[tonyg]

The only times I've hit my head on the ground as an adult have been coming off my bike. (Not that often, but still, three times is more than the zero for all other causes combined.) In each of those three times, I've been profoundly grateful that I've been wearing a helmet.

Adults tend not to fall over, voluntarily or in-, while walking around. There are plenty of ways to fall over and hit one's head while cycling that aren't under one's own control at all.