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

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.

  • “Consistent with an improvement in traffic safety, we find that the legalization of medical cannabis leads to a decrease in auto insurance premiums on average of $22 per policy per year. The effect is stronger in areas directly exposed to a dispensary, suggesting increased access to cannabis drives the results. In addition, we find relatively large declines in premiums in areas with relatively high drunk driving rates prior to medical cannabis legalization. This latter result is consistent with substitutabil-ity across substances that is argued in the literature.”
  • “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

  • RESOURCES: Marijuana Use and Highway Safety, Congressional Research Service, 2019 | Should per se limits be imposed for cannabis? Equating cannabinoid blood concentrations with actual driver impairment: practical limitations and concerns, Humboldt Journal of Social Relations, 2013 | Imposing per se limits for cannabis: Practical limitations and concerns, 2013 | Cannabis and psychomotor performance: A rational review of the evidence and implications for public policy, Drug Testing & Analysis, 2012 | Cannabis and Driving: A Scientific and Rational Review, 2011
  • “Previous studies have failed to demonstrate a clear relationship between impairment and speci c concentrations of ∆9-THC in blood or oral fluid. In agreement with these studies, the results of the present work showed that a majority of a group of 30 test subjects had pre-smoking ∆9-THC blood concentrations that exceeded the legal limits currently in place in five U.S. states (Illinois, Montana, Ohio, Nevada, and Washington), in the absence of impairment. The results also showed that post-smoking duration of impairment appeared to be inversely related to baseline blood ∆9-THC concentrations. … These findings provide further evidence that single measurements of specific ∆9-THC blood concentrations do not correlate with impairment, and that the use of per se legal limits for ∆9-THC is not scientifically justifiable at the present time.”
  • “Within a sample of nearly 200 regular cannabis users instructed to smoke cannabis as they do at home to achieve a usual level of intoxication, the aims of this study were to determine, with respect to driving outcomes, the (1) magnitude and time course of effects, (2) effect of cannabis with different THC amounts, (3) possible tolerance effects, and (4) accuracy of self-perception of impairment. … There was no correlation between blood THC concentrations collected 15 minutes after smoking and simulator performance at 30 minutes or any other time point even under our highly controlled conditions. In the real world, the time from consumption to a law enforcement stop and subsequent blood collections is highly variable, and the current results reinforce that per se laws based on blood THC concentrations are not supported.”
  • “Current breath and body fluid mechanisms used for cannabis testing fail to have the same degree of scientific support as alcohol. … Oral fluids and breathalyzers may have a place in the toolbox of law enforcement to identify a history of cannabis use. However, they are unreliable indicators of THC- related driving impairment and impose significant threats to civil liberty. The per se legal standard for cannabis intoxication while driving promotes the use of unreliable testing methods, and the zero-tolerance standard is inconsistent with the trend toward legalization, both of which will likely lead to over-prosecution.”
  • “It is difficult to connect the presence of marijuana or concentration of tetrahydrocannabinol (THC), the compound responsible for marijuana’s psychoactive properties (the “high”), to impairment in driving performance for an individual person.”
  • “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. … It is evident that the risks from driving after using cannabis are much lower than from other behaviors such as drink-driving, speeding or using mobile phones while driving. With the medical and recreational use of cannabis becoming more prevalent, the removal of cannabis-presence driving offences should be considered (while impairment-based offences would remain).”
  • “The current investigation used meta-analytic techniques to characterize the relationships between THC-related biomarkers, subjective intoxication, and impairment of driving and driving-related cognitive skills in regular and occasional cannabis users. Results indicate that blood THC, 11− OH-THC and 11− COOH-THC concentrations, oral fluid THC concentrations, and subjective ratings of intoxication are relatively poor indicators of cannabis-induced impairment. The use of per se limits as a means of identifying cannabis-impaired drivers should therefore be re-considered. Indeed, it seems there is a significant risk of unimpaired individuals being mistakenly identified as ‘cannabis-impaired’ (and vice-versa) under this approach.”
  • “In summary, current evidence from the above studies suggests that efforts to establish per se limits for cannabis-impaired drivers based on blood THC values are still premature at this time. Considerably more evidence is needed before we can have an equivalent ‘BAC for THC.’ The particular pharmacokinetics of cannabis and its variable impairing effects on driving ability currently seem to argue that defining a standardized per se limit for THC will be a very difficult goal to achieve.”
  • “Current research … indicates that biological THC concentrations are not strongly correlated with impairment, so per se laws that criminalize driving above specific thresholds do not appear to be justified as stand-alone policy.”