5 ms·

Even after reading the piece, I'm still struggling to understand the mechanics of it all. Under normal conditions, when people walking in front of you stop, yo

by jawns 10y ago

Even after reading the piece, I'm still struggling to understand the mechanics of it all. Under normal conditions, when people walking in front of you stop, you stop also so you don't bump into them. Makes sense, right? So you would think that as long as everybody obeys that simple rule of movement, this should never happen. But having been caught in a much less serious "crush" once, I know it's not that simple. In my case, I was at a similar event -- religious in nature, lots of people from all around the world, and mostly traveling in groups -- and although everybody was polite and non-aggressive, as groups traveled through the crowds, nobody wanted to be separated from their groups, and that caused major issues. For instance, a lot of groups traveled while holding hands in an effort to remain together. But imagine how much more difficult it is for a bunch of worm-like people chains to travel through a crowd than it is for discrete individuals to do so. The surge I was caught in was little more than a momentary push forward, but it was very scary to lose control over my ability to move freely. I would hope that with most people having cell phones nowadays, which gives them the ability to more easily regroup after separation, it should be a priority in cases like this to urge people to not form chains and to devalue staying with their group if it affects the flow of traffic.

mkaziz 10y ago
Thinking about the sheer numbers might help imagining the mechanics. 2 million pilgrims crowded into very specific places at or near Makkah that year. For reference, the population of Chicago is 2.7 million.
visakanv 10y ago
I once saw a reddit comment that explained it really, really well: https://np.reddit.com/r/worldnews/comments/3pcvfb/saudi_arabia_hajj_disaster_death_toll_at_least/cw5vxtm?context=3 https://np.reddit.com/r/worldnews/comments/3pcvfb/saudi_arab...
- Analemma_ 10y ago
  There's also a good New Yorker article that made a lot of the same points, but in the form of a longform article, if you prefer that: http://www.newyorker.com/magazine/2011/02/07/crush-point http://www.newyorker.com/magazine/2011/02/07/crush-point. The bottom line is that past a certain "person density", crowds are actually governed by a lot of the same laws as fluid dynamics. When you introduce a "wall" in front of the flow, the people at the back don't even know it because they're too far away to notice. They can't know to "stop" any more than water at the back of a pipe can suddenly stop if the water ahead of it encounters an obstacle.
  - cloudjacker 10y ago
    60% water, checks out
mmanfrin 10y ago
The problem is that people don't stop immediately. You stop, the person behind you stops just a little bit later, the person behind them a little bit later than that. As you're walking the gaps in between people shrink. Finally the person in front of you stops, you try to stop but the person behind you is close and bumps in to you a bit as you try to stop, you bump the person ahead of you who has stopped because they can't move forward, which squeezes the person ahead of him. Repeat for thousands of people and you have a collection of tiny small bumps and pushes that squeeze people to the point of death.
mbrameld 10y ago
At a certain density a crowd of people starts behaving more like a fluid than a group of individuals.
- semi-extrinsic 10y ago
  And the GP has rediscovered that viscosity is much higher for a fluid made up of long-chained molecules (think motor oil) than a fluid made up of short-chained ones (think gasoline).
lisper 10y ago
> as long as everybody obeys that simple rule of movement, this should never happen. That's true, but beyond a certain density this is no longer possible because there is no space between you and the next person. Here's a video that shows this phenomenon in rather graphic detail (warning: disturbing imagery): https://www.youtube.com/watch?v=eu4DlwE0Bu8 https://www.youtube.com/watch?v=eu4DlwE0Bu8
noobermin 10y ago
>Under normal conditions, when people walking in front of you stop, you stop also so you don't bump into them. Makes sense, right? By that logic, there should be no car crashes.
- duaneb 10y ago
  > By that logic, there should be no car crashes. Under normal conditions with rational drivers, there shouldn't be any car crashes. It's when the assumptions fail—distractions, obstacles in the middle of the road, weather, inebriation—that people crash.
ihsw 10y ago
> I would hope that with most people having cell phones nowadays, which gives them the ability to more easily regroup after separation, it should be a priority in cases like this to urge people to not form chains and to devalue staying with their group if it affects the flow of traffic. While true, the reception at such events is usually non-existent due to the sheer volume of people with devices. Even with a healthy quantity of cellular base stations or wifi access points, there will still be serious issues and connections will be spotty at best.

by jawns 10y ago

mkaziz 10y ago

Thinking about the sheer numbers might help imagining the mechanics. 2 million pilgrims crowded into very specific places at or near Makkah that year. For reference, the population of Chicago is 2.7 million.

visakanv 10y ago

I once saw a reddit comment that explained it really, really well: https://np.reddit.com/r/worldnews/comments/3pcvfb/saudi_arabia_hajj_disaster_death_toll_at_least/cw5vxtm?context=3 https://np.reddit.com/r/worldnews/comments/3pcvfb/saudi_arab...

Analemma_ 10y ago
There's also a good New Yorker article that made a lot of the same points, but in the form of a longform article, if you prefer that: http://www.newyorker.com/magazine/2011/02/07/crush-point http://www.newyorker.com/magazine/2011/02/07/crush-point. The bottom line is that past a certain "person density", crowds are actually governed by a lot of the same laws as fluid dynamics. When you introduce a "wall" in front of the flow, the people at the back don't even know it because they're too far away to notice. They can't know to "stop" any more than water at the back of a pipe can suddenly stop if the water ahead of it encounters an obstacle.
- cloudjacker 10y ago
  60% water, checks out

mmanfrin 10y ago

The problem is that people don't stop immediately. You stop, the person behind you stops just a little bit later, the person behind them a little bit later than that. As you're walking the gaps in between people shrink. Finally the person in front of you stops, you try to stop but the person behind you is close and bumps in to you a bit as you try to stop, you bump the person ahead of you who has stopped because they can't move forward, which squeezes the person ahead of him. Repeat for thousands of people and you have a collection of tiny small bumps and pushes that squeeze people to the point of death.

mbrameld 10y ago

At a certain density a crowd of people starts behaving more like a fluid than a group of individuals.

semi-extrinsic 10y ago
And the GP has rediscovered that viscosity is much higher for a fluid made up of long-chained molecules (think motor oil) than a fluid made up of short-chained ones (think gasoline).

lisper 10y ago

> as long as everybody obeys that simple rule of movement, this should never happen. That's true, but beyond a certain density this is no longer possible because there is no space between you and the next person. Here's a video that shows this phenomenon in rather graphic detail (warning: disturbing imagery): https://www.youtube.com/watch?v=eu4DlwE0Bu8 https://www.youtube.com/watch?v=eu4DlwE0Bu8

noobermin 10y ago

>Under normal conditions, when people walking in front of you stop, you stop also so you don't bump into them. Makes sense, right? By that logic, there should be no car crashes.

duaneb 10y ago
> By that logic, there should be no car crashes. Under normal conditions with rational drivers, there shouldn't be any car crashes. It's when the assumptions fail—distractions, obstacles in the middle of the road, weather, inebriation—that people crash.

ihsw 10y ago

> I would hope that with most people having cell phones nowadays, which gives them the ability to more easily regroup after separation, it should be a priority in cases like this to urge people to not form chains and to devalue staying with their group if it affects the flow of traffic. While true, the reception at such events is usually non-existent due to the sheer volume of people with devices. Even with a healthy quantity of cellular base stations or wifi access points, there will still be serious issues and connections will be spotty at best.