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Comment by nacho-daddy | Hacker News Reader

Crowds of a high enough density push people in orbital waves, with both clockwise and counterclockwise oscillations pulsing at 18 second intervals through the crowd. Monitoring these motions in realtime is possible and may help prevent crowd crush events. Also: At high densities crowds act like sticky springs, with the ground adding the sticky.

18 seconds before the next wave of people crushes you. 18 seconds to get out. 18 seconds could save your life.

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I read it kind of quickly, but didn’t their analysis suggest that the 18s period in question may have been specific to the Plaza Consistorial—that is, that the specific oscillation period may depend on the size (and possibly shape) of the confinement area?

> To address the role of confinement, we perform an additional series of measurements after the festival opening, when a security team splits the crowd into two halves (Fig. 3e and Supplementary Video 4). We find that the two decoupled crowds still oscillate but at a higher frequency. We measure the shortest dimension of the crowds before and after the festival opening, and the extent of the region of space where the 2010 Love Parade crowd featured chiral oscillations (Methods). Figure 3h shows that the velocity spectra of these markedly different crowds peak at the same value when rescaling the frequency w by the inverse of the confinement length L. This result strongly suggests that the period of the emergent oscillations depends on confinement, and is not an intrinsic 'material' property.

Two things that would deserve clarification (although outside of the scope of the research here) are:

(1) When is it time to leave? At what threshold? Is it around an arm's length of free space between people?

And (2) Where to go? Straight line towards an exit? Perpendicular to the crowd? Exactly against the flow? Along the flow but towards edges? Probably depends on a lot of parameters..

I don’t recall where, but for some reason I remember learning that if people start to bump into you too many times, that’s a warning shot to get out. It was something like “bumps per second”.

A naive application of fluid dynamics would suggest that the answer to 2 is to go perpendicular to the flow until you reach a wall, then go against the flow towards an exit.

In the middle of a crowd/fluid your motion is determined by the humans/molecules around you. The closer you get to a wall, the fewer particles have an influence on you. Just make sure it's not a wall people are moving towards.

Not sure if anyone has studied how well this holds up to humans. Human crowds have very fluid-like behavior, but of course they don't behave perfectly like a liquid in a pipe

> Human crowds have very fluid-like behavior, but of course they don't behave perfectly like a liquid in a pipe

Hypothetically, how would the dynamic change if it were a crowd of cats instead?

Non-cat fluids don't usually spontaneously draw knives and start cutting themselves apart.

Is there anything to the notion that you might prefer to be batted around in the “soft body” portion of the dense-crowd “fluid” rather than pinned between several tons of soft bodies in motion and a hard limit? You seem rather more crushable than an individual molecule might be…

(edit: “crowd crush safety expert” Paul Wertheimer, who came to some notoriety through studying mosh pits from within them, apparently suggests—at least in situations where the crowd is moving toward a clear focal point—you’re right: he recommends the edges

https://www.cnn.com/2021/11/13/us/how-to-stay-safe-at-concer...

That said I note that the authors of TFA draw a distinction between dense-crowd situations undergoing unidirectional flow and the steady, confined dense crowd whose behavior they describe.)

I wonder if we couldn't have the equivalent of the water barrels near the start of guard rails, but for crowds?

Maybe giant inflated or crushable areas around an exit that could take up slack?

maybe tie the crush event with something else, like opening extra doors, turning on lighted warning signs or illuminating exits?

Doors with crash bars are kind of that. If there's too many people, they'll push up against them and force them open.

.,.......except at a Greatfull Dead concert where this phenominon is called the "seeing the gears" or "have you seen the gears" nobody gets crushed, everybody gets....where they need to be plus up in the stands, are human waves of people, who are also bouncing a multitude of beach balls around, some of which are giant, some of which are not filled with air everybody steping to the groove.....except the spinners, who are another phenominon, the tapers huddled, but looking for any likely person at all to spell them bobby just played the grammys......

Of note: If that 18 second rule became known to a significant number of people, the dynamic will change

I like your summary. Thanks!

Beautiful. Thank you.

Id bet that the 18 seconds is related to the amount of time between when people naturally shift on their feet/comfort pocket...

Meaning that, if you notice the frequency by which people shift their weight and move -- its likely in that window, assuming other externalities (bathroom, heat, thirst, anger, etc -- the physiological cycle of body comfort may be in that area of 18 secs)

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