A Crowd Has Physics

Not every crowd is dangerous. Concerts, festivals and demonstrations can be organized properly.

In 2000 Dirk Helbing, Illés Farkas and Tamás Vicsek published a model of how people move during an evacuation. It showed how, at high density, individual attempts to move faster can reduce the overall flow — the “faster is slower” effect.

The model was influential, but later researchers rightly criticized the habit of explaining crushes with the word “panic”: people in disasters often help each other, and the main danger comes from the geometry of the space, the density and physical pressure. A crowd is dangerous not because everyone suddenly goes mad. Sometimes an exit that is too narrow is enough for the system.

Danger appears when density rises, the flow stops, people move in opposite directions or the exit becomes narrow.

Before an event starts it helps to note:

• several exits;
• the edges of the space;
• barriers and dead ends;
• a meeting point with friends;
• how security is working;
• the direction of the main flow.

If it gets so tight that choosing your own movement becomes difficult, it is better to leave early. The sensible moment to leave a crowd usually arrives before everyone else agrees that it is sensible.

In a dense flow you must not try to walk straight against the movement. Keep your balance, hold your arms in front of your chest to protect breathing space, avoid walls and hard obstacles, and shift gradually toward the edge.

If you drop something, do not bend down for it. A phone is cheaper than a spine.

If you fall, protect your head and try to get up as fast as possible, using a stable support rather than grabbing a random person by the neck.

Panic is not an obligatory property of a crowd. People often help each other. But the pressure of a large mass arises with no ill intent, and it is hard to negotiate with in words.

The most accessible testing ground is the exit from a stadium after a match, or a train carriage at rush hour. Density rises not where there are many people but where the flow narrows: a staircase, a turnstile, a door, a corner. Someone inside the flow cannot see the narrowing — it is behind other people’s backs — and makes decisions by the density he feels around him, though the danger is created by the geometry ahead. That is why a spot at the edge with a view of the exit and a good spot in the middle are different purchases, and their prices differ too, only they are paid in different currencies.

Hence a rule that is hard to follow: leave while you can still choose a direction. The moment when the crowd stopped obeying is recognized by the fact that the direction is now chosen for you.