Earlier this year in Germany, the courts decided that a bicyclist shares responsibility for pain and suffering from head injuries received in an accident if they are not wearing a helmet(1). So although it is not required by law to wear a helmet, bicyclists may be entitled to substantially less compensation in the event of an accident.
Of course, any urban warrior amid the tons of pollution-spewing steel claiming ownership of the roads knows that the head is only a small part of the utterly naked exposure cyclists feel. Those of us wearing helmets are motivated mostly by "umbrella karma": if you wear your helmet, you won't crash, akin to the observation that it only rains when you forget your brolly.
But even the most committed safety advocate faces that awkward moment, trying to figure where to stash their melon-mantle at the shop or during the train stretch of a bike-ride-bike commute. The Stash provides a solution. The sides of this helmet fold in, significantly reducing the helmet's volume to the point where it could be stored in a corner of a briefcase or backpack. A video of the helmet design in the anvil-drop test demonstrates that the unique concept surpasses the safety requirements for impact reduction. So far the only retailers mentioned for the Stash are UK based. The helmet has Europe's CE certification but seems to lack the safety credentials for sale in the USA. The list price is just under 50 pounds sterling. Stash is the perfect companion to your freaky folding bike, but will work just fine with the standard selection of folding bikes as well.
(1) OLG Düsseldorf, Aktenzeichen: I - 1 U 182/06