Yeah, but why are red/yellow/green "universal" colors for driving? I don't think it's because of some innate human love for those particular meanings of the colors. In fact, I'd bet that in a society without cars the red octagon would not be so "universally" understood to mean "stop."

Basic colour meanings of red and green transcend many genres of human existence, from business (using a red pen, or being "in the red") to entire gamuts of electronics and engineering, red warning stickers, etc.

I agree that the octagon is arbitrary. But that's beside the point, because it's self-evidently arbitrary. Nobody assumes there must be a deeper meaning behind a basic shape. The iconography of a floppy disk is not a basic shape.