How Crowds Can Kill You

People mean well, but in high density situations we begin to risk our safety. Learn how a crowd can turn into a deadly disaster.

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Have you ever been standing in a crowd before? Say you were waiting for Weird Al to come out from backstage after a show. And then suddenly he does, and all of a sudden it seems like all the people around you multiply exponentially. And as far as you can see, you're surrounded by people. And all of them start to press forward. And you, against your own will, are propelled forward. And you're pushed into the person in front of you, and you propel them forward. And you stop and think, "Uh oh!"

Well if you thought that, you were thinking very clearly. Because you were in a very dangerous situation: something called a high density crowd. A high density crowd is one where there's 6 or more people per square meter. It can get a lot more packed than that, but that's the lowest level threshold. The reason that's the threshold is because, when you have 6 people per square meter, individuals in the crowd start to lose their ability to move on their own accord. Things get more and more packed, and the crowd tends to behave a lot like a fluid.

So there's a couple of ways that you can actually die in a high density crowd. The first one is called a crowd crush. When you first start to get to about 6 people per square meter, the individuals lose their ability to move around. The next step is that you lose your ability to move your arms from your sides. And as people pack in further and further, the pressure from all sides keeps your lungs from inflating and deflating, which means you lose your ability to breathe.

What's amazing (and horrific) is that people suffocate in crowds because they're squeezed so tightly by the people pressing against them. That's a crowd crush.

Another way you can die in a crowd is what's called progressive crowd collapse. So, say you have a bunch of people crowded together in a high density crowd. And one of them falls down. That creates a hole in this crowd, and the people who were formerly leaning against this person who just fell down start to fall down. And so on and so forth. A domino effect is created. People start to pile up and the ones on the bottom are literally pressed to death by the humans who have piled up on top them.

So why don't people just get up and go, get out of the crowd? Well, the short answer is, they can't.

One mark for humanity is that crowd researchers have shown that when individuals are given information - say, something like, "Oh, someone ahead is being crushed to death" - they respond positively by, say, backing up and alleviating the pressure on the crowd in front.

The problem is humans aren't ants. We don't transmit information through crowds like ants do. And so people end up dying in crowd crushes and progressive crowd collapses because the people in the back are pushing forward.

Another common misconception are mass panics and stampedes. There's this idea that people stampede over one another and that's how deaths occur in crowd crushes. This is really off point. In fact, if you have enough room to rush out over your fellow humans to get from Point A to Point B, there's probably enough room for those fellow humans to get out of the way. So, stampedes really don't cost any lives whatsoever. That's not the problem with crowd crushes.

And the same with mass panics. Very rarely do entire crowds panic and move in a panicked way. In fact, you can suffocate in a crowd crush in a very calm crowd that's just entered a bottleneck in a narrow corridor, and trying to get out of an exit. People just quietly die pinned up against their fellow human beings who are leaving the place.

What do you do if you find yourself in a crowd? Well, get out. That's the best thing you can do. But, as you're entering a crowd, crowd researchers suggest that you pay attention, stop talking, and listen ahead for people calling for help, or saying "Move back!" or any other indication that there's a crowd crush going on. In that case, get back as far as you can.

If you find yourself in a crowd, and it starts to surge forward, follow the crowd movement, but move to the side as much as possible. Just stay out of crowds. How about that?
SOURCES:

http://www.theguardian.com/world/2015/oct/03/hajj-crush-how-crowd-disasters-happen-and-how-they-can-be-avoided

http://www.sciencealert.com/crowd-expert-says-tragedies-like-hajj-crush-aren-t-stampedes-like-we-think

http://www.newyorker.com/magazine/2011/02/07/crush-point

Aron, J. (2011). Mass sway reveals risk of a crushing crowd. New Scientist, 211(2824), 23.

Harding, P., Gwynne, S., & Amos, M. (2011). Mutual Information for the Detection of Crush. Plos ONE, 6(12), 1. doi:10.1371/journal.pone.0028747

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