Research and science are great, but anecdotes are fun, too, so I thought I should contribute a couple.

On one of the routes that I used to ride regularly, there was a lamppost with a traffic light control box bolted to it right at head height. On two occasions, I came around the post from the opposite side, didn't see the box, and ran headfirst into it, fast enough to take me right off the back of the bike. The first time, I realized that I needed to pay more attention to known hazards on my route, and the second time REALLY emphasized that point.

Going home from a concert at 2:30 AM, I was distracted enough to fail to realize that a 5" high curb was not a painted line on the pavement. I found out when I tried to merge across it, and had the bike taken from under me. Normally, I am able to manage this sort of fall without incident or injury, but this time I was taken completely by surprise, and did not keep my head off the pavement.

In both of these cases, I was wearing my usual headgear - a fedora or a cowboy hat. Both of these instances made me think seriously about that choice. After careful thought and vigilant monitoring of any repercussions (mild headache for an hour or so in the first case, and a cut above the eyebrow from my glasses in the second), I realized several things. Among them: 1.My head can take some abuse. 2.If I had been wearing a helmet in these instances, I would have needed to purchase a replacement helmet. 3.If I had been wearing a helmet in these instances, and had subsequently spent the money on a replacement, I would certainly have felt the need to tell people about how the cost was worth it because of the injuries that the damaged helmets had prevented - I might have even sworn that I owed them my life.

I have hit my head, hard, in various situations, and have even been knocked unconcious for a brief time, though never as a result of a fall from a bike. Based on my personal experience, I wear a helmet when working pulling pipe out of a well, when riding in a large, officially sanctioned bike ride, and possibly in a few other circumstances. I do not expect to be able to protect myself completely from hitting my head as I move through a world with many hard surfaces, but I am not too worried about that, and I am confident that my daily life is low risk enough that I do not need protective gear on a daily basis. If only the local law enforcement agencies had some respect for that judgement, life would be a bit easier.