Until recently we thought crowds behaved as fluids

Dr Keith Still, a mathematician, explains how he disproved the theory that crowds behave like fluids

A crowd consists of many individuals each exploiting their best opportunity. Although each of these individuals are doing what’s best for them, they are not aware of what is best for the crowd.

Keith Still was stuck in a queue with 10,000 other people trying to get into a concert and the queue was moving in a very strange way.

"If you imagine an eggtimer where the grains of sand are flowing in the centre, then that’s one type of fluid flow, granular flow where you can really understand this in terms of frictional forces and particles. Then here I was in the centre of the crowd and I was moving much slower than the edges. So this seemed to be back to front to the standard thought."

Clearly a crowd isn’t fluid. At this point, he really got excited:

"You begin to understand that you’ve found something that is very different from what everybody else has been seeing."

The mathematics needed to be inverted in some way. Keith's research in the last few years has really been to understand the nature of crowd flow as opposed to the nature of granular or fluid flow. A different type of mathematics needs to be used to solve these sorts of problems. The only way you can really build models of these types of phenomena is to go out in a crowd and walk around. He explains:

"You time yourself, you use metronomes to see if your pace is keeping consistent, you use security cameras, video cameras, use as much material as possible in order to record the event and then you can sit down in the lab afterwards and analyse what it is you’re seeing. I must have taken thousands and thousands of photographs, many hundreds of hours of video footage. I have a double garage and I can’t get the car into it – that’s the amount of material I have collected!"

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