The "risk compensation" hypothesis suggests that individuals who use safety measures during an activity will offset that safety measure with an increase in risky behavior.
A recent study of California motorcyclists compared risky behaviors between helmeted and un-helmeted riders. The results were that riders who had been drinking were half as likely to wear a helmet as those had not been drinking. However, when these riders were segregated into drinking and non-drinking groups, helmet use was not associated with the rider being the cause of a crash, with traffic violations such as running a red light, or with running off the road.
When riders used a helmet, they were less likely to be involved in an accident. Riders are also more likely to wear a helmet during a long trip, presumably from an intuition that a longer ride may increase the potential for an accident. The study therefore found no "risk compensation" behavior from helmeted motorcyclists.
Ouellet JV. Helmet use and risk compensation in motorcycle accidents. Traffic Injury Prevention. (February 2011).