I have two pieces of relevant experience. Firstly I hit my head on the side when I was about 16 on a bike with no helmet, in a slow speed accident. I had unpl

I have two pieces of relevant experience. Firstly I hit my head on the side when I was about 16 on a bike with no helmet, in a slow speed accident. I had unpleasent concussion, about 10 minutes of retrograde amnesia and 5 minutes or so of post traumatic amnesia. Some weird vomiting symptom about 3 days into recovery which would have been cause for serious concern if it repeated itself. I think I was off school for a whole week. I passed through high school with my usual marks (top end of the middle), and onto an undistinguished but advanced university education. Another friend was involved in a human versus baseball bat assault (skull fracture I think) around the same time and had big problems for a few years. I also worked in brain injury rehab for 4 years or so in my mid 20s. During that time I would have interacted with hundreds of patients with severe traumatic brain injuries. During that time I saw one paitent with a bike related TBI. Bike at full speed versus cow. I don't remember if he was wearing a helmet but his potential as a person was dramatically reduced, although nothing easy to observe casually. I used to be for compulsory bike helmet laws. These days I think the indirect risks of a reduced number of cyclists on the road (as evidenced by the australian experience) outweigh the direct risks of a small number of TBIs. I wear a helmet when I have longer distances to travel, or need to make use of dangerous main roads (I tend to cycle on wide underutilised sidewalks where possible). For short distances I don't wear a helmet. It's a risk, but a small one.