The Theodor-Heuss-Brucke carries traffic between Mainz and Wiesbaden, but it is also one of the best places to feel the scale of the Rhine at Mainz. From the bridge, the old city, riverfront, and opposite bank line up in a way that makes the city's geography immediately clear.
It is most rewarding on foot as part of a Rhine promenade walk. Pause midway for the view, then continue along the water rather than treating the bridge as a destination in itself.