A new study conducted by researchers who examined drivers that visited emergency departments has found that marijuana use alone is not associated with higher odds of car accidents. In fact, the study revealed that high self-reported acute cannabis use was actually linked to lower odds of a crash. On the other hand, alcohol use, whether used alone or combined with marijuana, showed a clear correlation with the likelihood of a collision.
The researchers collected data from emergency departments in Denver, Colorado; Portland, Oregon; and Sacramento, California. They measured drivers’ blood for THC and metabolites, recorded alcohol levels through breathalyzer tests or clinical care, and conducted interviews with the drivers.
While most advocates for marijuana legalization acknowledge that marijuana can impair a driver’s ability to operate a car safely, this new study found that the mere use of cannabis did not correlate with higher rates of motor vehicle collisions (MVCs). The authors of the study stated that “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.”
Interestingly, the analysis conducted by the researchers showed that drivers who used more marijuana were actually less likely to be involved in car accidents. The study concluded that THC levels are not a reliable indicator of driving risk and suggested that measuring actual impairment would be a better test.
The study further highlighted the complexity surrounding per se DUI limits on THC. Using strict cut-offs to determine the influence of cannabis use on driving proves challenging from both a scientific and legal perspective due to factors such as regular cannabis use patterns, time and means of measurement, and usual use.
One limitation of the study is that it only included drivers who agreed to participate. Participants may have had less concerning drug use behaviors when it came to events like motor vehicle collisions where they might be concerned about being at fault. Additionally, self-reported use could be biased in favor of a weaker relationship between cannabis use and car crashes.
The study, published in the April 2024 issue of the journal Accident Analysis and Prevention, involved researchers from various 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), and Portland State University.
As more states consider legalizing marijuana, concerns have been raised about higher rates of marijuana use by drivers and potential risks to public safety. However, research shows that the relationship between cannabis use and impaired driving is not as straightforward as it may seem.
A study from 2019 found that individuals who drive at the legal THC limit are statistically no more likely to be involved in an accident than individuals who haven’t used marijuana. Furthermore, accurately testing drivers for impairment is a challenge. Efforts are being made to develop an objective standard to measure marijuana impairment and a related field sobriety test to ensure highway safety.
In conclusion, this latest study adds to a growing body of research that suggests marijuana use alone does not increase the risk of car accidents. On the other hand, drinking alcohol has been shown to have a clear correlation with the likelihood of a collision. The complexity surrounding THC levels as an indicator of driving risk highlights the need for a focus on actual driving behaviors and clinical signs of intoxication when determining if a driver is under the influence.
