I believe the percentage of cyclists who die from head injuries is something like 75%. I don't know the breakdown of auto-related fatalities regarding seatbelts.

How many stories do you hear where the person says something about how the helmet split and saved their life? Or maybe the helmet broke and they got a nasty concussion but think how much worse it would have been! And so on and so on.

The fact is, there's no evidence that the injury would have been any worse had they not been wearing a helmet. You cannot study something that never happened. In fact people are still having nasty head injuries even wearing bike helmets. I believe this is for lots of different reasons, not the least of which is the fact that helmets are designed for low speed impact which many collisions are not.

When controlled for population decline, no place that has implemented compulsory helmet use has ever been able to show any decrease in injury or death even as the rate of helmet use increases dramatically.

This failure to demonstrate efficacy and the inevitable dramatic decrease in ridership is why the British Medical Association has declared its opposition to mandatory helmet use.