Added a poll. You could add a few more options, but I think this pretty much covers the major cases.

At one time, I only wore a helmet when I was at a race that required one. Then I started wearing a helmet when I was going out on a "fast ride," but not slow rides. Then one day I was out on a "slow ride," and we decided to sprint to a yellow sign. Didn't make it, and was transported to a medical facility in an ambulance. After that, I always have worn a helmet. I figure with the pricing structure in medical care today, that incident would have cost me thousands in deductibles and co-pays. Any commonly available helmet would have reduced my injuries to the point where I could have ridden home. Seems like a fairly reasonable investment.