In 1966 as a young engineer I attended the opening by the Queen of the Severn Bridge.
After the ceremony the public were allowed to walk on to the bridge and a vast crowd did so, advancing over the four-span Aust viaduct.
Vertical vibrations were felt, and in a short time the crowd was walking in step which enhanced their amplitude. It did not take long for some people to become alarmed and to turn and rush back off the bridge, although the amplitude of the vibration was very small.
So 'lock-in' does occur when pedestrians cross a structure capable of vibrating vertically at frequency close to that of footsteps (NCE 22 February).
John Curtis (F) firstname.lastname@example.org