I have three scary bridges.
The first is probably only scary to me. I had recurring dreams for a while of the end of the world coming and these two twin gateways of doom featured. I eventually came face to face with them, the Delaware Memorial Bridge. I must have seen them on a trip down 95 as a kid and called them back up. And lit up at night, they look like Cirith Ungol in the LotR movies.
Second is only due to my personal experience with the bridge. Millard E. Tydings Memorial bridge over the Susquehanna on I-95. Crossed it during the edge of a tropical storm with lightning striking the bridge, wind gusts and no visibility.
My real vote is the outer roadway lower level eastbound on the Queensboro Bridge, though. I didn't even know it existed until I got lost trying to find the entrance to the normal lower level. (My wife gets skittish on bridges, so I go lower level middle lane.) I got onto the bridge and its a single lane, a former trolley path and very narrow, swerves near the bride abutments, and was the single worst experience I've had in a car. It's the mirror of the pedestrian path and is NOT for the faint of heart.