Why Some Bridges Appear to Wobble in Unison

London’s Millennium Bridge opened in June 2000. Within hours, engineers faced a surprise: as hundreds of people crossed together, the bridge started to sway so much that some staggered or grabbed railings to keep their balance. The steel bridge was built to modern standards and was structurally sound, but it swayed sharply. Up until then, most believed that regular foot traffic couldn't possibly get a 350-meter-long bridge swinging like that. The Millennium Bridge made clear that large crowds don’t have to walk in perfect step for a bridge to develop a dramatic sideways motion.

The phenomenon behind this is called “synchronous lateral excitation.” When the bridge began to move, pedestrians instinctively adjusted, swiveling their hips to keep steady. Those little corrections did something odd: they lined up people's footsteps with the bridge’s swaying almost perfectly, which made the whole structure move even more. People weren’t trying to coordinate. Their automatic balancing reflex just happened to sync up, and the effect grew stronger with each person joining in.

Look at the math and it makes sense. The light swinging of the bridge acted like a metronome. Once enough people shifted their stride to match the motion, the energy fed into the bridge outpaced anything built in to dampen the swinging. That’s why new hardware, heavy dampers and tuned mass absorbers, has to be added to some footbridges after problems show up, even if engineers' calculations said the bridges should be safe in theory.

Precise marching isn't required. Military protocol even tells soldiers to "break step" when crossing bridges, a rule in place for exactly this reason. All it takes is a few hundred distracted commuters and the laws of physics do the rest.