I messengered for 13 years and wore a helmet a grand total of about 10 days in that period. Never clunked my head once in that time. 3 years ago, while riding to the shop, I got smoked by an airport limo. Among my broken in 2 places back and broken shoulder was a VERY bad concussion. It took me about 10 weeks for my back to heal before I was able to THINK about riding again, let alone actually do it. The concussion took much longer....dizzyness...head-rushes. I wear one most of the time now....altho it should be ALL the time. The fact is you never know when some jack-ass cager is gonna be having a bad day or just not paying attention. Why take the chance?