 The bridge's movements were caused by a 'positive feedback' phenomenon, known as synchronous lateral excitation.The natural sway motion of people walking caused small sideways oscillations in the bridge, which in turn caused people on the bridge to sway in step, increasing the amplitude of the bridge oscillations and continually reinforcing the effect; the maximum sway was around 70mm.On the day of opening, the bridge was crossed by 90,000 people, with up to 2,000 on the bridge at any one time.