Monday, February 2, 2009

Protecting us with Physics

The annual Hajj ritual in Mecca has been made safer in recent years by, of all things, Physics. In a recent post, I wrote about the huge crowds at the recent presidential inauguration and how relieved I was that stampedes did not break out. Popular Mechanics recently wrote about work done by Dirk Helbing, a professor of sociology at ETH in Zurich, and his colleague Anders Fredrick Johansson with officials in Washington DC. The group made changes in the planned layout of barriers using the physics-based approach of Helbing and Johansson, shown to be highly successful in the prevention of stampedes. This same approach has been used to make the Hajj, an equally large gathering, a much safer event than it has been in the past.

In an even earlier post, I wrote about the shared space movement, an approach to traffic control that advocates the removal of signs, street markings - even curbs. The shared space approach can be understood by comparing traffic to a flock of birds or school of fish. When birds flock, no one bird is in charge, yet the group moves in an orderly fashion. The flock functions as it does because each bird senses the position and direction of motion of its neighbors and adjusts its own flight to match. The result is self-organization, a smooth and even dance-like movement of the flock.

Both of these approaches to crowd control are based on the idea of self-organizing traffic flows, a concept borrowed from physics where it is used to explain the behavior of complex systems as diverse as the turbulent flow of fluid through pipes or the swarming of insects. Shared space advocates “are trying to use the concept of self-organization to improve performance,” Helbing explains.

Crowds of people move the same way as flocking birds, explains Dr. Helbing, if given the opportunity to interact with each other. The ability to make eye-contact is key to the success of shared-space. Drivers and pedestrians have to acknowledge each other’s presence, then adjust their behavior to those around them.

Helbing believes the shared-space approach can be taken much further by designing traffic systems that use insights about self-organization from physics. “I am convinced that if you know about complex systems, you can make sense of some of this. Perhaps, though, the idea of controlling the system is wrong. Personally, I think that guided self-organization is better.”

The annual Hajj ritual that attracts millions of Muslim pilgrims to Mecca is one area where Helbing’s ideas about guided self-organization have had surprising and profound impact. In recent years, tragic stampedes have occurred during the Hajj, leading to the deaths of hundreds – 251 people died in 2004 and 362 more in 2006, all trampled under the feet of panicking pilgrims.

Helbing’s work has attracted the attention of Saudi authorities who hoped to prevent more such tragedies. After studying video footage of the crowd at Mecca, Helbing noticed that the movement near a narrow entryway onto the Jamarat bridge was smooth like slow-moving water until just before the stampede broke out. The video, which can be viewed here, shows that, as the crowding near the bottleneck becomes heavier, the flow goes from smooth to a stop-and-go forward motion Helbing calls “shock waves.” Suddenly, without warning, a different kind of movement begins– rapid back and forth motions in a direction perpendicular to the flow of the crowd.

This lateral motion, explains Helbing, is a signature that the flow is about to go turbulent, like water flowing through a hose when the faucet is suddenly turned on full-blast. Indeed, shortly after lateral movements appear in the crowd, panic breaks out. The video is difficult to watch. Even though the footage shows only dots on the screen, it is hard to escape the knowledge that the dots are people, some of whom are falling under the feet of others and dying.

Helbing recommended a redesign of the entryway to the bridge, including some one-way lanes. These changes in design take into account the self-organization of the crowd Helbing observed in the video data. Since implementing Helbing’s recommendations, no further stampedes have occurred. Wired wrote about this work last year.

I, as one of those little particles who self-organized ourselves into an orderly flow on the National Mall in Washington DC two weeks ago, am very happy that Dr. Helbing and his colleagues have continued to find ways to apply interesting theories from physics to the problem of controlling a large crowd. What could have been a day marked by tragedy that has become all too common was, instead, a fully joyous occasion.


  1. Can't believe I missed this before and I'm glad I saw it today. FASCINATING!


  2. Hi, Gay - thanks for the comment (and for the tweet!)