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Bustling Through The Physics of Crowds

Text: We humans spend a lot of time moving in crowds. In a series of three panels, we see students move through a school hallway with lockers. Fans wearing baseball caps cheer from bleachers at a game. People on a subway platform rush to get in and out of a train.
TEXT: Sometimes we share the same goals: leaving a stadium when the game ends, getting on the subway after work. Many people heading the same direction exit through a wide doorway at a stadium.
TEXT: Sometimes we are at odds: trying to get to an airport gate just as a large group disembarks. A person struggles to move in the opposite direction of everyone around him.
TEXT: Sometimes it’s necessary to evacuate large groups quickly, without people getting hurt in the process. Good design of exit routes and doorways is literally life-saving. Many people calming walking through a doorway with a big clear EXIT sign above it.
TEXT: A lack of emergency exits at the 2021 Astroworld Festival in Houston may have led to 10 deaths and hundreds of injuries People in a crowd in front of a stage press against each other and nearby barriers.
TEXT: Designing safe spaces — that also allow people to move efficiently — is the focus of much of the research into crowd behavior. Social science is key to this work. By the late 20th century, physicists and engineers had developed many tools to understand how people move through public spaces.
A roughly rectangular space seen from above that has several exits and interior structures; five arrows come down from the top, each taking a different path to an exit. TEXT: People may be complicated as individuals, but crowds aren’t random. We move toward exits, avoid obstacles, and look for efficient paths determined by our goals.
Three different people thinking of different goals: an airport gate, a trash can, a bathroom, an exit. The foremost, Alice, is a woman with blonde hair in a bowl cut looking to the upper right. TEXT: While social science can reveal how we make decisions based on our own perception and circumstances, physics examines the universal features and collective behaviors, providing analogies from fluids, gases, and particles.
Physicist Alessandro Corbetta speaks; nearby, Alice runs with outstretched arms. The bubble above her head contains the face of a cute Pomeranian. TEXT: Building models from real-world crowds isn’t simple, though. Alessandro Corbetta (Physicist, Eindhoven University of Technology, The Netherlands): “As physicists we face a substantial challenge in trying to model human crowds, a system that seems almost impossible to cast in the language of mathematics.”
Water flows from a sink faucet toward a coffee mug held underneath it; the water swerves to avoid the mug and flows into the sink. TEXT: In a seminal 2003 review, Australian engineer Roger L. Hughes referred to human crowds as “thinking fluids.” Hughes: “There is a strong similarity to classical fluid dynamics, but the present flows ‘think,’ which gives them some intriguing properties.”
At an intersection of two hallways; a barrier prevents two arrows that are going in opposite directions from hitting each other. TEXT: Hughes was instrumental in adapting the math for fluids to human crowds. Among other things, he investigated how well-placed barriers can increase both crowd motion and safety.
TEXT: Many of the questions in this field are very practical. Physicist Mohcine Chraibi stands in a doorway pushing outward against the doorframe using both arms and one leg, contorting the frame. Behind him, two people peer out through the doorway. TEXT: Mohcine Chraibi (computer scientist, Forschungszentrum Jülich, Germany): How wide can a door or bottleneck be, and what is the maximum flow that we can have?
A row of elongated cubes moves through a doorway that’s precisely the size of one cube. TEXT: Chraibi’s Pedestrian Dynamics group conducted experiments with small groups on an artificial train platform and took video footage of train stations in Germany. The flow of pedestrians through a bottleneck turns out to increase linearly with its width.
Two rows of elongated cubes move through a doorway that’s precisely two-cubes wide. TEXT: Double the width of a doorway, and the flow through it is twice as fast. The German government incorporated these results in various regulations, and other EU nations have expressed interest in the research.
Physicist Chraibi gestures at two people behind him who are both politely encouraging the other to go through a doorway first. TEXT: Cultural factors also play a role, which researchers are studying by comparing results between different nations. Chraibi: “In some countries, it’s socially not acceptable to overtake people.”
Two lines of police with shields and raised batons press against people gathered in the foreground. TEXT: Some researchers have safety explicitly in mind. Mass religious gatherings, sporting events, concerts and protests (especially when police or military respond with force) are all susceptible to overcrowding and deadly accidents.
Safety engineer Jian Ma draws our attention to a person laying in a road crushed by a crowd. TEXT: Jian Ma (safety engineer, Southwest Jiaotong University, China): “My long-term goal is to answer the question about why and how crowd trampling disasters happen, which I believe is also the most important question this field can answer.”
Physicist Corbetta; behind him several people including Alice from earlier float and bob among the waves in a body of water. TEXT: Hughes’ “thinking fluids” metaphor is good for situations where human messiness can be smoothed over. Corbetta: “If you look at the dynamics repeating over and over again, in similar conditions, for one year, you can find physical features that are reproducible and universal, not connected to one specific individual.”
