Mehdi Moussaid: “We must raise awareness of the dangers of crowds” – Science et Avenir

154 people were killed in the Itaewon area of ​​Seoul in a riot that turned into a disaster. Tens of thousands of people gathered here to celebrate Halloween. The victims of the disaster came from more than ten countries, from Australia to Vietnam through France, the US, China and Japan. The drama is a reminder of how “crowds are dangerous,” researcher Mehdi Moussaid, a crowd behaviorist at the Max Planck Institute in Berlin, explained to Science et Avenir. (Mehdi Moussaid also hosts Fouloscopie, a YouTube channel where he promotes scientific data on crowds.)

“Two-way flow is likely to increase the effect of the crowd”

Sciences et Avenir: What is the key parameter that will hasten a catastrophe like the one in Seoul?

Mehdi Mussaid: Crowding, that is, the number of people gathered in a given area. From 6 people per square meter, the situation becomes very unpleasant. At 8, 9, or 10, we reach a level of contraction where people are pressed against each other and can no longer breathe.

The drama unfolded on a street four meters wide and 40 meters long.

Judging by the images, people were moving in both directions: a two-way flow would likely enhance the effect of the crowd. This is exactly what the organizers of an event, for example, a concert, try to avoid by creating circulation corridors leading to the stage, to the toilets, etc. Unfortunately, this approach is difficult to implement in the urban environment that we are talking about: there are bars and restaurants everywhere. It is impossible to favor movement in the direction of such and such a place.

Does this bloodthirsty mafia phenomenon in South Korea compare with other past episodes?

In 2006, a giant stampede in Mecca claimed the lives of almost 400 people. Four years later, it was in Germany, in Duisburg, that a mass movement turned into a tragedy, as a result of which 19 people died. It happened during the love parade. The organizers showed transparency by immediately providing free access to surveillance cameras, which filmed the panic movement in large numbers. This data has been well used by the research community and has accelerated our understanding of the crowd, whose movements obey the laws of fluid mechanics.

“Trapped in the crowd, you need to conserve oxygen”

The danger of the crowd remains unknown…

Indeed, educational work is being carried out on this topic, especially among the youngest. Children are taught to be careful not to drown, they are taught to carefully cross the road. But we don’t talk to them about the crowd. However, this is the message to be conveyed: crowds are dangerous. And if you find yourself in a crowd in which you begin to feel uncomfortable, you need to get away from it. This way you take cover and also protect the others, as your departure has reduced the density of the crowd.

But what advice can we give to those who are trapped?

You must conserve your oxygen. This means not screaming, but rather breathing normally. By folding your forearms across your chest, you can protect your chest and avoid compression. It is also very important to avoid falls. You must keep your balance, otherwise, if you lie down on the ground, there is a risk of falling over. In a domino effect, other people fall on you, and it becomes impossible to get up.

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