Marijuana and Psychomotor Performance


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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.

  • “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.

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.”
  • “[The] implementation of recreational cannabis laws was associated with increases in traffic fatalities in Colorado but not in Washington state. … Findings suggest that adverse unintended effects of recreational cannabis laws can be heterogeneous and may depend on variations in implementation of these laws (e.g., density of recreational cannabis stores).”
  • “Following the recent release of 2018 roadway fatality re- ports by the US Department of Transportation, we analyzed data from more states over a longer period of commercial sales to get a better understanding of the relationship between legalization of recreational marijuana and traffic fatalities. … By analyzing additional experimental states over a more recent time period, we have provided additional data that legalization of recreational marijuana is associated with in- creased traffic fatality rates. … Our conclusions, nonetheless, are limited by adjusting for only 3 state-specific factors that may have changed during the study period. It is possible that another confounder, rather than marijuana legalization and commercialization, caused the observed increase in roadway deaths.”

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