Carroll Street Bridge
Spans the Gowanus Canal.
Contrary to appearances, the Carroll Street Bridge is NOT a suspension bridge, but rather a retractile bridge, which rolls back horizontally on wheels set on steel rails to allow shipping to pass. Built in 1888-89, it is one of the oldest bridges in New York City and the oldest of four such bridges in the whole country! One-way eastbound across the Gowanus Canal.
And yes, the pictured cyclists are going the wrong way on this one-way bridge.
This curious sign is posted above the Bridge: