I have seen many people carrying rather than wearing their helmets. We have a mandatory helmet law here in Australia, and I think it's just to convince people that they are wearing one if they see the cops (don't ask me how that's supposed to work).
As far as the "helmets cause head injuries" argument goes, are we talking proper use of helmets or improper use of helmets? All I'll say is that mine saved me from some pretty serious head injuries last year.
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I am clinically insane. I am proud of it.
That is all.