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Nobody really knows how many people will throng the nation's capital for Barack Obama's inauguration, but the city's population is expected to double or triple at least. What should you do if you find yourself in a crowd crush?
Don't go with the flow. The way people die in crowd crushes is not from trampling but from asphyxiation—the force of five people moving forward is enough to collapse the lung of an adult on the receiving end. The best way to avoid that fate is to move gradually sideways or backward, out of the human flow, at the earliest sign of trouble. The trick is to know what those signs are. Once you are in obvious peril, it's usually too late.
The earliest warning of a crowd crush is the absence of organizers, police officers (especially mounted police, who have the long view), barriers, or signs and loudspeakers. If you don't see any of these, consider turning back. Crowds are rarely belligerent, but they can become deadly if, for example, there's no way to announce that someone has fallen down and everyone must take a step back.
Huge crowds can be very safe, and small crowds can be deadly. In general, four people per square meter is a safe ratio. If you see more than that—especially in a moving crowd—it's a good idea to get out of the way. Otherwise, if someone jostles you, you won't have room to stick a foot out to stabilize yourself. If you fall, other people may trip over you, creating a pileup. Meanwhile, the rest of the crowd will continue to surge forward, unaware of your situation, and the pressure will build.
Another, more overt sign of danger is the sensation of being touched on all four sides. That's the time to work your way to the margin of the crowd. After that, the last opportunity to escape may be when you feel shock waves travel through the crowd. This happens when people at the back push forward, but the people at the front have no where to go.
If you feel the crowd sway like this, you are in serious danger. Wait until the crowd stops moving and then inch your way sideways and backward, zigzagging to safety. Just as you might swim back to shore in the ocean, try to navigate during the pause between waves.
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Explainer thanks traffic engineer John J. Fruin and G. Keith Still of Crowd Dynamics Ltd.