I'm a little wary of relying on this, because it seems like the kind of effect that would go away if no-helmet was more normalized. In other words, if we have a

I'm a little wary of relying on this, because it seems like the kind of effect that would go away if no-helmet was more normalized. In other words, if we have a (local) culture that says "bicycling is sufficiently dangerous as to require a helmet", then drivers (even the ones who don't themselves cycle) are going to have that in their heads, and it's going to influence their behavior: "if I accidentally strike this person not wearing a helmet they're going to get really hurt, I'd better be extra careful". If the consensus is that lacking a helmet is not particularly problematic, I suspect we'll find drivers not acting specially too.