The role of illicit drugs on driving, and particularly of cannabis and driving, is the object of increasing awareness. While there is increasing evidence of their effect on psychomotor performance and increased risk of involvement in traffic accidents, limited information is available concerning factors that can predict the likelihood of driving under the influence of cannabis. The present study aims to determine the past year prevalence of driving under the influence of cannabis, and of being a passenger in a vehicle driven by a person under the influence of cannabis, as well as to examine the correlations with a broad range of potential risk factors. A total of 2500 people, aged between 14 and 70 and living in Castille and Leon (Spain), were surveyed in 2004 with regard to their consumption of alcohol and illicit drugs. Among those who reported cannabis use in the previous year, further assessment was carried out. 15.7% of those surveyed reported cannabis consumption in the previous 12 months, of whom 9.7% reported driving a vehicle under the influence of cannabis during this period, on average eight times. One out of five (19.9%) reported being a passenger in a vehicle driven by a person under the influence of cannabis, on average five times in the previous 12 months. The predictors of driving under the influence of cannabis were the population size of community, the number of drugs consumed, reference to cannabis-related problems and to being a passenger in a vehicle driven by a person under the influence of alcohol. The data show that cannabis consumption and driving is common, and requires more attention from policy makers.