Driving under the influence of alcohol or cannabis is a major public health concern, as both are major risk factors for motor vehicle accidents (MVAs). Prevalence levels for both driving-risk behaviors have increased in Canada in recent years, despite punitive laws and enforcement aimed at impaired driving. Young drivers are a major risk group, due to their common substance use and MVA involvement. Data from a cohort of N= 102 high frequency cannabis users (18-28 years old, 70 males and 32 females) who were also active alcohol users, recruited by mass advertising from university student populations in Toronto, indicated that a significantly (p= 0.009) higher proportion of the sample (35.0 percent) had driven a car while under the influence of cannabis than had driven while under the influence of alcohol (4.9 percent) or of a combination of cannabis and alcohol (3.9 percent) in the 30 days prior to the assessment. Multiple explanations of this finding are possible. First, law-enforcement and practical deterrence effects for alcohol- versus cannabis-impaired driving in Canada may be substantially different. Second, cannabis users may generally believe that the impairment effects of cannabis are limited, and frequent users may specifically believe in their ability to control cannabis's effects on driving. Implications for interventions and policy are discussed. (Published Abstract)