A mighty cheer erupts from El Sadar — Pamplona's massive soccer stadium. Ossasuna, the local team, is winning.
Iker Zuriguel estimates there are some 20,000 to 25,000 people inside the stadium clapping, stomping, and yelling. He's hovering just outside the match — but he's not here for the game.
"We came here because there is a crowd," says Zuriguel, an applied physicist at the nearby University of Navarra. He studies the movement of such crowds to optimize their flow and comfort — and improve public safety.
"A lot of people trying to move too fast in a huge crowd can be dangerous," he says — whether it's happening at a concert, pilgrimage, or sporting event like this one. It can lead to injury, trampling, and, in the worst scenarios, fatalities.
Within seconds of the game ending, a river of fans in red shirts spills out of the doors of the stadium and onto the street.
Zuriguel chooses a spot on the corner where the crowd is denser. He wades through the throng, pointing out people walking directly behind individuals whom they don't know, but who are moving in the same direction they want to move in. They're not conscious of this so-called lane formation — they just do it.
"It's easier following the ones that are walking just to avoid colliding with people that [are] coming in the opposite direction," he says.
Each of these individuals may think they can make their own decisions about how to move through the crowd. But within a mass of people like this, certain patterns emerge. And Zuriguel is interested in understanding what those patterns are.
"As an individual, we can think and we can react," he says. "But when we start increasing the density, sometimes it's difficult to do what we want to do because the crowd is there."
And this is where the laws of physics, rather than free will, start to take over.
Going granular
Zuriguel didn't always study crowds. For a long time, he focused on the movement of particles instead of people. He'd do experiments that involved sending grains and ball bearings down little silos.
But then, 20 years ago, a paper in the journal Transportation Science caught his eye. A team of researchers had put 20-some people in a room and told them to evacuate as quickly as possible, as if there were an emergency. The striking thing was that when the researchers placed an obstacle near the exit door — in this case, a board — the flow improved.
"The room is evacuated faster," says Zuriguel.
But he says the paper triggered a controversy. It wasn't evident if the board was helping purely because it was changing the physical dynamics involved, or if it was perhaps changing people's behavior or psychology. And knowing which factor was behind this phenomenon could help make people safer in the real world.
So Zuriguel decided to take a more rigorous approach to the problem. He first turned to his grains, placing a small obstacle above the silo exit.
"I still remember the feeling of doing the first experiment with the obstacle," says Zuriguel.
Up to that point, the grains had always clogged the outlet every two or three seconds. With the obstacle, they kept going for three minutes.
"And then I say, 'Oh my God,'" recalls Zuriguel. "It was, for me, unbelievable." After re-running the experiment to be sure, he invited his colleagues to come have a look.
"Grains have no psychology at all," he says. In other words, the result could be explained by physics alone. Somehow, the obstacle was reducing pressure from building at the exit, preventing the grains from jamming up.
Counting sheep
Zuriguel next turned to sheep.
"Sheep came to my mind because my grandpa was [a] shepherd," he says. And as his grandfather knew well, sheep follow one another — even if they have to shove their way through a narrow opening.
Zuriguel and his colleagues worked with a shepherd in the mountains outside the city of Zaragoza to run a hundred sheep at a time through a doorway to receive food on the other side. The result was this: "flow, jam, clog, flow, jam — this kind of behavior," says Zuriguel.
When they put a concrete cylinder a few feet from the door, the number of sheep passing through improved ever so slightly. But just like with the grains, it cut both the duration and number of the longest clogs — which can be the most dangerous — by more than 90%.
At last, Zuriguel was ready to try this with people. But when he used a one-ton obstacle and asked student volunteers to shove their way to the exit, "the experiments were becoming a little bit dangerous," he says. "People [were] pushing a lot."
He was ready to drop the whole thing altogether when a colleague told him about a captain in the Spanish army he knew who liked the project and was eager to participate. "A group of 200 soldiers are available for [you] to do experiments to push as hard as you want, like an exercise," Zuriguel recalls the captain telling him.
Zuriguel thought of that study from two decades ago with 20 people and a board; a group of 200 could offer a far more powerful test.
So they set it up. The instructions the soldiers were given were, "Try to go out of this barracks as fast as possible — as if there were a fire, as if your life was in danger," says Zuriguel.
But there was no effect of the obstacle on either the flow rate or the clogging.
To be fair, humans aren't shaped like sheep or grains. And unlike in the study with the board, Zuriguel believes that all those forceful soldiers built up so much pressure on the exit, shoving as hard as they could in the presence of an obstacle, that shear forces may have taken over, spinning them about and making a clean exit harder.
"People [are] facing backwards and they want to rotate a lot," he says. "We believe that these rotations, this mess, is preventing the efficiency of the obstacle."
Zuriguel continues to look for ways to improve a crowd's rapid departure from a small space.
In the meantime, he has also wanted to analyze crowds in the real world — and he hasn't had to go far to do it.
"I have an amazing laboratory two miles from my office," he says with a smile.
Running experiments before the running of the bulls
Zuriguel stands in the heart of Pamplona in a plaza a bit smaller than the area of an Olympic swimming pool.
Every morning for eight days in early July, the world-famous running of the bulls passes through here. "The bulls go uphill there and they turn around here," he says.
But before all that, things kick off with the festival of San Fermín. All that morning, people pour into this little plaza.
"The density grows, grows, grows," says Zuriguel. "Every single moment is people pushing you." The crowd can get so tight that people are lifted off the ground beneath them. "What everybody is waiting [for] is the starting of the festival at noon. Just a single firework — boom — at noon."
At this point, some 6,000 people are crammed so tightly that Zuriguel says it can be hard to breathe. For several years, he and his colleagues have filmed the movements of the masses from a balcony above.
"I've been within this crowd before, thinking many times, 'That's chaotic. It's a mess,'" he says.
But within this apparent chaos, the footage revealed a pattern: Each person in the crowd repeatedly traced out a rough circle on the ground about the size of a car. "That was really surprising," says Zuriguel. "I wasn't expecting this."
These orbital motions consistently lasted 18 seconds, a length of time the research team says is likely due to the shape of the plaza. In fact, just after the firework, when a traditional band cuts through the middle of the crowd, splitting the square in two, those same orbital motions also cut in half, to eight or nine seconds.
Zuriguel is now exploring the handful of pressure waves that ripple through this crowd where people push against each other from the back of the plaza to the front. Such waves elsewhere can be deadly, but Zuriguel says there's never been an injury or fatality here during the festival of San Fermín.
The analysis isn't finished yet, but he thinks those orbital motions, along with most everyone facing the same direction towards the city hall, might help break up the pressure waves.
"If we understand the motion, if we understand why this happens," Zuriguel says, "I think we will be able to apply some strategies in other places to prevent these kind of disasters" — thereby translating the jitters of a sangría-soaked crowd into recommendations that may well save people's lives.
Copyright 2025 NPR
