Three weeks ago, I had a bicycle accident in Onondaga park. I was just coming out of a long sprint on a track bike, and stupidly, I let go of the handlebars, got upright to relax my back, and pedaled hands free for a while, watching the landscape, still going fast, until my right foot slipped... I went from 20 miles an hour to zero instantly, on tarmac. I fell backwards sideways. It went so fast, I still can't figure out exactly the details. My shoulder hit first, then my head, my side, my hip, my leg. The heaviest of the blow was absorbed by the shoulder. I ripped my favorite fleece jacket. All I could think of in that fraction of a second in mid air was: thankfully I'm wearing a helmet, and: is my helmet strong enough? It was. While my whole body felt chewed up, as if jumping off a rolling car, my head got nothing. After the blow I laid on the road motionless for a minute, unable to catch a breath. I had heard a "crack", so I thought maybe I had broken something. I moved one leg, the other, one arm, ow... got up painfully, completely chocked, looked for my glasses that had flown nearby, couldn't find them... it was dusk. No cell phone. Couldn't walk. Couldn't breathe. My bike was lying further, the front wheel twisted and wobbly. I straightened it the best I could, and remounted to go back home. I suffered a separated shoulder and some seriously bruised ribs. Three weeks later I still can't take a deep breath without hurting or getting up without feeling like someone is stabbing me in the chest. But it could have been far worse if I hadn't been wearing a helmet. At the very least I would have left a piece of scalp on the road. At worst I would have had a concussion or a fractured skull.
In the end I am thankful. Although I've been riding hands free since I was a child, I will not do it anymore. Lesson learned. Also, after the second chest trauma I endured this year (the previous one from a snowboarding fall), and thinking back about all the times when I rode recklessly downhill, and all the near accidents with cars and people crossing my path when they were not supposed to, I will take it easy. In the end it's a matter of probability. If you ride often, you will eventually get into an accident. If you ride fast often, you will get seriously hurt. And if you ride fast often without a helmet, you are going to get killed.
By the way, I find most bicycle helmets unbearably dorky, so I wear a skateboarding helmet. Better protection, better look, ventilation not so good. It's a trade off.