The safety fence on the Calvert Street Bridge. This is a relatively modern addition to the bridge - they added it in I think the 1980s because it is a very long way down into Rock Creek Park from the bridge, and a number of suicides happened off the bridge, so they made it much harder to climb over.
@grahamp Yes, it's a visually striking object in itself, especially when taken as an abstract concept like this, but knowing why it's there does add a very different emotional undertone to it, doesn't it?