Helmet saved my life. Head first into a tree at around 12-15mph (wasn't really looking at the speed). Enough force to break my spine all the way through in two places and 7 break ribs. Helmet was broken in at least 5-7 places all the way through; no head trauma. Well, I say no head trauma, but I believe there was very little - perhaps some nerve damage causing one pupil to be smaller than the other (yes I have been to the doctor for that after the fact). I'll let you decide whether the helmet kept me from a fatal skull fracture or at least some type of brain damage. Think about the force it would take to break a spine all the way through and 7 ribs on a man that is 6'1" and roughly 185lbs - pretty fit, good muscle tone, some fat around the mid-section.
How can you know when you fall that you won't hit a guard rail head-first, or careen off the road and uncontrollably smack a tree? My accident happened so fast I wasn't even able to put my hands up. Some people are just accident prone; I'm NOT that guy. I had never broken a bone - other than fingers and toes - and I've never sprained an ankle, never had stitches or staples; this was NOT going to happen to a guy like me. Just some food for thought.
The "Helmet saved my life" line - I truly believe that. The doctors truly believe that. You don't have to. You can say I'm delusional (maybe it's a symptom of a head injury).