Cody Dock is one of my favourite little spots-that-most-people-have-never-heard-of in London, and here is what looks to be a brilliant idea for a bridge there, by someone called Thomas Randall-Page:
That’s the bridge in its two possible states. Left, people can cross it. Right, bigger-than-small boats can go under it.
I tried to contrive a verbal description of how it works, but have failed. It’s all to do with rotating the square shaped bridge in such a way that its centre of gravity stays steady. But, (a) take my word for it that it’s very clever, and (b) follow the link and see how right I am.
The world does not seem to be building many new big bridges, but it is still contriving little bridges of great inventiveness, if only because they’re cheaper.