Movable bridges were usually cheaper to build than fixed bridges because they didn't require the same heavy foundations and structure.
These Lift bridges are counter-weighted and usually operated manually or by using a windlass like these are..
The bridge above was a bit different from most..... it as its own traffic light system... one press of a button and the lights change an d the traffic stops. Once through another press and the bridge lowers, the lights change to green and off gose the boat and any traffic on their way.
For this bridge the boats windlass came into play
A windlass is an apparatus for moving heavy weights. Typically, a windlass consists of a horizontal cylinder (barrel), which is rotated by the turn of a crank or belt. It was just a matter of a few turns before the counter balance comes into play and the bridge opened