Whether or not you wear a helmet, clearly depends on the context. If riding slow on an upright style bike, it is unlikely your head will hit the ground, as demonstrated in the video clip at the top of this post. (We thank the clip’s maker for not going downstairs and alerting riders of ice on the path).
If you are riding fast though, in a forward leaning position, your head will almost certainly hit the ground when you fall. Let salacious compilations of filmed crashes in bike races, like the one I have pasted below, be a warning to all of you hipsters.
Australian helmet laws were devised at a time when almost all cycling was fast and froward leaning. The people making those laws could not have foreseen a Dutch style bike craze sweeping the world. If you know a politician, do send them this link. Many thanks to the incomparable Brian Jones for sharing the first of these clips via twitter.