I always wear one when on a bike, used to not wear my helmet when adjusting & testing out bikes up & down the street in front of my house. One day I was testing an old Schwinn and still not sure what happened but I ended up on the ground and hit my head pretty hard. I think a pedal hit the ground as I was cornering. Could have been
much worse, luckily no lasting damage (physically at least
), but now I always wear my helmet no matter how short the ride.