Generally speaking, the evolution process has adapted the human form to survive the bulk of impacts that nature and gravity are likely to throw at it. The viatal organs are englase in a bony yet flexible cage. The brain is suspended in a shock absorbing fluid inside a rigid dome of bone. Of cource, cavemen will have fallen off clifffs etc, but even the mighty mother nature can't account for everything.
Indeed, nature can't account for cycling the velocities involved are un-natural in the sense that evolution has never accounted for this in the current design of the human form. It has the ptoential to cause the type of injury that nature never had in mind when it started tinkering with home erectus (snigger) and is why some of us choose to wear helmets.
accidents will happen, and the mdoern environment with stairs, laddrs, Stella Artois etc is also beyond the scope of evolution. Nevertheless, in terrms of transportation the human body is still best able to protect itself from injury in it's designed, eveolved form, walking upriggt on 2 legs.
don't wear a helmet on a bike, I'm fine with that if that's your choice. but justifying not wearign a lid (not that you need to justify it to me anyway) by pointing out epdestrians dont wear them is pretty funny, when this is the human bodies designed (or adapted) nature mode of motion.
Now, seatbelts for toilets might be an idea...
