Marijuana and Psychomotor Performance


Operating a motor vehicle under the influence of cannabis is a criminal offense in every state, irrespective of cannabis’ legal status under the law.

In assessments of actual on-road driving performance, subjects typically demonstrate only modest changes in psychomotor performance following THC administration

  • “Although laboratory studies have shown that marijuana consumption can affect a person’s response times and motor performance, studies of the impact of marijuana consumption on a driver’s risk of being involved in a crash have produced conflicting results, with some studies finding little or no increased risk of a crash from marijuana usage. Levels of impairment that can be identified in laboratory settings may not have a significant impact in real world settings, where many variables affect the likelihood of a crash occurring.”
  • “Most marijuana-intoxicated drivers show only modest impairments on actual road tests. … Although cognitive studies suggest that cannabis use may lead to unsafe driving, experimental studies have suggested that it can have the opposite effect.”

The combined administration of cannabis and alcohol typically has an additive influence upon psychomotor performance, 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.”

By contrast, THC positive drivers, absent the presence of alcohol, typically possess a low — or even no — risk of motor vehicle accident compared to drug-negative drivers.

  • “We undertook a systematic search of electronic databases, and identified 13 culpability studies and 4 case–control studies from which cannabis-crash odds ratios could be extracted. … Taking the role of study biases into account, we have shown that the best epidemiological evidence concerning the risk of crashing after using cannabis (as indicated by testing positive to THC) is compatible with the null hypothesis that the recent use of cannabis has no effect at all (such that the cannabis-crash OR  =  1.0).”
  • “In this multi-site observational study of non-fatally injured drivers, we found no increase in crash risk, after adjustment for age, sex, and use of other impairing substances, in drivers with THC <5ng/mL. For drivers with THC≥5ng/mL there may be an increased risk of crash responsibility, but this result was statistically non-significant and further study is required. … There was significantly increased risk in drivers who had used alcohol, sedating medications, or recreational drugs other than cannabis. … Our findings … suggest that the impact of cannabis on road safety is relatively small at present time.”
  • “As noted above, even if cannabis impairment is present, it creates (unless combined with alcohol or other drugs) only a fraction of the risks associated with driving at the legal 0.08 BAC threshold, let alone the much higher risks associated with higher levels of alcohol. … The maximum risk for cannabis intoxication alone, unmixed with alcohol or other drugs, appears to be more comparable to risks such as talking on a hands-free cellphone (legal in all states) than to driving with a BAC above 0.08, let alone the rapidly-rising risks at higher BACs.”

By comparison, operating a vehicle with multiple passengers or after consuming even slight amounts of alcohol are behaviors associated with greater risk of motor vehicle accident

Data has not substantiated claims of an uptick in marijuana-induced fatal accidents in states that have regulated the use of cannabis for medical purposes, and some data has identified a decrease in motor vehicle accidents.

  • “We examine the relationship between traffic fatalities and state marijuana laws using data from 1985 through 2019 and Poisson difference in difference models that allow effects to vary over time. … We find lower state traffic fatalities following the implementation of MMLs [medical marijuana laws], consistent with earlier work. This is true whether we employ a simple MML indicator or a continuous indicator of the permissiveness of state medical marijuana laws. … Controlling for prior MMLs, we find no evidence of a statistically significant association between RMLs [recreational marijuana laws] and traffic fatalities. Further, we find no evidence of an association between traffic fatalities and cross-border recreational legalization. … Identifying the effects of RMLs is complex, and the available data is yet limited. The effects of liberalization in other states with different histories, policies, and norms may differ from the effects associated with liberalization. Liberalization may eventually be shown to lead to more fatalities, at least under some sets of circumstances, as more and different states legalize recreational use and more data accrues. However, as of 2019, we find liberalization has been associated with lower traffic fatalities, not higher.”
  • “While attention has been given to how legalization of recreational cannabis affects traffic crash rates, there was been limited research on how cannabis affects pedestrians involved in traffic crashes. This study examined the association between cannabis legalization (medical, recreational use, and recreational sales) and fatal motor vehicle crash rates (both pedestrian-involved and total fatal crashes). … We found no significant differences in pedestrian-involved fatal motor vehicle crashes between legalized cannabis states and control states following medical or recreational cannabis legalization. Washington and Oregon saw immediate decreases in all fatal crashes (-4.15 and -6.60) following medical cannabis legalization. … Overall findings do not suggest an elevated risk of total or pedestrian-involved fatal motor vehicle crashes.”
  • “This paper reports a quasi-experimental evaluation of California’s 1996 medical marijuana law (MML), known as Proposition 215, on statewide motor vehicle fatalities between 1996 and 2015. … We found that legalizing medical marijuana in California led to a sustained reduction in statewide motor vehicle fatalities. … California’s 1996 MML appears to have produced a large, sustained decrease in statewide motor vehicle fatalities amounting to an annual reduction between 588 and 900 vehicle fatalities.”

By contrast, data assessing the potential impact of adult-use legalization access on motor vehicle accidents is more mixed. Initial reports published in the years immediately following legalization consistently showed no change in accident trends, while more recent studies assessing longer time periods report inconsistent findings.

  • “Utilizing provincial emergency department (ED) records (April 1, 2015-December 31, 2019) from Alberta and Ontario, Canada, we employed Seasonal Autoregressive Integrated Moving Average (SARIMA) models to assess associations between Canada’s cannabis legalization (via the Cannabis Act implemented on October 17, 2018) and weekly provincial counts of ICD-10-CA-defined traffic-injury ED presentations. … Implementation of the Cannabis Act was not associated with evidence of significant post-legalization changes in traffic-injury ED visits in Ontario or Alberta among all drivers or youth drivers, in particular.”
  • “We performed a prospective observational study on the use of cannabis and other illicit drugs in the trauma population at a lead Canadian trauma centre in London, Ontario, in the 3 months before (July 1 to Sept. 30, 2018) and 3 months after (Nov. 1, 2018, to Jan. 31, 2019) the legalization of cannabis in Canada. … We found that the rate of positive cannabinoid screen results among patients with trauma referred directly to our trauma service was similar in the 3 months before and [in] the 3 months after the legalization of recreational cannabis in Canada. … In the subgroup of patients whose mechanism of trauma was a motor vehicle collision, there was no difference in the rate of positive toxicology screen results or positive cannabinoid screen results between the two periods. … These preliminary single-centre data showing no increased rates of cannabis use in patients with trauma after legalization are reassuring.”
  • “A retrospective analysis of data collected at trauma centers in Arizona, California, Ohio, Oregon, New Jersey, and Texas between 2006 and 2018 was performed. … The data were analyzed to evaluate the trends in THC and alcohol use in victims of MVC [a motor vehicle crash], related to marijuana legalization. … There did not appear to be a relationship between the legalization of marijuana and the likelihood of finding THC in patients admitted after MVC. … There was no apparent increase in the incidence of driving under the influence of marijuana after legalization.”

Proposed per se thresholds for THC are not evidence-based and may result in inadvertently criminalizing adults who previously consumed cannabis several days earlier but are no longer under the influence