Calming the crowds in Mecca

Every year just before the festival of Eid upwards of three million Muslim pilgrims converge on the city of Mecca in Saudi Arabia, where they engage in a series of religious rituals, collectively known as the Hajj (literally ‘pilgrimage’ in Arabic).

All able-bodied Muslims are obliged to make the pilgrimage to their holy city at least once during their lifetime.

Once arrived in Mecca, pilgrims are largely occupied in processing between and around various sites of religious significance, symbolic ‘journeys’ intended to arouse a spiritual state and focus contemplation on the divine.

And like any movement of large numbers of people, crowd control is a real safety concern, with numerous deaths and injuries having occurred from trampling and crushing.

University of Melbourne engineering graduate Abdul Kamareddine, a Muslim man originally from Lebanon, has conducted research studying the flow of large crowds in an attempt to understand the mechanisms governing the way they behave.

He used the Hajj as a case study for crowd behaviour and, perhaps surprisingly, found that crowds flow better when objects are placed in their way. He also found the shape of the object makes a difference, and that crowds have a tendency to ‘self-stratify’.

“At the Hajj, there are two specific rituals with large crowd attendance that were of interest to my research,” Dr Kamareddine says. “They are Tawaf, which is a circulation around a cube shaped structure (Kaaba) and Al-Mass’a, which is a walk between two nearby Hills.

“Pedestrians on curved paths, such as that which occurs around the Kaaba (pictured), continually stratify themselves according to their tolerance for crowd density. This means those people who are more tolerant of high densities take a shorter path on the inside curve of a crowd, while those less tolerant of density will flow to the outside of the crowd, which on curved paths means they travel a longer distance.

“Such stratification occurs over a distance, which we call the stratification distance scale, and is generally of the order of the radius of curvature of the flow. 

“Once it has organised itself into this strata, a pedestrian crowd will typically flow smoothly around an obstacle.”

However, flows past a smaller obstacle with a distance scale less than the stratification distance scale, lead to some temporary breakdown in this stratification. The flow develops patches of turbulent behaviour with different pedestrian types responding differently to the obstacle.

“For instance the flow between the Safa and Marwa Hills is poorly stratified because of the lack of curvature of the flow between the Hills.

“At the start of each turning point by each hill, the change in curvature leads to turbulent behaviour as the stratification reforms, after its breakdown between the Hills, for the flow around the Hills,” Dr Kamareddine explains.

Problems were also identified at the Kaaba, where a glass and metal enclosure nearby, the Station of Abraham, was interrupting flows and causing pilgrims to need more time making Tawaf. This in itself resulted in much higher crowd densities, causing safety limits to be exceeded.

Researchers used Global Positioning Systems to record data at both ritual sites, and returned to Matlab in the University’s Department of Infrastructure Engineering to perform computer modelling, where they began working with slaters.

“Our research is the first to use slaters as biological entities to simulate crowd flows, because they best replicate the behaviours of calm crowds of humans,” he says. (Ants, which are often used in crowd modeling experiments, were in this situation unsuitable as they tend to display more panicked behaviours.)

Dr Kamareddine says observations of both pilgrims in Mecca and slaters in Melbourne led to several recommendations to ease the journey of pilgrims, including adding an obstacle at key exits, which assists with the self-stratification process.

“We also recommend authorities consider changing the location of the Station of Abraham for a smoother flow of pedestrians around the Kaaba, and that people should flow in one direction.”

Whether these recommendations will be implemented is uncertain, however Dr Kamareddine says “management of crowd size and flow around the Kaaba is crucially needed to reduce the risks of a crowd disaster.”

A system utilizing state of the art programmed cameras has so far been introduced.

www.eng.unimelb.edu.au