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

I find this bit quite important. So helmets are ineffective over 12mph... wow that's useful. Most people cycle above that speed I would assume.

If you are doing 20mph and have time to brake a bit, then impacting at under 12mph is quite probable, I would have thought.

If there is a car involved, then the chances of the impact being over 12 mph are probably quite high, I would have thought.