In May 1987 the Golden Gate Bridge had a 50th birthday party. The bridge was closed to automobile traffic so people could enjoy a walk across the spectacular span.

Organizers expected perhaps 50,000 pedestrians to show up. Instead, by some estimates, as many as 800,000 thronged the bridge approaches. By the time 250,000 were on the bridge engineers noticed something ominous: the roadway was flattening under what turned out to be the heaviest load it had ever been asked to carry. Worse, it was beginning to sway.

“Though crowds of people do not generally walk in step, if the bridge beneath them begins to move sideways — for whatever reason — the people on it instinctively tend to fall into step the better to keep their balance,” Henry Petroski writes. “This in turn exacerbates the sideways motion of the structure, and a positive feedback loop is developed,” making matters worse and worse.

This time disaster was averted. The authorities closed access to the bridge and tens of thousands of people, caught in pedestrian gridlock, made their way back to land, a process that for some took hours.