I'm still waiting for the Dahon Pango helmet to appear (actually for sale instead of just in reviews). Folding helmet, sounds like a contradiction in terms doesn't it? But of course a cycle helmet is not really designed as a hard rigid shell, but as a padded shell, which deforms under pressure, and reduces impact on the skull (and more importantly shock and rapid deceleration on the brain inside). The shell doesn't have to be that rigid. The outer shell can provide a limited amount of protection for 'intrusion' injuries (i.e. hitting a point with your head, that could produce a small local hole in the skull), and protects the foam shell in day to day use. Hence folding helmets (like folding bikes) make more sense in practice than intuition. Quite a lot on the science of cycle helmets here (sorry its wikipeadia). In particular one problem with cycle helmets is that they can contribute to 'rotational' injuries, and also increased compulsion to wear cycles (e.g. via legislation) seems to be associated with an increase in fatal accidents. Like many things cycle helmet use can be counter intuitive. However I do wear my helmet nearly all the time (just like I always wear a safety belt when I'm driving). I feel pretty naked if I don't do either. Rather a nice blog entry from that den of cycling danger(!), Copenhagen.
I do have a folding helmet, its called a Stash, it works by folding the sides in, which makes for a smaller package. Its not too bad, but its rather large for my head, and not particularly elegant. I actually don't like it that much in practice due to the fit, and still prefer the skate boarder style helmet I usually use (its also got a smoother outer shell, so ought to be better for rotational injures). The skater style helmet has relatively few ventilation holes, so does provide better protection against intrusion injuries than the usual cycle helmet design (which is really designed for cycle sports - where the cyclist is generating a lot of heat). At least the skater helmet is relatively robust, and I suspect many cycle helmets are damaged when they are carried around, when the think outer membrane is cracked.
I do like the Pango's design, which appears to be closely based on a climbing helmet design by Edelrid (the Madillo Foldable Climbing Helmet). A climbing helmet mainly provides protection from falling stones, which can cut or stun a climber. This isn't quite the same as the requirements for a cycle helmet, where the primary aim is to reduce deceleration injures inside the skull. However you can see that the fold design could be adapted to a foam filled helmet.
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