Remember kids... always wear a helmet. (The almighty bikeradar helmet thread)

Appears to be a very common misunderstanding about the often quoted 12mph impact speed helmets are supposedly reliable for. The velocity with which your head might hit the ground has nothing to do with the speed you're doing at the time, be it 5 or 50mph.

Imagine standing infront of a brick wall and banging your head against it. Just because you're stationary doesn't mean the impact speed is zero.

Of course...its obvious but I hadn't really thought of it like that. I guess it varies by crash...rider might go in arms first and slow down slightly etc