For years, the CDC (Disease Control) and NHTSA (Traffic Safety) government agencies have declared that bike helmets reduce the chance of a head injury by 85%. But that is changing.
Apparently that 85% number was the result of some casual statistical analysis, based on emergency room visits by cyclists who were or weren't wearing helmets when involved in an accident.
The Washington (DC) Area Bicyclists Association (WABA) petitioned the government to quit using that number as it is likely inaccurate even though it is often presented as "fact."
You might ask, "Why would a bicycle advocacy organization discourage helmet use?" And that's a good question. And they have two good answers:
1) If people think helmets stop all head injuries, they won't demand better helmets, and
2) Legislators may feel it makes sense to require everybody to wear one.
(The WABA was instrumental in getting a state helmet law passed for riders 16 and under, but they feel a law requiring all riders to wear a helmet on all rides would discourage bicycling. I can agree with that.)
Read more HERE.
Personally... I always wear a helmet. I'm not too concerned whether it makes me 85% safer, or "only" 50% safer. Even when I'm working on my bike and take it for a test ride in front of the house, I feel weirdly vulnerable if the bucket isn't attached. And it has paid off - my adult son always wears a helmet when he bicycles (or motorcycles), and granddaughter Mackie gives me a scoldin' if she sees me riding without one. In my 27+ years of transportation cycling, I've only been really, really, REALLY glad I had that helmet on two times! In either of those cases, I'm pretty confident my bike-riding would've been cut suddenly short, if not for the helmet. And when your head is flying toward that hard pavement, it's too late - there's no "do-over."