Running of the bulls could help scientists prepare for catastrophe | Science

The running of the bulls in Pamplona, Spain, provides a unique laboratory experiment. Where else can scientists know exactly when and where people will be fleeing for their lives? Now, a study of the event reveals exactly how crowded a street can get before people start to trip while running at top speed.

The findings could help researchers model better ways to help people escape building fires and other catastrophes, says Jorge Laval, a traffic theorist at the Georgia Institute of Technology who was not involved with the study. “I thought it was superinteresting.”

For the uninitiated, the running of the bulls is part of an annual festival in which human participants rush down a series of narrow streets alongside the charging animals. Although it’s a voluntary and somewhat contrived danger, Daniel Parisi of the Buenos Aires Institute of Technology, the study’s first author, says people’s flight is real enough to offer rare insight into how humans behave as they try to escape danger. About 16 people have been killed since the festival’s inception in 1910, and hundreds more have been injured. Most of the fatalities have come from being gored by a bull, with the most recent one occurring in 2009, but other injuries commonly result from falls and pileups.

To capture data on the runners, Parisi and his colleagues set up a pair of cameras suspended above the streets of Pamplona to get a bird’s-eye view of the event. They recorded two runs from consecutive days of the festival in 2019. The cameras caught each runner’s speed and direction, as well as the density of the crowds.

The cameras show a bunch of people jogging around somewhat slowly, spacing themselves out along the streets. When the bulls arrive, a big clump of runners sprints past the camera just ahead of the animals. People scurry to the side as the bulls overtake them.

Unexpectedly, the data revealed that the speed of the runners increased with the density of the crowd—up to a point. That’s the reverse of what researchers have typically found when studying the flow of people or traffic: Usually people slow down when crowds are more dense.

The authors say the results may be thanks to runners’ desire to change velocity during different parts of the event. In a crowded subway station, for instance, everybody wants to walk at a normal, steady pace; adding more people just slows everyone down. But in the streets of Pamplona, runners wait for the bulls, sprint for a short time as they go by, then return to a low velocity again, creating a wave of density and speed around the animals.

Eventually, density did spell disaster in Pamplona, too. Once the crowd swelled to about one or two people per square meter, the maximum velocity of runners dropped off sharply, often because people started to collide and fall over. Once the crowd surpassed two people per square meter, virtually no one could sustain speeds above a light jog (about 2 meters/second), the team reports today in the Proceedings of the National Academy of Sciences.

Precisely understanding the speed limits imposed by crowding could help researchers build better models of pedestrian movement and improve evacuations for fires and other emergencies, Parisi says.

“When people are stressed and running for their lives, there’s a sweet spot at very low densities when you can get a very high amount of flow,” Laval says. The best way to apply this information, he says, is to continue to create policies and building designs that disperse people and keep density low, perhaps by widening evacuation routes or by limiting the number of people in an area to begin with.

Laval and Parisi both agree that the best thing to do in an emergency situation is to avoid running or pushing all together, just like your elementary school teacher told you. If everybody stays upright, the system is less likely to get completely gridlocked.

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