The Science of How We Walk

An emerging field of research is devoted to studying pedestrian behavior


The study of walking around the city is usually tied to safety. Research suggests that people who talk on the phone while cruising the sidewalk fail to process their surroundings as fully as those walking undistracted, as we previously reported. People who walk and talk cross the street with less time to spare than those who don't; they even have less time to spare than people who walk and solve a complex math problem. Other work has found that drunk walking can be potentially as dangerous as drunk driving, at least statistically speaking.

But even silent, sober strolls are starting to catch the eyes of scientists. Last month the Economist published a great piece about the emerging science of typical pedestrian behavior — a field that combines elements of physics, engineering, statistics, and cognition to understand just how we'll take that next step:

More than any other way of getting around—such as being crushed into a train or stuck in a traffic jam—walking appears to offer freedom of choice. Reality is more complicated. Whether stepping aside to avoid a collision, following the person in front through a crowd or navigating busy streets, pedestrians are autonomous yet constrained by others. They are both highly mobile and very predictable.

Much of the piece focuses on the research of Mehdi Moussaid, a crowd scientist at the Max Planck Institut for Human Development in Berlin. A great deal of Moussaid's work looks at how pedestrians respond to sidewalk traffic. When a person is walking straight toward another, for instance, a decision occurs whether to go right or left to avoid a collision. The decision has nothing to do with driving customs; in Britain, walkers avoid to the right despite driving on the left. Still people end up choosing the proper side through the some sort of implicit social understanding, Moussaid concluded in a 2009 study [PDF]:

[W]e suggest that the left/right bias can be interpreted as a behavioural convention that emerges because the coordination during evading manoeuvres is enhanced when both pedestrians favour the same side (Bolay 1998; Helbing 1991). It is therefore advantageous for an individual to develop the same preference as the majority of people. Through a self-reinforcing process, most people would use the same strategy in the end. As both sides are equivalent in the beginning, the theory predicts that different preferences emerge in different regions of the world, as it is actually observed.

Not every society reacts to pedestrian congestion the same way. A recent comparison of Germans and Indians revealed that although people from both cultures walk "in a similar manner" when alone, their behavior varies greatly in the presence of others. As one might expect given the densities of their respective countries, Indians need less personal space than Germans do, according to the researchers. As a result, when Germans encountered traffic during a walking experiment, they decreased speed more rapidly than Indians did. "Surprisingly the more unordered behaviour of the Indians is more effective than the ordered behaviour of the Germans," the study concludes [PDF].

Moussaid has found that it's a natural tendency to clump together on the sidewalk. In a 2010 study published in PLoS One, Moussaid and colleagues reported that 70 percent of walkers travel in groups — a custom that slows down pedestrian flow by about 17 percent. That's because when pedestrian groups encounter space problems on the sidewalk they flex into V-shaped clusters that "do not have optimal 'aerodynamic' features" just so they can continue to talk, according to the researchers.

Other scientists are more interested in the course of an urban journey. In the January issue of Transportation Research Part F, Greek engineer Eleonora Papadimitriou presents a model of typical street-crossing behavior by city pedestrians. Papadimitriou found that people tend to cross major streets either at the beginning or the end of long trips through the city. People who walk fast tend to postpone crossings as long as possible; they're also more likely to cross in the middle of the block. Mid-block crossing rises in frequency with the length of a trip, one-way roads, and curbside parking. As the number of traffic lanes increases, however, we become more likely to cross at an official juncture. 

In the end the study of basic pedestrian behavior still comes back to public safety. Understanding how we walk can increase the safety of crowd movements: it can help architects design more effective building exits and help event planners prepare for mass gatherings like the Olympics or the World Cup. While Moussaid began his work from a physicist's perspective, he has since adjusted his views to include cognitive factors of pedestrian behavior [PDF]; as a result, he now suspects that changes to the visual field, such as lighting systems, could help direct people through crowds in an emergency. Papadimitriou, meanwhile, believes that a better understanding of urban crossing behavior will ultimately help city road networks — too often "oriented towards motorists' needs" — adjust to the needs of pedestrians.

Top image credit: Amy Johansson /

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