The buzzword is "risk compensation". There's a whole article on Wikipedia about it (and lots of studies) but, to make a long story short, "measures, designed to improve traffic safety, may bring along negative consequences in a way that individuals increase the riskiness of their driving behaviour because they feel safer (Dulisse, 1997)"
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 )