I've had a serious head injury, though not from cycling. Hurting your limbs is painful and may cause you to lose mobility. Brain damage affects your very ego and ability to experience the world - permanently. I have a headache and fatigue as I write this comment, my eyes aren't quite focused on the text like they used to be, and I am considerably less intelligent than I was. This will probably last for the rest of my life - for most of my existence.

It's difficult to put into words just how soul-crushing it is to have your inner monologue disrupted, slowed, and impaired. To sit in a class and listen, just as you used to, yet not comprehend or learn. To sit in front of your IDE expecting to accomplish a once-trivial task, yet feel nothing but overwhelming nausea and your working memory being overloaded.

It hammers home that consciousness is not special and that we're nothing more than biological machines.

Protect that consciousness. It is precious and it is vulnerable. Wear a helmet.