway is obstructed—even if the obstruction is a minor one. The queueing effect can further limit the crowd’s movement: people inch forward as they wait, creating the illusion of progress, but also further compressing the crowd. If the crowding is not relieved, crowd turbulence can cause people to lose their footing and fall. Once they fall, they may not be able to regain their footing, and so they are then trampled by the rest of the crowd as it passes over them. Most deaths, however, do not result from trampling, but from asphyxiation: people are jammed into together so tightly that they cannot breathe.

Even very large groups of people can usually navigate through entrances and exits with ease, but if crowding becomes so great that the group feels threatened, a crowd can panic. The most obvious solution–limit the size of the crowd by constricting the size of the entry points so that they are smaller than the size of the exits—is not always the best solution; it can backfire because overcrowding is likely at the entry points. Instead, the space should be designed so that it can accommodate a large, mobile group.  It should feature, for example, “pressure relief valves”—structures that can be opened should the densities become too great—and be completely free of any obstacles (e.g., stairwells, a vendor’s booth, or a misplaced trash barrel) that may cause a bottleneck. The movement of the individuals in the space should also be regulated, just as the movement of automobiles on highways are regulated by lanes, signals, and well-designed access ramps. Steps can be taken to disperse people evenly in the space and over time. Just as most concert vendors moved away from general admission seating after the Cincinnati tragedy to assigned seating, issuing tickets linked to specific time and locations in the space can minimize excessive clumping within the group.  But, perhaps the best solution is consider Thomas Hardy’s advice and remain far from the madding crowd.

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