Operating a motor vehicle under the influence of cannabis is a criminal offense in every state, irrespective of cannabis’ legal status under the law.
Acute cannabis intoxication may influence in a dose-related manner certain psychomotor skills, such as reaction time, necessary to operate a motor vehicle safely. However, these effects tend relatively short-lived and are far less dramatic than changes in psychomotor performance associated with drivers under the influence of alcohol. In studies of either on-road or simulated driving behavior, subjects under the influence of cannabis tend to drive in a more cautious and compensatory manner — such as by reducing speed and engaging in fewer lane changes — while subjects under the influence of alcohol tend to drive more recklessly.
“The purpose of the present pilot study was to investigate the effects of therapeutic cannabis use on simulated driving. It was found that therapeutic cannabis reduced overall mean speed with no effects on straightaway mean speed, straightaway lateral control, or brake latency.”
“Smoked cannabis (12.5 percent THC) led to an acute decrease in speed in young adults. … There was no clear effect of smoked cannabis on lateral control. … There was no evidence of residual effects … over the two days following cannabis administration.”
“The compensatory behavior exhibited by cannabis-influenced drivers distinctly contrasts with an alcohol-induced higher risk behavior, evidenced by greater percent speed.”
“Subjects seemed to be aware of their impairment after THC intake and tried to compensate by driving slower, alcohol seemed to make them overly confident and caused them drive faster than in the control sessions.”
“Experimental research on the effects of cannabis … indicate … that any effects dissipate quickly under one hour. Furthermore, while drivers feel high, they actually tend to compensate for their feelings.”
“THC’s effects differ qualitatively from many other drugs, especially alcohol. For example, subjects drive faster after drinking alcohol and slower after smoking marijuana. … Very importantly, our city driving studied showed that drivers who drank alcohol over-estimated their performance quality whereas those who smoked marijuana under-estimated it. … “[S]ubjects in the marijuana group were not only aware of their intoxicated condition, but were … attempting to compensate for it. These … findings … support … the common belief that drivers become overconfident after drinking alcohol and … that they become more cautious and self-critical after consuming low doses of THC, as smoked marijuana.”
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
“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.”
“When alcohol and THC were combined the odds of an unsafe driving action increased by approximately 8 to 10 percent for each 0.01 BAC unit increase over alcohol or THC alone.”
“Driver culpability exhibited the expected positive association with alcohol use (OR 13.7) and with combined alcohol and cannabis use (OR 6.9,). There was only a weak positive association between cannabis use (with no other drug) and culpability (OR 1.3).”
“The effect of ethanol and cannabis taken simultaneously is additive. … When THC and ethanol were detected together the risk of being judged impaired was increased markedly.”
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.”
“As a whole, the evidence from the 13 sets of no-alcohol culpability study counts imply that the raised crash risks associated with cannabis are low on average for drivers with THC-values above typical study thresholds.”
“The primary objective of this study was to analyse whether there is a significant association between driving under the influence of cannabis and unfavorable traffic events. … 24 studies were included in the meta-analysis. … Our analysis suggests that the overall effect size for driving under the influence of cannabis on unfavorable traffic events is not statistically significant.”
“[O]ur study focuses on the contribution of cannabis to motor vehicle crashes among drivers at zero or low BACs. Our study indicates that, albeit marginally, cannabis contributes to fatal crash responsibility in the absence of alcohol.”
“Acute cannabis intoxication is associated with a statistically significant increase in motor vehicle crash risk. The increase is of low to medium magnitude (OR between 1.2 and 1.4).”
“For both sober and drinking drivers, being positive for a drug was found to increase the risk of being fatally injured. When the drug-positive variable was separated into marijuana and other drugs, only the latter was found to contribute significantly to crash risk.”
