Originally Posted by
sudo bike
I disagree that it has a large impact on brain damage. Brain damage is usually due,
AFAIK, to the brain bouncing around inside the skull, not direct damage to the skull itself (which bicycle helmets can help with). Here's some anti-helmet propaganda talking about it
http://www.cyclehelmets.org/1139.html 
. I don't think rotational injury can just be waved off, as it is important to brain damage... just what degree helmets may contribute to this is not really known very well (also no real consensus, as far as I can tell). Again, I don't think it is so much that it is impossible to design a helmet that would be more effective (as noted, some designs are trying to address rotational injury, such as a sliding "scalp" layer), I just think bicycle helmets, particularly the most popular kind that just about everyone wears, are woefully inadequate if their purpose is to prevent brain injury.
What it really comes down to, for me, is that I feel I'm pretty unlikely to get in a crash, and if I am, it probably isn't going to be one where a helmet will help me much (getting hit by a car).
There's actually two questions in the articles being quoted: first is that if helmets reduce seriousness of injuries in a crash, and second is if wearing a helmet makes you safer in general. Injury reduction is directly related to protective qualities of a helmet, but in comparison the trends in accident numbers are only partially related to wearing a helmet - general riding habits, amount of riding done, safety thinking etc. have a much larger effect on that. A helmet doesn't protect you from getting into an accident, it can only reduce the injuries.
So when talking about reducing severity of head and brain injuries, what we do know is that cyclists and motorcycle riders get head and brain injuries, we have a good model of the injury mechanisms, and we have good test-based evidence that helmets reduce damage by lessening the forces involved in these kind of situations. We also have good data from hospitals - from crashes where injuries are serious enough to ward a hospital visit - and these show that helmets lessen the head and brain injuries when the crash is severe. I think these are pretty good indicators - it's hard to imagine a person getting serious brain damage in a crash and not be taken to a hospital immediately or soon after! In these studies, rotational brain damage is included in the "serious brain damage" group - they do not differentiate between direct and rotational origin of brain damage, only if the damage exists or not. So, even if helmets would increase rotational injury, they are shown to cause a reduction in total number of serious brain damage, meaning that your chances are better when wearing a helmet.
There has been studies into cyclist-vehicle collisions, and did show that helmets do reduce the seriousness of skull and brain damage in case of these accidents
