I've been doing some research into this symbol (part of ISO 7010), which I'd like to use:
but I can't find out whether I'm allowed to or not. Is the glyph in the public domain? What about similar commonly-used symbols?
Apparently, Wikimedia Commons considers it freely usable for all purposes, i.e. effectively in the public domain.
Specifically, they base this on the wording on the Japanese Foundation for Promoting Personal Mobility and Ecological Transportation website (the "green man running through the door" sign was originally designed in Japan, by one Yukio Ota, before it became an international standard), which says (emphasis mine):
To be safe, I would suggest basing your sign on the specific design available from the Eco-Mo Foundation website, just to avoid any possibility of anyone else claiming copyright, however tenuous, on any variant design. Also, as noted on the website, you should not pass off the sign as your own design nor use it in a trademark. Of course, if you want to be sure, you could always e-mail the Eco-Mo Foundation and ask.
Moved from comments below: The same answer presumably applies to all the other symbols found on the Eco-Mo website. Beyond that, I'd be hesitant to generalize in the absence of evidence. I haven't been able to find a blanket statement by, say, the ISO saying that all their standard symbols would be free to use (for any purpose, or at least their intended one), even if the fact that they are standardized symbols does strongly suggest it.
Furthermore, many standard symbols may have local laws (or even international treaties) regulating their use independently of copyright, which can further complicate things. For example, the Red Cross symbol is specifically protected by the Geneva Conventions and implementing national legislation, despite it being essentially ineligible for copyright due to its simplicity, and the various Red Cross societies have been known to occasionally sue misusers of the symbol.
The issue is not one of copyright so much as legal requirement. International and national standard pictograms are not owned; they are mandated (or advised, depending on the jurisdiction and the application). In the US, for example, there are OSHA requirements for the workplace. The EU has its own signage regulations which are based on the ISO 7010 standard.
Specialized signage companies (note that the OSHA link specifically advises the use of such companies) produce signage which is exactly in compliance with local regulations as regards size, color and the exact art. The evolution of standard signs is interesting in itself; many of them have a long history, particularly in electrical and fire safety.
As with almost all government-funded or government-produced items, these are free for anyone to use, but if you are creating a signage program for a warehouse you had better look up the standards and adhere to them. Also in line with standard government practice, you pay for the books, catalogs and codes if you want paper copies, but regulatory guidance is often available online. If you Google "safety signs and signals", you'll find a useful PDF document that I can't link here because its URL has a numeric IP address.
Here is an extract from page 9 of the PDF (from the UK Health and Safety Executive) that specifically addresses pictograms:
Many sign companies have labels or other types of warning signs they have developed themselves, and they retain copyright on the complete finished product, but the pictograms themselves, where they are part of national or international standards, do not bear royalty or attribution copyright requirements, nor do they bear a copyright mark.