This was by far the best article I have ever read on helmets. It is still a ready reference on my bookshelf. I agree with the stand taken by Motorcyclist. A helmet needs to absorb the impact to work effectively. Unfortunately, both the DOT and Snell standards are aimed at penetration, not the forces transmitted to the head. The European standard seems much better, IMO. As long as we live here, though, we are going to have to survive with the helmets available, and that means DOT and Snell.
I will disagree on one small point, Bruce, you do not want the helmet
shell to break apart. It needs to stay intact to protect you from sharp edges and objects, and to provide abrasion resistance until your movement stops. The inside of the helmet should collapse and show significant damage if it does its job. They used to, but that is more uncommon these days. Without a standard or test results for a helmet's force absorption qualities, I'm afraid we must take pot luck, and hope for the best. There is a lot of variation between brands and even between helmet models. The Snell standard does seem to result in the stiffest helmets on average, but after many years of racing, and some beneficial use of my helmets, I still prefer them. I have always worn a Snell helmet, and I have always survived.
One thing is certain, make sure it fits! Too loose allows your head to move quite a way, until it contacts the liner firmly, and stops suddenly. Unfortunately, you brain keeps moving. I had a friend that was killed in a race car that way. I was knocked unconscious from a helmet that I later found was too loose. It is like having loose seat belts, they aren't as effective because they can then cause injury by themselves. Having a helmet that fits snugly and properly, contacting your head evenly all around, helps the helmet to do its job.
-Scotty