Cannabis is a commonly abused drug whose use cuts across social class, is linked to cognitive impairment, and may be a major contributor to intoxication-related accidents - either alone or with alcohol. However, cannabis intoxication is little studied in relation to driving compared to alcohol. Not only does the current NHTSA Strategic Plan for Behavioral Research prioritize understanding how drugs other than alcohol contribute to traffic crashes, it has recently become more pressing to understand the effects of cannabis because of increasing rates of legalized medical and/or recreational use, that will likely result in more cannabis intoxicated drivers. Social and legal policy will be unable to effectively address the many concerns about driving safety raised by more frequent and widespread use of cannabis without new research to better determine the parameters within which cannabis use does, or does not, increase automobile accident risk. The purpose of this study is to better describe specific, driving-related cognitive impairments caused by acute cannabis intoxication, their persistence over time, underlying functional brain anatomy, and relationship to performance on a state-of the art validated simulated driving task in which the investigators have prior experience. In a randomized, counterbalanced, double-blinded fashion, the investigators will administer two cannabis doses and placebo of smoked cannabis (paced inhalation using a vaporizer) to 48 regular cannabis users and 48 occasional cannabis users on 3 separate occasions. Following cannabis dosing cognitive and driving impairment will be assessed longitudinally for several hours using a combination of fMRI and neuropsychological tests, to clarify relationships between subjective and objective measures of intoxication and of impairment, that include expert assessment of THC and its metabolite levels in blood and saliva. This study takes place in Hartford, Connecticut.