Marijuana use alone is not associated with higher odds of car accidents, according to a new study by researchers who looked at drivers who visited emergency departments. In fact, high self-reported acute cannabis use was actually associated with lower odds of a crash.
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Alcohol, meanwhile—whether used by itself or combined with marijuana—showed a clear correlation with odds of a collision.
To arrive at the results, researchers gathered data from emergency departments in Denver, Colorado; Portland, Oregon; and Sacramento, California. They obtained drivers’ blood and measured it for THC and metabolites, recorded alcohol levels as measured by a breathalyzer or in the course of clinical care and conducted interviews with the drivers.
While most legalization advocates do not dispute that marijuana can impair a driver’s ability to safely operate a car, the new study found that the mere use of cannabis did not correlate to higher rates of motor vehicle collisions (MVCs).
“Cannabis alone was not associated with higher odds of MVC, while acute alcohol use alone, and combined use of alcohol and cannabis were both independently associated with higher odds of MVC,” authors wrote.
Strikingly, drivers who used more marijuana were actually less likely to crash, according to researchers’ analysis.
“Stratifying by level of self-reported or measured cannabis use, higher levels were not associated with higher odds for MVC, with or without co-use of alcohol,” they wrote. “In fact, high self-reported acute cannabis use was associated with lower odds of MVC.”
In light of the results, the nine-author research team concluded that THC levels are a less-than-reliable indicator of driving risk, suggesting that a better test would be to measure actual impairment.
“Alcohol use alone or in conjunction with cannabis was consistently associated with higer [sic] odds for MVC. However, the relationship between measured levels of cannabis and MVC was not as clear,” the study says. “Emphasis on actual driving behaviors and clinical signs of intoxication to determine driving under the influence has the strongest rationale.”
As for per se DUI limits on THC, the study says that using “strict cut-offs of drug levels to gauge the influence of cannabis use on driving remains complex from a scientific and legal perspective, as the implication of measured levels are complicated by usual use, time and means of measurement, and regular cannabis use patterns.”
Authors noted that one limit of the study could be that it only included drivers who agreed to participate. As such, participants “may have less concerning drug use behaviors, particularly those related to events such as MVC where they may be concerned about being at fault.”
Self-reported use might also be biased “in favor of a weaker relationship” between cannabis and car crashes, they said.
Authors of the study, published in the April 2024 issue of the journal Accident Analysis and Prevention, represented a range of institutions, including the Oregon Health and Science University, the University of Colorado School of Medicine, University of California Davis, the Insurance Institute for Highway Safety (which also funded the study), Portland State University and others.
As more states have considered legalizing marijuana in recent years, many have expressed concerns that the policy change could lead to higher rates of use by drivers and, in turn, greater risk to public safety. But research shows the relationship between cannabis use and impaired driving isn’t as simple as it might seem.
A study published in 2019, for example, concluded that those who drive at the legal THC limit—which is typically between two to five nanograms of THC per milliliter of blood—were statistically no more likely to be involved in an accident than people who hadn’t used marijuana.
Adding to the complexity is the difficulty of accurately testing drivers. Last summer, a congressional report for a Transportation, Housing and Urban Development, and Related Agencies (THUD) bill said that the House Appropriations Committee “continues to support the development of an objective standard to measure marijuana impairment and a related field sobriety test to ensure highway safety.”
In 2022, Sen. John Hickenlooper (D-CO) sent a letter to the Department of Transportation (DOT) seeking an update on that status of a federal report into research barriers that are inhibiting the development of a standardized test for marijuana impairment on the roads. The department was required to complete the report under a large-scale infrastructure bill that President Joe Biden signed, but it missed a deadline and wasn’t unclear how much longer the project will take.
Earlier this month, scientists said they’ve identified an alternative way to test for recent marijuana use that’s significantly more accurate than standard THC blood tests and are actively working to build on that research.
In a prior study, the researchers also assessed driving ability during a simulation, and notably they found that daily cannabis consumers had an average five times the THC concentration in their blood after the 30-minute mark compared to occasional users—yet the latter group “showed evidence of decrement in their driving skills, whereas that wasn’t statistically significant in the daily users.”
The Congressional Research Service determined in 2019 that while “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.”
Another study from 2022 found that smoking CBD-rich marijuana had “no significant impact” on driving ability, despite the fact that all study participants exceeded the per se limit for THC in their blood.
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