 Vibration was attributed to an under-researched phenomenon whereby pedestrians crossing a bridge that has a lateral sway have an unconscious tendency to match their footsteps to the sway, exacerbating it.The tendency of a suspension bridge to sway vertically when troops march over it in step was well known, which is why troops are required to break step when crossing such a bridge.An example is London's Albert Bridge, which has a sign dating from 1873 warning marching ranks of soldiers to break step while crossing.