USA has 12.9 traffic deaths per 100 000 people yearly with 330 million people.
Europe as a whole has 7.4 with 746.4 million people, and developed EU countries are around or under 5. Surface area is actually quite similar 9.3 million vs 10 million km2.
There are countries with population densities higher and lower than US in there, and ALL BUT ONE OF THEM have less traffic deaths than US. It's Bosnia and Herzegovina by the way. And it's at 13.5.
USA is crazy unsafe for a developed country, and it barely matters if you compare with sparsely populated Canada (5.2) or Sweden (2.0), or densely populated Germany (3.7) or Japan (2.1).
It's not about population nor population density. It's not about wealth. It's not about population distribution.
12.9 still seems very low to me. What makes the lower number better? I wouldn’t want the difference between 12.9 and 7.4 if it means I need to give up on driving cars and take slow inconvenient public transit or be limited to where public transit takes me. Cars are freedom.
I would also argue the US is more successful than literally every other country in part because of fast road infrastructure. So maybe they’re all just making the wrong tradeoff.