The field of pedestrian and evacuation dynamics (PED) aims to explain the way people behave when moving within the built environment, both individually, and as part of a crowd. For example, research conducted in this field may aim to explain why a queue forms around exit barriers in a particular pattern in a train station during rush hour.

Whilst PED is useful for understanding crowd behaviour under normal circumstances, it is particularly beneficial for investigating crowd behaviour during emergency evacuations. In these events, large numbers of people are required to leave a building in response to a threat, so it is useful to know how long it might take for everyone to leave.

Crowds often seem to behave in peculiar ways, such as when two groups walking in opposite directions stay separate from one another whilst passing, rather than mixing together. Computer simulation models of crowds to try and replicate this ‘emergent’ behaviour through modelling a crowd (or individuals that form the crowd) with simple rules.