Crowd collapses and crowd crushes are catastrophic incidents that can occur when a body of people becomes dangerously overcrowded. When a body of people reaches or exceeds the density of per square meter (about 2.5square feet per person, or a square 1foot 7inches to a side), the pressure on each individual can cause the crowd to collapse in on itself, or become so densely packed that individuals are crushed and asphyxiated.
At this density, a crowd can start to act like fluid, sweeping individuals around without their volition. Such incidents are invariably the product of failures of organizations, and most major crowd disasters can be prevented by simple crowd management strategies. Such incidents can occur at large gatherings such as sporting, commercial, social, and religious events. The critical factor is crowd density rather than crowd size.
Crowd collapses and crushes are often conflated with human stampedes.
Background
One study has calculated that there were 232 deaths and over 66,000 injuries in the ten years between 1992 and 2002 as a result of such activity, and it is believed by crowd scientists that such incidents are both vastly under-reported and increasing in frequency. One estimate is that only one in ten crowd injuries occurring in doorbuster sales are reported, while many, if not most, injuries at rock concerts go unreported.
Dynamics
The average individual occupies an oval floorspace approximately , or and at densities of individuals can move freely without contact. Even if people are moving quickly, at this density it is possible to avoid obstacles and the chance of a crowd-related incident is minimal. Even at the risk is low; however at densities of the possibilities for individuals to move become limited, while at higher densities () individuals become pressed against each other, and can be unable to move of their own volition. At this point a crowd can begin to act like a fluid, with individuals moved about by the pressure of those around them, and shockwaves can pass through the crowd as pressures within the crowd change. This can be highly dangerous, although in some cases this experience is actively sought, such as at rock concerts or football matches, where the excitement, camaraderie, and literally 'going with the flow' is an essential part of the experience, and activities like dancing and moshing are common. The dangers inherent in these conditions is that the crowd will collapse in on itself, or become so densely packed that individuals are crushed and asphyxiated.
Crowd collapses
A crowd collapse occurs when a crowd is so dense that each individual is touching others all around, and is to an extent supported by those around. This can occur whether the crowd is moving or stationary. If a person then falls, the support to those around is lost, while the pressure from those further out remains, causing people to fall into the void. This process is then repeated, causing a bigger void, and will progress until the pressure eases; meanwhile, those who have fallen are at risk of being smothered by the weight of bodies on top, or being trampled as the crowd is swept over them. An example of a progressive crowd collapse was the 2015 Mina stampede in Mecca, Saudi Arabia during the Hajj when over 2,400 people were reported to have died.
Crowd crushes
At even higher densities (up to ) a crowd can become so packed that people are crushed together to such an extent they can no longer breathe, and are asphyxiated. Such crowd crushes can occur when a moving crowd is funnelled into a smaller and smaller space, when it meets an obstacle (such as a dead end, or a locked door), or when an already densely packed crowd has an influx of people, causing a pressure wave toward those at the front of the crowd. In this situation those entering may be unaware of the effect on those in front, and continue to press in. Examples of crushes are the Hillsborough disaster in Sheffield, South Yorkshire, England in 1989 and the Love Parade disaster in Duisburg, North Rhine-Westphalia, Germany in 2010.
Crowd "stampedes"
The term stampede is usually used in reference to animals that are fleeing a threat. Stampede events that involve humans are extremely rare and are unlikely to be fatal. According to Keith Still, professor of crowd science at Manchester Metropolitan University, "If you look at the analysis, I’ve not seen any instances of the cause of mass fatalities being a stampede. People don’t die because they panic. They panic because they are dying". Paul Torrens, a professor at the Center for Geospatial Information Science at the University of Maryland, remarks that "the idea of the hysterical mass is a myth".