The bridge is swaying slowly and absorbing energy from vehicles passing over and wind rushing by it. Each “impulse” is dampened so that the bridge can’t shake itself apart (reinforced oscillation) In real time the bridge is barely moving and even when it does the energy is taken out of the system instead of allowing it to reflect back and forth across the bridge. The design was done by the same guy who designed the Tacoma Narrows. The bridge does seem to move up and down fairly frequently, but at a slow rate. Maybe from the distribution of weight changing constantly, plus other factors like temperature could affect it too.”]
Source: https://gizmodo.com/manhattan-bridge-doesnt-look-very-safe-in-this-video-5322405