I'm curious what testing and what people. Unless it's an as-yet uncontacted tribe in the Amazon rainforest, I'm not convinced that they successfully made a universally understood sign of danger.

And even if the message gets across, I will reiterate: when archaeologists understand that a message says "entering here will kill you," it only makes them want to enter more. Future post-post-apocalypse archaeologists will treat our nuclear waste disposal sites with as much care as a 19th century British scholar would have treated the pyramids. We're a curious bunch. Best we can hope for is that we keep making Geiger counters