Question:
Try this experiment. Let a fellow motorist T-Bone you in your car at 30 mph. Replace the glass you busted with your head and try again with a helmet. Which causes less injury?

Stupid people with nothing to protect, refuse to wear automotive helmets when motoring. I'm tired of my insurance going up over their stupidity.

It's been demonstrated by insurance sponsored studies that per hour, motorists get more head injuries per capita than cyclists. But even with this greater risk, they're too stupid to wear helmets. Cycling may be dangerous, but motoring is even more so.

When are motorist going to get a clue and quit bouncing their heads against protective glass and metal?

Answer: Traumatic brain injury, even in mild forms, can require extensive institutionalization and/or extensive rehab, so the cost of a severly brain injury per incident is very high. I find the argument that costs are "passed on to us taxpayers" to be shallow - some people (the doctors, hospitals and therapists) will gain by servicing those with head injuries, and although it's not a zero sum game, the real dollar costs of head injuries is not all "lost." In some respects costs of any kind, whether for head injury, smoking, lack of exercise, bad eating habits, failure to obtain insurance, or even unprotected sex are "passed on" in the forms of higher prices, greater taxes, etc., but the costs of head injury to society in the form of lost productivity for the injured individuals are real and measurable. Thus at a minimum, the societal cost of head injury can be measured as the net present value of the reduction in earning capacity of the injured for the time span of the disability.

To compute the net societal cost or benefit of bike helmets, take the NPV of the cost, factor in the probability of occurrence and you will get the "break even" cost for a helmet. If the cost of the injury is (on average) less, then helmets are a waste.