Crowds can be dangerous just because they exist and without any ulterior motive or bad intentions. Just by having large numbers of people in a small area, hazards are created. Here are a few of those hazards, along with advice on how to recognize them and how to minimize them if you’re in charge.
If people are packed into an area, you need to look at how tightly packed they are. A normal human body has a “footprint” of about 2 square feet (roughly a foot deep and two feet wide), so if you have more than four people per square yard or square meter, they're going to be touching each other. That’s when the dangers start to appear, because people can’t turn or move without involving another person. This is often a precursor level to more serious densities once the doors open or someone shouts “Fire”.
At six people per square yard, the individual can no longer move freely and the crowd will take on more fluid motions, where force at one point in such a crowd can travel like a ripple through a pond. If a person were to fall down at this density level, they’d knock down several others which would cause a domino effect (“crowd collapse” is the technical term). This is the point where managers need to start thinking about waist-high barriers to act as tidal breaks; such barriers won’t stop a crowd, but can stop the worst of the transmission of forces through it.
At between seven and eight per square yard, you don’t have to worry about falling and being crushed because you can’t fall over. The bad news is that overheating from the bodies packed around you will cause some to faint, but the only way to get them to help is to lift them overhead and “crowd-surf” them to the edges.
At nine people per square yard it becomes hard to breathe, since bodies are pressed tight together and the effect is like being buried in sand or grain: exhalation is easy, but once chest volume reduces it becomes a serious struggle to inhale and push against the material/bodies that have moved to fill in the space. At this point, it’s too late to do anything but get out anyway you can. Crowds at this level have lifted and crushed horses sent in to help disperse the crowd, in addition to causing thousands of deaths and injuries in a single incident. This PDF, from a company that specializes in crowd control, has an impressive list of crowd incidents.
Look at the space you’re in and where the flow of people leads. Wide pathways that lead to limited entrances or exits is a bad sign.
Doorways have to open out, away from the building, according to most fire codes. This is to prevent a crowd from slamming into closed doors and being unable to open them. This works when the crowd is inside looking to get out, but if traffic is flowing the other way it could create a dam for the flood of bodies to jam up against. Doors that swing both directions and are sized for the expected crowds are becoming more common, but older buildings may not have been updated.
Does the flow of bodies change directions? Corners and stairs are dangerous because people will find that the outside of a corner is larger than the inside, causing pressure when they hit the straight portion and have to sort out who goes where. Stairways have the potential for people falling over hand rails, since that is the only free space that they can fall into.
Is the crowd moving or static?
- Static crowds are uncomfortable, and don’t normally become dangerous until they start to move.
- Moving crowds are okay if they have a short path to room to spread out.
Is the crowd worked up or angry? If either, get away as fast as possible. It doesn’t take a riot to get people crushed in a crowd; the annual pilgrimage to Mecca, Saudi Arabia (not a poor area) has caused some of the worst crowd incidents and killing thousands. Fervor -- whether religious, political, or tribal -- can turn a peaceful assembly into a mess in a heartbeat, and once the “leaders” show up and start working the crowd, it’s time to be elsewhere. If the “vibe” is positive and folks are in a good mood it may be safe to stick to the edges, within sight of the exits.
Once the crowd density reaches a certain point (exactly which point is in dispute), it stops being a collection of individuals and starts to behave like a simple organism. Communications between parts is vital to the organism moving well and reacting properly to forces acting on it; otherwise, it becomes a collection of smaller organisms that are working against each other.
All in all, I prefer to avoid gatherings of people if it is going to involve more than a few dozen bodies. Gun shows, small-venue concerts, and such are about my limit. Major industry conventions in the past were uncomfortable, and as much as I enjoyed Las Vegas I doubt I’d ever go back at my own expense. I live in an area renowned for its politeness and hospitality, but I’ve seen too many groups turn ugly in a short period of time for me to be totally relaxed around people I don’t know.