Le comportement des foules en conditions EXTRÊMES

In the early 1960s, urban planner John Freen pioneered a scientific approach to understanding crowd dynamics, moving beyond mere aesthetics. Through extensive time-lapse photography and meticulous data collection in the New York subway, Freen observed a mathematical relationship between crowd density and movement speed. At low densities, speeds were variable, averaging 1.4 m/s. However, as density increased, movement became slower and more uniform, with speeds dropping significantly. This discovery, termed a fundamental diagram, revealed that crowd speed is inextricably linked to density. Freen's work sparked further research globally, confirming this relationship across various cities like Tokyo and Paris, and even extending to car traffic. The consistent pattern across different contexts suggested a fundamental law governing collective movement. Freen is now considered a pioneer in this modern scientific discipline. To further explore these dynamics, researchers sought to understand behavior under extreme conditions – high densities coupled with high speeds – an area previously considered inaccessible. This "inaccessible region" on the fundamental diagram represented a theoretical impossibility where crowds exhibit both high density and high velocity simultaneously. While normal conditions dictate that one can move faster with fewer people or slower through a dense crowd, never both. The study of this inaccessible region was hampered by the difficulty of observing such extreme events. Panic situations, while demonstrating high density and speed, are unpredictable and impossible to pre-study. This led researchers to seek a place where crowds would deliberately approach danger for the thrill of escaping it. They found this unique environment in the San Fermín festival in Pamplona, Spain. The San Fermín festival attracts around a million visitors over nine days. While the initial "Chupinazo" event can generate extremely high densities, it's the daily "encierro," or bull run, that provides the most valuable data. During the encierro, six bulls and six steers are released into the city streets, running towards an arena, passing through crowds of revelers. This inherently dangerous ritual, despite its controversial nature, offers a rare and predictable scenario of a dense crowd fleeing danger at high speeds. A Spanish research team, led by Iker Ziguenza, began studying the encierro in 2022. They deployed cameras to capture the event, focusing on the density and speed of the crowd. Initially, as crowds gathered before the bulls' arrival, they observed typical high-density, low-speed behavior. However, as the bulls approached, a shockwave of density and speed emerged, pushing participants into the previously inaccessible region of the fundamental diagram. Analysis of individual trajectories revealed that most festival-goers attempted to avoid this region, with attempts to accelerate in dense crowds quickly being constrained. When individuals did enter this forbidden zone, their speed abruptly dropped to zero, often resulting in falls. These falls are particularly dangerous in a stampede, as they can trigger a cascade of subsequent falls, leading to serious injuries and fatalities. This phenomenon is not unique to San Fermín; similar deadly crowd collapses have occurred in events like the 2022 Seoul Halloween crush, where hundreds of people were trampled. To explain these falls, researchers delved into biomechanics, specifically stride length. They found a correlation between the desired speed and the required stride length. When the available space in front of a person, determined by crowd density, is less than the required stride length for a certain speed, that speed becomes physically impossible. By comparing the space available with the required stride length for different speeds, researchers identified a direct link to the "inaccessible region" of the fundamental diagram. Essentially, attempting to move at speeds that require stride lengths longer than the available space leads to falls. This is not a psychological phenomenon but a matter of simple mechanics. The research also explored high-density scenarios without the element of fleeing danger, such as the "Chupinazo" event. Here, densities can reach six people per square meter. While the fundamental diagram predicts near-zero velocities at such densities, observations revealed a different phenomenon: movement in all directions, characterized as turbulence. This is a known effect in compressed crowds, caused by waves of pushing and shoving propagating through the crowd, creating a shaking effect. However, a more surprising discovery emerged from detailed analysis of individual movements during the Chupinazo. Researchers observed regular, circular movements, resembling vortices. Spectral analysis confirmed oscillatory motion with an angular velocity of 20 degrees per second, meaning individuals completed a rotation every 18 seconds. Intriguingly, these vortices could rotate either clockwise or counterclockwise. This phenomenon, initially thought to be a cultural peculiarity, was also detected in archive footage of the 2010 Love Parade disaster, which also resulted in fatalities. This suggested a more widespread underlying mechanism. While voluntary circular movements exist (like pilgrims circling the Kaaba), the San Fermín and Love Parade vortices were not intentional. Physicists studying "active matter" – collections of self-propelled entities – found a parallel in experiments with microscopic beads. When confined, these beads spontaneously form small vortices due to random collisions and amplification of small deviations. This phenomenon, observed in non-living matter without intentionality, mirrored the human crowd behavior. The researchers hypothesize that in dense crowds, arbitrary pushes and shoves, even slight ones, can lead to a slight majority pushing in one direction. This creates a feedback loop, escalating into a coherent rotary movement – a "symmetry break." This implies that in these extreme crowd conditions, individuals are not making conscious decisions but are behaving more like particles obeying physical laws. This fundamental research has immediate practical applications. The oscillation frequencies of these vortices are measurable in real-time using spectral analysis of overall crowd movement. This provides a potential early warning system for dangerous crowd behavior, allowing for alarms to be raised before turbulence and cascading falls occur. The San Fermín festival continues to be a crucial site for ongoing research into crowd dynamics, promising further discoveries in the future.