I use the same formula I used when I rode a motorcycle. 15mph+any kind of over the handlebar crash=potential for landing on or hitting my head on something at ~15mph from 6 feet in the air. 20mph is +5. Check local laws and do what you want, but I recommend a helmet.

Oh, this is about haircuts? Helmet hair not a problem. 1 or 2 on sides, knuckle high scissor cut on top. Some version of short or flattop for ~35 years.