[prmoustache]

You are very unlikely to crash at 15-20kph and even less likely to hit your head at such speed in a very upright position.

In fact the odds aren't bigger than when running. Do you think runners should wear helmets?

I chose to wear or not my helmet depending on the conditions. Running errands nearby on my girlfriend's lady bike? I don't use it. Riding on mostly separated bicycle paths at my girlfriend's slow pace, same. Riding 3h on the roads or trails on my road and mountain bikes, I wear it.