The article on London's Millennium Bridge (NCE last week), states that only 20% of people on a bridge can coordinate movements.
Some 20 years ago I carried out a small investigation in the UK on the possibility of vandals being able to excite damaging vibration of pedestrian footbridges, as part of my responsibility to ensure my company's footbridges were safe. We concluded that vandals are less likely to be so disciplined and had no reported problems. Co-ordination by music, such as at football stadia, could be in a different category.
The amplitude and frequency of the bridges was quite simply measured by a moving plotter with a tube fixed to the ground at mid-span. The plotter was actually built from Meccano, and we discovered that the greatest response, though of low amplitude, was created by a terrier dog trotting across the bridge unaccompanied!
Clive Lane (F), technical director, CU Lighting, Charles House, Great Amwell, Ware, Herts