The other drivers on the road shouldn't be a factor here as it's not the thing being studied. A human driving and a self-driving car driving the same route are

The other drivers on the road shouldn't be a factor here as it's not the thing being studied. A human driving and a self-driving car driving the same route are both equally exposed to danger from the cars around them. The study should control for that variable. But I think you're getting at a really interesting question. If the ratio of self-driving cars increases in your zip code does your insurance, as a human driver, decrease? I.E. is if safer to drive next to a self-driving car or a human driven car?