On the helmet issue I'm ambivalent. There was a study a while back that indicated that drivers drive more aggressively around cyclists wearing helmets...giving them 6 inches to a foot less space on the road. I imagine this is probably less pronounced in places where there are lots of cyclists, as I suspect the brain finds a regular niche for them rather than analyzing each cyclist as a new event. I would think it would take more brain power to make the "Wearing a helmet?" assessment each time and that it would eventually be optimized away by drivers that deal with cyclists every day. In SF, for example, I've found that drivers have no hesitation in driving 6 inches away from me and my bike...so I think they're only giving me the "wearing a helmet" margin, no matter what. I'd obviously rather the niche go the other direction, and drivers would give every cyclist a reasonable amount of space on the road, but that doesn't seem to be what happens. I'm probably not going to convince drivers in Mountain View to respect pedestrians in cross-walks either.
The type of bicycles most popular in America all have a very forward-leaning riding position, compared to Dutch bikes. If you get into a crash at speed there is a good chance you are going to be pitched over the handlebars. A good friend of my brother was in a coma for 3 months after crashing into another bicyclist, flying over the handlebars and landing on his head. An ex girlfriend had the same sort of crash, but she had a helmet on and managed to avoid brain damage.
On a Dutch style bicycle, a headfirst over the handlebars crash is nearly impossible because the riding position is completely upright, and laid back over the rear wheel.
Helmet necessity seems like more of a factor for the type of bike you are riding vs. cars. In all the cases where I've seen bike/car accidents, the person was wearing a helmet, and they were dead anyway.
OTOH suppose a driver has a cyclist on his right and another cyclist on his left, and only the right one is wearing a helmet...better not to wear a helmet in that case I think.
