[fulafel]

Potential reasons for the surprising helmet-accident correlation:

(edit: inserted 0'th reason)

0) It could be true but irrelevant: We mainly want to reduce severe injuries, accident rate only peripherally interesting in relation to this.

1) Is the direction of causality estabilished? Is it possible that a trend of increasing accident rates results in lawmakers passing helmet laws?

2) But generally it stands to reason that cycling helmets can't actually reduce accidents, just reduce their impact. And it also sounds reasonable that helmets would give bikers a small amount of additional risk appetite.

3) Maybe helmet laws encourage casual or impaired cyclists (uncoordinated people, children, people with bad awareness, etc) to get on bikes, and/or repel the confident types who choose to do other exercise rather than wear sweaty and dorky looking helmets. So the bicycling acuity of the bicycler population is reduced.

4) Is there data picking at play? This cites only 2 studies reporting increased accident rates, and both of them were in USA, so the data is not very good for drawing general conclusions. Is there something in the local circumstances or bicyclist demography? How many studies can you find where this increased accident rate doesn't show?

[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.

[tobylane]

>3) Maybe helmet laws encourage casual or impaired cyclists (uncoordinated people, children, people with bad awareness, etc) to get on bikes, and/or repel the confident types who choose to do other exercise rather than wear sweaty and dorky looking helmets. So the bicycling acuity of the bicycler population is reduced.

I was expecting the exact opposite. It's the casual or new cyclists who stop cycling because of a helmet law, this is the most common finding of all the investigations into a possible helmet law I've seen. It then leads into 3.5) the type of people who cycle when a helmet is required take more risks/spend more time among cars and have less fellow cyclists keeping the drivers aware/honest/giving.

[stouset]

Drivers are also documented as passing faster and more closely to helmeted cyclists.