There’s not many options for what it could be, so it’s pretty easy to figure out. The color is most important, with some additional info like utility (which are few and welol known) and width.
I wonder how they factored in that red-green color blindness affects up to 8% of males. Should color really be the most important? Would be interesting to see what percentage of accidents are attributed to misreading a symbol vs a color vs anything else.
It's probably old enough that they didn't. Plus as said elsewhere, where NOT to dig. If looking to intersect it someone who can see the color correctly will have to identify the location.