[fjsolwmv]

Helmetless scares car drivers into keeping a safe distance.

A helmet law wouldn't encourage a new biker who already had a helmet option.

[anfilt]

It's not the first some time some one has thought this. Probably, one of earlier studies regarding that point: http://www.drianwalker.com/overtaking/overtakingprobrief.pdf

I think it would be interesting if conducted with multiple riders, and different areas. Combine this with the fact there are studies showing increase car and bicycle crashes with helmet wearers. It definitely plausible.

There is also an other study I remember seeing that noticed helmet wearers took more risks. So there could be more variables at play.

http://journals.sagepub.com/doi/10.1177/0956797615620784

[fulafel]

This could be too. It should be in the data: countries where cyclists mainly share/don't share roads with cars. In my part of the world, nearly all car-bike collisions are at intersections.