(Traffic deaths / km driven) was NaN for most of human history. It's a dumb metric with dumb units.

There should be a well-defined unitless quantity that's real-valued across human history, say normalizing by population size and total human travel. I am not claiming that fixes the NaN, just that the NaN is a smell.

Another smell is that everyone being horribly maimed but never killed would not be a victory for safety though the above metric says it'd be great.

Well it's also a good metric if you are trying to make an argument that the US has unsafe infrastructure compared to other countries like the article does.