This post has been updated to include some additional very useful comments – scroll to the bottom!
A coupla posts back I mentioned I don’t wear a helmet all the time. I like to dress exactly how I like to be dressed for the day and not have my outfit stop me from riding a bike (and, instead, drive). Which prompted some of you to want to lock me away from your children (should I corrupt them into not wearing theirs). But many more of you shared some very good info about where the research and laws are all at.

It’s always good to have informed ammunition when someone confronts you with their outrage that you’re slipping through the streets on two wheels unhindered. (I should point out again, I do wear a helmet on long trips (ie not on familiar roads in my neighbourhood), when I’m riding off-road etc.)
So, a bit of a cheat sheet for you, based on comments from the original post:
There are safety studies about the issue. This one is a good overview. But a few interesting factoids that I pulled from the link…
* Cycling is no more dangerous than being a pedestrian.
* UK research has pointed out that it “takes at least 8000 years of average cycling to produce one clinically severe head injury and 22,000 years for one death“.
* Six times as many pedestrians as cyclists are killed by motor traffic, yet travel surveys show annual mileage walked is only five times that cycled; a mile of walking must be more “dangerous” than a mile of cycling…” The proportion of cyclist injuries which are head injuries is essentially the same as the proportion for pedestrians at 30.0 % vs. 30.1 %.
* No randomized controlled trials have been done on the safety of bike helmets.
This from Dr Paul Martin (“a medical specialist…involved in the resuscitation of trauma victims and their subsequent management in the operating theatre”):
A bicycle helmet will not prevent brain damage if the forces involved are so great that you would sustain brain damage without a helmet. You will certainly prevent superficial injuries but that’s about it. In fact, this is a major reason why they’re good to wear for sport cycling. There is nothing worse than having to abandon a race with a scalp laceration. I do not wear a bicycle helmet most of the time. When competing, I’ll wear one. Do you wear a helmet when in a car? If not, why not? The risk of a serious head injury in a car is much greater than on a bicycle. If there was a mandatory car-occupant helmet law would you all comply? 
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