I know that there is no substitute for a properly fitted helmet, but at what point is "close" no longer better than "none"?
I would like to get an extra helmet to have on hand for when somebody wants to ride on my bike with me, and I can't see carrying a collection of every size helmet around with me just in case. I know too small might be uncomfortable, but I'm not talking about all-day rides, just quick jaunts. I also know too big can fly right off in a pinch and be useless...
Is there an average size that would be most likely to fit any random person trying one on? What do you guys/gals do?