“Summary estimates of relative risk of accident involvement associated with the use of various drugs. Based on meta-analysis: Cannabis and auto injury: best estimate adjusted for publication bias, OR = 1.10”
“The highest risk of the driver being severely injured was associated with driving positive for high concentrations of alcohol (≥0.8 g/L), alone or in combination with other psychoactive substances. For alcohol, risk increased exponentially with blood alcohol concentration (BAC). The second most risky category contained various drug-drug combinations, amphetamines and medicinal opioids. Medium increased risk was associated with medium sized BACs (at or above 0.5 g/L, below 0.8 g/L) and benzoylecgonine. The least risky drug seemed to be cannabis and benzodiazepines and Z-drugs.”
“The study concludes that drug use, especially alcohol, benzodiazepines and multiple drug use and drug–alcohol combinations, among vehicle drivers increases the risk for a road trauma accident requiring hospitalization. … No increased risk for road trauma was found for drivers exposed to cannabis.”
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
Drivers with two or more passengers in the car possess a crash risk of more than two-fold (OR=2.2).
Driving with BAC levels .05 and .08 are more than six times more likely (OR=6.40) than of a sober driver to be responsible for a fatal motor vehicle accident
“Even minimally ‘buzzed’ drivers (BAC=0.01 percent) are 46 percent more likely to be officially blamed for a crash than are the sober drivers they collide with.”
“[Tobacco] smokers had a 1.5-fold increase in risk for motor vehicle crash over non-smokers. Also, an increased tendency to smoke while driving was associated with greater risk of motor vehicle crash.”
“Significant increased risk of motor vehicle accidents was found in subjects taking antidepressants within 1 month (adjusted odds ratio (AOR) 1.73, 95% confidence interval (CI) 1.34, 2.22), 1 week (AOR 1.71, 95% CI 1.29, 2.26), and 1 day (AOR 1.70, 95% CI 1.26, 2.29) before MVAs occurred.”
“[O]verall odds for collision involvement in the preceding year among current [tobacco cigarette] smokers for 2002-2014 was 1.27 times that of nonsmokers.”
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.
“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.”
“[O]n average, medical marijuana law states had lower traffic fatality rates than non-MML states. …. Medical marijuana laws are associated with reductions in traffic fatalities, particularly pronounced among those aged 25 to 44 years. … It is possible that this is related to lower alcohol-impaired driving behavior in MML-states.”
“State-specific estimates indicated a reduction in opioid positivity for most states after implementation of an operational MML. … Operational MMLs are associated with reductions in opioid positivity among 21- to 40-year-old fatally injured drivers and may reduce opioid use and overdose.”
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.
“[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.”
“In the five years after legalization, fatal crash rates increased more in Colorado and Washington than would be expected had they continued to parallel crash rates in the control states (+1.2 crashes/billion vehicle miles traveled, but not significantly so. The effect was more pronounced and statistically significant after the opening of commercial dispensaries… [This finding]… stands in contrast to earlier studies finding decreases in traffic fatalities following medical marijuana legalization. … [T]hese unexpected findings raise the possibility that legalization of medical and recreational marijuana represent two distinct policy exposures rather than ‘escalating doses’ of the same exposure and pose very different risks. This is an area in need of further study.”
“We find that states that legalized marijuana have not experienced significantly different rates of marijuana- or alcohol-related traffic fatalities relative to their synthetic controls. … In summary, the similar trajectory of traffic fatalities in Washington and Colorado relative to their synthetic control counterparts yield little evidence that the total rate of traffic fatalities has increased significantly as a consequence of recreational marijuana legalization.”
“We found no significant association between recreational marijuana legalization in Washington and Colorado and subsequent changes in motor vehicle crash fatality rates in the first three years after recreational marijuana legalization. … [W]e also found no association between recreational marijuana legalization and total crash rates when analyzing available state-reported nonfatal crash statistics. … Post–recreational marijuana legalization changes in motor vehicle crash fatality rates for Washington and Colorado also did not significantly differ from those for the control states.”
“We (the state of Colorado) have not experienced any significant issue as a result of legalization. … We have actually seen an overall decrease in DUI’s since legalization. So, the short answer is: There has been no increase since the legalization of marijuana here.”
“In Colorado, despite limited traffic data, the Department of Public Safety reports the following: [T]he number of summons issued for Driving Under the Influence [DUI] in which marijuana or marijuana-in-combination[ 138] with other drugs [was recorded] decreased 1% between 2014 and 2015 (674 to 665).”
