Oddly, I was reading about this sign design just last week. It's not commonly known here in the UK and from our perspective looks remarkably and worryingly similar to the ionising-radiation warning sign, down to the same colours and proportions; more than enough to cause a double-take in a crisis.

This led me to discover that the black-yellow radiation warning sign common in Europe is usually magenta-yellow in North America, so I now understand why the black-yellow fallout sign might not be as immediately confusing to an American.

Radiation warning sign history: https://pubarchive.lbl.gov/islandora/object/ir%3A106177