A metal canister with a brain on the label blows a cloud of air within which float different people’s faces. They include a brown skinned woman in a hijab, a young black man named Joe, an older latinx woman, and an older man with glasses and bushy mustache. TEXT: Researchers can also look at things in finer detail, where the physics analogy is more like a “thinking gas” or even “thinking atoms.” Individual properties like speed combine with collective quantities such as pressure and density to govern behavior, while “social forces” account for specific aspects of our humanity.
A hand holds its thumb over the end of a hose trying to prevent water and small people from spurting out but one person and drops of water have escaped and shoot into the air. TEXT: The smoothness of crowd flow — or lack thereof — is analogous to viscosity; barriers exert imaginary forces that get weaker the farther away people are; people at the front of a thick crowd feel “pressure” from behind.
Physicist Corbetta peers over the edge of the panel, looking toward the panels that follow. TEXT: Modeling crowds requires identifying where behavior and thought patterns modify purely physical interactions. Corbetta: “People aim to be efficient. My interpretation of a ‘thinking fluid’: you have ‘particles’ that are smart in the way they choose the route.”
People walking and thinking of different goals: a slice of pizza, a trash can, the exit. TEXT: Molecules don’t have goals. People within crowds generally do, even if they don’t all have the same goals.
Four people stand in a clump talking on phones or playing games, Alice follows a green arrow that goes around the clump. TEXT: People generally don’t like bunching up too much, so sometimes choose longer but less crowded paths.
Two inattentive people walk toward a man sitting on a bench feeding birds; they each bump comically into the bench and fall over. TEXT: Atoms typically bounce off barriers and other obstacles in predictable ways, but we (usually) don’t.
A big strong man, The Brute, in athletic gear jostles three different people as he shoves his way past them. His force sends them backwards or flailing to the side. Joe is seen careening through the air. TEXT: “Social forces” disobey Newton’s laws. A rude person exerts a larger force on those around them than they experience from those they shove past.
Water pours into one end of an ice cube tray, ice cubes emerge from the other end of the tray and climb out, marching in two lines. TEXT: As with non-thinking matter, crowds sometimes spontaneously undergo “phase transitions,” akin to water becoming ice. People moving in opposite directions will form lanes, as individuals tend to follow people moving in the same direction while avoiding those going the other way.
Four marathon runners including Alice surf or run along moving waves of water. TEXT: Similarly, researchers have applied imaging techniques developed for fluids to marathon runners, identifying motions like waves and turbulence in the crowds.
Silhouette of a tight crowd with people struggling; a silhouette of a large structure representing one of the pillars of the Jamarat bridge in the background. TEXT: These theoretical and experimental results are crucial for finding points of congestion. During the 2006 Hajj in Mecca, hundreds of Muslim pilgrims were crushed to death partly due to the way crowds move unevenly through the various ceremonial spaces.
Joe, now pulling a rolling suitcase, a family of three and a bewildered Tourist in a bucket hat, sunglasses, and a selfie stick all walk along a sidewalk. TEXT: Sometimes individual characteristics matter: where we direct our attention, whether families are part of the crowd, if people are carrying luggage.
A wooden artist’s model of the human body representing the Tourist, wearing his hat, sunglasses and a Bahama shirt with a suitcase and selfie stick. TEXT: This scale of work is very granular. The physical models can include body orientation or other three-dimensional characteristics, in a simplified way.
People, some dragging suitcases, going up and down two staircases with handrails. One person has tripped on a suitcase and fallen to the bottom of the stairs. TEXT: Ma and his colleagues looked at the effects of luggage. Student volunteers walked up and down specially constructed stairs. By varying the numbers of students carrying luggage, the researchers built a mathematical model for optimum traffic.
People on an escalator; The Tourist struggles to keep a tower of luggage from falling over onto the people further down the escalator, who are holding their arms up in fear. In the background, Alice calmly rides an elevator with a suitcase. TEXT: His group concluded that there’s less congestion if people with bulky luggage use elevators in metro stations. Their simulations even showed that people should stand, not walk, on escalators, for issues of safety and congestion.
A pack of people moving unified in a single direction. TEXT: While many experimental setups like Ma’s — where researchers often give instructions to participants — are psychologically different from real-world scenarios, it’s the combination of theory and experiment that has driven the field forward.
Water gushes from a faucet like a river and moves around the same coffee mug from earlier; the mug holds a traffic cone. TEXT: Pure physics and math reduce us too much to toys, while “thinking fluids” and related analogies provide a way to bring specific human messiness into the models.
The water from the faucet becomes a river, The Brute, Alice, The Tourist, Joe, and the Pomeranian float calmly in inner tubes on the water. TEXT: This blend of theory and experiment is where the physics of crowds is making its current impacts, informing governments around the world — to save lives and help smooth the flow of crowds. END.

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Using tools from fields like fluid dynamics to better understand how groups of people move around can improve flow and make large gatherings safer.

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