Never let your government get away with claiming that #bikeHelmets are "the first rule of bike safety".
Rule number one is infrastructure, and the 2nd is air+brakes+chain mechanical soundness of the bike, upright geometry of the bike, traffic awareness, ride with fingers on your brake levers and having practiced emergency stops, twenty is plenty... Helmets are for stunts or a footnote to "don't fall on your head" rule that applies to walking moreso than biking.
Helmets optional makes sense for jogging and bicycling below 10 MPH where the risks are equivalent. I wish I could remember where I read that number. I wouldn't have remembered it except from a reasonably rigorous source.
Above 20 MPH is another story. I've taken two spills at speed and was glad I was helmeted in both cases. With each fall I learned another safety rule. For example, don't pretend you have aero bars on when you don't.
My approach is that I wear a helmet if I'm pushing the limit of my ability - in terms of speed, distance, terrain (for me that's winter riding in ice and snow. If I did mountain biking I'd use a helmet then too).
For cycling as a form of utilitarian transport about town, during the ~8 months when ice isn't a concern, I don't generally bother with a helmet.
😀🚲
Ward Cunningham
dragonfrog