As with all bridge structures, the Millennium Bridge is subject to a degree of movement. However, when large groups of people were crossing, greater than expected sideways movements occurred….The research indicated that the movement was caused by the sideways loads we generate when walking. Chance correlation of our footsteps when we walk in a crowd generated slight sideways movements of the bridge. It then became more comfortable for people to walk in synchronization with the bridge movement. This instinctive behavior ensures that the sideways forces we exert match the resonant frequency of the bridge, and are timed such as to increase the motion of the bridge.
BBC • ARUP | London Millenium Bridge