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.
"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
"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."
"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)."
"Adjusted odds ratios between drug class use and crash risk, adjusted for demographic variables: age, gender and race/ethnicity: THC = 1.05"
US National Highway Traffic Safety Administration, Drug and Alcohol Crash Risk, 2015
"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."
"[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."
Data has not substantiated claims of an alleged uptick in marijuana-induced fatal accidents in states that have regulated the use of cannabis for either medical or recreational purposes. In fact, some studies have identified a decrease in motor vehicle accidents following legalization
In 1996, NHTSA reports that there were an estimated 37,500 fatal car crashes on US roadways. But by 2014, during which time a majority of states legalized medical cannabis and several others passed laws either reducing or eliminating marijuana-related adult use penalties, this total fell to just under 30,000.
"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."
"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."
"[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."
"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
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
"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 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."
"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."