St Paul's Cathedral is at one end of the bridge, with Tate Modern at the other
London's Millennium Bridge "wobbled" so much when it opened because of the way people walked across it, a study by civil engineers has claimed.
The £18m Thames crossing was famously shut after two days in 2000 as police were worried it swayed so much.
This was because people naturally move from side to side when walking rather than in a truly straight line, research at the University of Bristol suggested.
The same problem arose on other bridges tested as part of the study.
The 1,067ft (320m) crossing links St Paul's Cathedral on the north bank of the river to Tate Modern in the south.
It was designed by the architect Sir Norman Foster with sculptor Sir Anthony Caro and engineers Arup.
About 100,000 people used it on its inaugural weekend but many found it swayed alarmingly, leading engineers to conduct safety checks.
The crossing, the first in more than a century, was designed by Lord Foster
It had been thought the structure's "wobble" was caused by having so many people walking at the same pace as each other, effectively moving in unison.
But the researchers in Bristol found that pedestrians' footsteps were never synchronised in this way because people naturally lean in different directions to maintain their balance.
When hundreds of people were crossing the bridge at the same time, their natural movements would contribute to the "wobble", the civil engineers said.
The study, published in the Royal Society journal Proceedings of the Royal Society A, tested other crossings, such as the Clifton Suspension Bridge in Bristol.
It found the phenomenon occurred on different kinds of bridge design and so could not be blamed on the structures themselves.