The case for helmets on bicycles is less clear cut than it is for more dangerous forms of transit such as downhill longboarding. Among other things it makes car drivers drive more dangerously around you, but it also changes much more common and less dangerous accidents into more dangerous ones. Taking a tumble on a bicycle is generally not fun, but getting your neck twisted due to a helmet isn't improving things.

Of course I come at this from the perspective from a country where there's more miles of cycle lane than miles of car-only roads. Who knows which way the balance tips if you need to mix bicycle and car traffic. To be honest I would simply not ride a bicycle if that was required of me on a regular basis.