Here is a helpful fact sheet regarding Marijuana and Psychomotor performance prepared by the folks at NORML. Some highlights:
Operating a motor vehicle under the influence of cannabis is a criminal offense in every state, even the ones, like Oregon, where it is legal to use for recreational purposes.
In assessments of actual on-road driving performance, subjects typically demonstrate only modest changes in psychomotor performance following THC administration.
The combined administration of cannabis and alcohol, however, has an additive influence, which can lead to significantly reduced performance and increased odds of accident.
- “The present study examined the effects of combinations of smoked cannabis (12.5% THC) and alcohol (target BrAC 0.08%) on simulated driving performance, subjective drug effects, cardiovascular measures, and self-reported perception of driving ability. … Combinations of alcohol and cannabis increased weaving and reaction time, and tended to produce greater subjective effects compared to placebo and the single drug conditions suggesting a potential additive effect. The fact that participants were unaware of this increased effect has important implications for driving safety.”
- “Relative to drivers testing negative for both alcohol and marijuana, the adjusted odds ratios of fatal crash initiation were 5.37 for those testing positive for alcohol and negative for marijuana, 1.62 for those testing positive for marijuana and negative for alcohol, and 6.39 for those testing positive for both alcohol and marijuana.”
- “Relative to drivers who tested negative for both alcohol and marijuana, the estimated odds of fatal crash involvement increased 16 fold for those testing positive for alcohol and negative for marijuana, 1.5 fold for those testing negative for alcohol and positive for marijuana, and over 25 fold for those testing positive for both alcohol and marijuana.”