I am a researcher in the UK, and this is pretty much happening here. It's required by the national body which the government uses to fund research (some info here) and that requirement gets passed on to the subject area-specific research councils.
Basically, work I publish needs to at least be available on a University-level preprint server ("green" open-access); many publications allow this now. For publishers that don't, the research councils have arrangements with research institutes to pay the fees for the final published versions to be publicly available ("gold" open-access). It's not ideal as we're still overpaying the publishers, but it's a compromise that sets a pretty clear direction. In addition, I'm required to make research data readily available. Rumour has it that my research council will soon be picking random papers, trying to get the data, and kicking buttocks where they encounter a problem.
I don't think legislation is necessary; policy at the funding level seems to be doing the trick, and is a bit more flexible.