Parapet can also refer to the fencing they might construct along a bridge. I remember when I was visiting Sydney, they had a massive metal parapet all along the bridge, which really spoiled the view. I wasn't sure what the point of it was at first, but then I realized it was to stop people from jumping off the bridge.
I wonder if it is actually effective at all. I tend to think if someone wants to do that kind of thing, a fence around a single bridge won't stop them.
But maybe it has deterred some people. Or maybe they just don't want those people falling into that particular part of the harbor!