The Hajj attracts millions of pilgrims from across the world every year and involves several complex rituals, which means it is always a potentially dangerous event.
In recent years, great lengths have been taken to ensure the safety of pilgrims, with, according to Saudi government sources, more than £200bn spent since 1992 on redesigning the infrastructure of the Hajj, which involves events at several sites in and around the city of Mecca. One key way that organisers plan for the safety of crowd events such as the Hajj, but also parades, carnivals and sporting competitions, is by using computer simulations to model large groups of people.
In a recent systematic review of computer models we drew upon the social identity approach, which suggests a distinction between physical crowds (where people are simply in one place) and psychological crowds (where people in a physical crowd share a common self-definition – a social identity).
A group of people at an event may all see themselves and each other as Muslims, Manchester United supporters, or music lovers, for example. This shared identity affects the behaviour of the crowd and is therefore imperative for understanding and predicting the crowd movements, including flow and congestion.