I shivered at the thought of being severely brain-damaged after being hit by a car while cycling about a month ago. I am now extremely humbled by the fragility of the human body. The vehicle that hit me was going really slow--a hard requirement of the lane. An apt cyclist can easily achieve 30 MPH (48km/h). That's enough to do a lot of damage itself. Now imagine a shock with a vehicle coming in the opposite direction at a mere 20 MPH (32km/h) (that’s not what happened to me BTW. I have no recollection of the accident, and no wish to get in touch with the driver. I don’t resent him at all, in fact he was extremely caring and wanted to ride with me in the ambulance but my mom was already occupying the only spot).
I'm terrible at physics but you guys and girls are probably not, so you make the calculations. To sum up, without a helmet a ridiculously "benign" accident at low speeds can literally impair you for life.
After the crash, my helmet went into pieces. I wish someone had got it so I can visualize the extent of my luck. It was an old helmet that should have been replaced at least 2 years ago. It cost me about 30 bucks and probably saved my life or cognition.
So, cyclists: own a helmet and use it whenever you're on the bike even if there are no cars around. A skilled cyclist can still crash all by himself/herself. And a car could appear from nowhere.
Some people get brain damage by falling in the bathroom. Why would you be safer waltzing around on top of a metal frame?