In 2000, charity participants took the first walk over the newly opened London Millennium Footbridge. As they walked their feet synchronized and the natural side-to-side motion caused the bridge to sway—much to the dismay of the walkers. Officials closed the footbridge until 2002 while they made modifications to stop the swaying.
Obviously the charity participants were frightened, but in a sense they brought it on themselves: because walking on a swaying surface takes about 5% less energy than walking on a stationary surface.
Ohio State University researchers wanted to look into the human behavior side of this equation. Why is it that, consciously or unconsciously, the charity participants fell into the same way of walking and kept walking that way as the bridge swayed beneath them?
The study found that when a few people walked on such a surface as the bridge, the optimal way to walk was without shaking it. Add enough people and the group will make the bridge sway to lower the group’s energy cost.
The research team is trying to discover a complete theory of why we walk the way we do. Unsurprisingly they found that stability is always the first concern, but the next priority is conserving energy despite the situation. The team has jokingly named it “the principle of maximum laziness” as their working theory suggests people usually adjust things like cadence or length and width of strides to save even a tiny it of energy.