“In monitoring the impacts of recreational marijuana legalization in Washington State, government researchers report that there was no trend identified in the percentage of drivers testing positive for marijuana (either marijuana only or marijuana in combination with other drugs/alcohol) for those involved in traffic fatalities and who were tested for drugs or alcohol
“[Marijuana] legalization is associated with a nearly nine percent decrease in traffic fatalities, most likely to due to its impact on alcohol consumption.”
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
“[E]pidemiological evidence supporting a specific per se limit for THC is scant. … Blood THC >2 ng/mL, and possibly even THC >5 ng/mL, does not necessarily represent recent use of cannabis in frequent cannabis users.”
“[T]he relationship between crash risk and the amount of cannabis consumed or the blood concentrations of THC is weak. … [B]lood concentration of THC is a poor index of driving-related risk or impairment.”
“[B]lood and other fluid levels of various cannabinoids correlate poorly with impairment. A person can have detectable cannabinoids, such as THC, in the blood or oral fluid, but not be cognitively impaired, since these levels may reflect on-going, low-level use, such as in medicinal cases, or a single use that occurred many hours or even days previously.”
“Research studies have been unable to consistently correlate levels of marijuana consumption, or THC in a person’s body, and levels of impairment. Thus some researchers, and the National Highway Traffic Safety Administration, have observed that using a measure of THC as evidence of a driver’s impairment is not supported by scientific evidence to date.”
“[B]ecause there is a poor correlation between ∆9-THC bodily content and driving impairment, the Commission recommends against the establishment of a threshold of delta-9-THC bodily content for determining driving impairment.”
“To contribute to the ongoing discussion about threshold limits of Δ9-tetrahydrocannabinol (THC) in road traffic, a driving simulator study with 15 habitually cannabis consuming test persons was conducted. … Consistent with previous studies, a direct correlation between the individual fitness to drive (amount of penalty points) and the THC concentrations … was not found. Therefore, determining a threshold limit for legal purposes based on these values alone seems to be arbitrary.”
“[B]lood THC levels drop very sharply over time-periods measured in minutes. Blood THC is not a good proxy either for recency of use or for impairment, and the dose-effect curve for fatality risk remains a matter of sharp controversy. … Moreover, the lipid-solubility of THC means that a frequent cannabis user will always have measurable THC in his or her blood, even when that person has not used recently and is neither subjectively intoxicated nor objectively impaired.”
“It is difficult to establish a relationship between a person’s THC blood or plasma concentration and performance impairing effects. … It is inadvisable to try and predict effects based on blood THC concentrations alone, and currently impossible to predict specific effects based on THC-COOH (metabolite) concentrations.”
“The interpretation of cannabinoid effects is even more difficult than identifying the presence or concentration of natural or synthetic cannabinoid markers in a diverse array of biological samples. Interpretation is complex because the onset, peak, and duration of effects are different based on whether the route of cannabis administration is inhalation, oral, or rectal, and on whether the individual is an occasional or chronic frequent cannabis users. …. Currently, science does not support the development of cannabinoid limits per se in motor vehicles drivers because of the many factors influencing concentration–effect relationships.”
“There is no one blood or oral fluid concentration that can differentiate impaired and not impaired. It’s not like we need to say, ‘Oh, let’s do some more research and give you an answer.’ We already know. We’ve done the research.”
“The alcohol laws are based on evidence concerning the decreased ability of drivers across the population to function safely at these BACs. … Such evidence is not currently available for concentrations of other drugs.”
“Individuals can vary widely in their sensitivity to THC induced impairment as evinced by the weak correlations between THC in serum and magnitude of performance impairment.”
“One of the program’s objectives was to determine whether it is possible to predict driving impairment by plasma concentrations of THC and/or its metabolite, THC-COOH, in single samples. The answer is very clear: it is not. Plasma of drivers showing substantial impairment in these studies contained both high and low THC concentrations; and, drivers with high plasma concentrations showed substantial, but also no impairment, and even some improvement.”
