Who are the copyright holders of common street and warning signs?

Does anyone know who the actual copyright holders are of common sign designs? I found for traffic signs, the Manual On Uniform Traffic Control Devices (MUTCD) explicitly forbids copyright of all traffic signs except for one.

04 Any traffic control device design or application provision contained in this Manual shall be considered
to be in the public domain. Traffic control devices contained in this Manual shall not be protected by a
patent, trademark, or copyright, except for the Interstate Shield and any items owned by FHWA.

The PDF contains all signs as vectors. I've seen collections of "official traffic signs" going for very very high prices on some websites (not eBay but professional sign supply companies), so it surprised me to see you can get the official ones for free.

I see conflicting reports for other items. ISO seems to hold the copyright of a lot of things like the new 21482 radiation hazard sign that's a triangle with radiation microwaving some poor stick figure and a skull that's already dead (so it doesn't matter if it's being microwaved!). Wikipedia contains the trefoil radiation symbol, the biohazard symbol, etc with "no copyright" but that's hardly an accurate/reliable source. ISO charges for their designs so I guess I would have to check each one against their database before using it. Things like DO NOT LOOK INTO LASER WITH REMAINING EYE signs with ISO symbols on them probably easily fall under parody protection.

Obviously it would be illegal to create a very authentic interstate or distance-to sign and put it where it could be confused for a real one, but I've had people mention they would like a personalized street sign or whatever. Also people like biohazard and radiation symbols for things like Halloween decorating. That kind of makes me wonder what the actual legality of putting a biohazard symbol on an empty barrel in a Halloween display would be, both from a copyright viewpoint and safety law viewpoint.