London's Millennium Bridge would have wobbled with as few as 160 people walking in time, new research shows.
The wobble was a chicken-and-egg problem, says Prof Strogatz
All that was needed was a smallish crowd to stride in step, to trigger the now infamous wobble, the research by Professor Steve Strogatz shows.
Writing in the journal Nature, Prof Strogatz said he hoped his analysis would help the design of safeguards.
After opening in June 2000, the £18.2m bridge was closed for 20 months while giant shock absorbers were fitted.
About 80,000 people crossed the suspension bridge spanning the Thames from the Tate Modern to St Paul's Cathedral on its opening day, with as many as 2,000 on it at any one time.
Prof Strogatz described the wobble as a chicken-and-egg problem.
"What came first, the bridge movements or the synchronised strides?" he said.
The bridge had two factors working against it, he explained: its flexible structure and its natural frequency was close to that of human walking.
"If the people are initially disorganised and random, if a few of them get into sync by accident, the bridge would become unstable," he said.
When a certain number of pedestrians was reached, the wobbling became marked enough to force everyone into stride.
Prof Strogatz, a mechanics expert from Cornell University in New York, US, said the analysis should help engineers solve the problem before they build a bridge.