I'm actually just talking about the expansion joints! Like one or two places that are metallic, the rest can be concrete. I havent biked over one but I expect that they are one of the least hazardous of the hazardous terrains - I'm comparing them mentally to pot holes and rail lines (not sure the right for them, street car lines that as it turns out are perfectly shaped to get a bicycle tire stuck in while at speed).

Just between you and me, fuuuuuck those metal grates on bridges. I'm grateful that I have almost always had the opportunity to get onto a good sidewalk/path instead of navigating a bridge with an 'interesting' surface.