With cycling, in particular, it's not just individuals making judgments about their own safety, either; a 2006 UK study shows that drivers passing cyclists give helmet wearers less clearance, perhaps out of some subconscious sense that they'd do better if they get hit, or maybe that, being risk-averse helmet-wearers, they're less likely to lash out and do something unexpected. (see http://www.helmets.org/walkerstudy.htm )