I always wear a helmet. The first thing I do after planting my backside in the seat is put on my helmet. It's always full-face and always on my head when I ride. My skull, no matter what anyone says, is softer than the concrete/asphalt I ride on. There are way too many people out there paying too much attention to their cell phones or reading the frickin' morning newspaper while driving for me to chance not wearing a helmet. I do not doubt my own abilities, I have serious doubts about the abilities of those around me.
