In the classroom exercises we illustrate the density v flow relationship using a loop of string. The loop has an area of 1 square metre.
At two people per square metre - free flowing movement, no restrictions - provided the two individuals are moving at about the same speed. If this were (for example) a parent and child the pair would be moving at the pace of the slowest individual (the child).
At densities lower than 1 person per square metre the crowd is free flowing and stable. As we increase the number of people per square metre (which is why we are using the string to maintain the density), people cannot take whole paces forward. Movement starts to become constrained and the flow rates drops as shown in the graph below.
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