Speeding up to slow down

Srinivasan developed two mathematical models to explain how travelers typically behave on moving walkways. He found that the “optimal locomotion speed,” or the walking speed that expends the least energy, is smaller on the walkway than on stationary ground. The second model showed that people walk in a manner that resolves conflicts between signals received from their eyes and legs.

When people step onto a conveyor belt, they slow their walking speed by about half the speed of the walkway because their “brain may get confused … the walking speed the eyes see is different from the walking speed the legs feel,” the study found.

This means that time saved by using conveyor belts may be almost negligible: an average of 11 seconds for a 100-meter stretch.

“I predict that people walk a bit slower on the walkway if they are not conscious of how fast they are going, but still move faster because the walkway gives them some speed,” Srinivasan said in an interview Sunday afternoon.

The study also found that travelers’ walking behavior varies with the speed of the walkway. Though people tend to move relatively quickly on a slow conveyor belt, they would likely stand still on a faster one.

“One reason to use moving walkways in airports is to reduce the passenger transit times between airport terminals,” Sranivasan noted in his study. “But standing still on the walkway instead of walking on the moving walkway or bypassing the walkway may reduce the overall transport speed compared to when there is no walkway, thus increasing the transit times.”

Srinivasan emphasized that the study has certain limitations.

“The paper assumes one type of person, not in a hurry, wanting to go from point A to point B,” said Srinivasan, now an assistant professor at Ohio State University. “As naturally as possible without thinking about it, what does he do? And that’s the question the paper answers.”

But answering this question is only a small part of Srinivasan’s larger goal.

“The bigger question … is the concept of a predictive theory of legged locomotion,” Srinavasan explained. “If you could predict how a person will move in any kind of novel situation, it would be really useful for all kinds of applications.”

He noted that Neil Armstrong and Buzz Aldrin, having landed on the moon, decided that “bunny-hopping” was a more efficient method of locomotion there than walking or running. Ultimately, Srinavasan said he aims to develop a theory that can predict such behavioral changes far in advance.

Mechanical and aerospace engineering professor Philip Holmes, who is currently co-authoring a paper with Srinivasan, said in an interview that the study’s strength lies in its novelty.

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“It’s very difficult to make accurate assessments of how much energy it takes to do simple tasks that we do all the time because there are so many ponderables,” Holmes explained. “[Srinivasan’s] ability is to make very simple mathematical models, to match them very carefully to the data sets that are available and then to pull out something unusual.”

“It’s not going to change the world, but it’s a nice little study,” he added.