Cannabis and Driving: What Studies Really Show

A study¹ from Ohio has reignited the conversation around cannabis, driving, and impairment. Researchers analyzed blood samples from hundreds of fatal crashes and discovered that nearly half of the deceased drivers tested positive for THC. The study’s authors highlight the need for stronger public messaging, warning drivers about the dangers of combining cannabis with driving. The research was led by trauma specialists affiliated with the American College of Surgeons and Wright State University.

But while the study underscores the importance of safe and responsible use, it also raises a deeper question: Does the mere presence of THC in the bloodstream truly indicate impairment?

What the Study Found

The study examined 246 fatal motor vehicle crashes in Montgomery County, Ohio, between 2019 and 2024. Researchers found that 41.9% of the deceased drivers tested positive for THC, with an average active concentration of 30.7 nanograms per milliliter (ng/mL).

Interestingly, those rates remained consistent both before and after the state legalized recreational cannabis.
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The Limits of Blood THC Testing

The study raises concerns about how such data are interpreted in the public and legal spheres. Just because THC is present in someone’s system, especially after death, doesn’t necessarily mean the person was impaired at the time of the crash.

Cannabis impairment doesn’t follow the same rules as alcohol. With alcohol, blood concentration and impairment correlate closely. A 0.08% BAC is well established as a point where coordination and reaction time are significantly reduced.

THC, however, is far more complex. It’s fat-soluble, meaning it lingers in the body long after consumption. Medical patients and frequent users can maintain measurable THC levels in their blood for days, weeks, or even months after use, without any psychoactive effect.

When we test for THC, what we find is presence, not necessarily impairment. And for medical cannabis patients who rely on daily or frequent use, THC concentrations are consistently higher than those of occasional users. That doesn’t make them unsafe drivers; it makes them consistent users with different body chemistries.

In fact, research has repeatedly shown that:

• THC levels in the blood do not consistently correlate with driving performance.
• Chronic users often show high THC concentrations long after their last use.
• Impairment is short-lived, often lasting only a few hours after consumption.
• Postmortem THC levels can sometimes appear higher than when the person was alive, depending on how and when the sample was collected. This process, known as postmortem redistribution, makes it difficult to determine true impairment.

That last point matters greatly. The Ohio study was based on postmortem samples, which can’t account for the exact time of use, metabolism rates, or behavioral signs of impairment. In other words, it’s scientifically impossible to determine whether THC contributed to a crash just because it’s present in someone’s system.

Why THC Driving Limits Miss the Mark

Despite these complexities, most U.S. states rely on per se THC limits, typically between 2 and 5 ng/mL, to determine legal impairment. If a driver’s blood exceeds that limit, they can be prosecuted for driving under the influence, even if no signs of actual impairment are observed.

These laws were created to promote road safety, but they fail to reflect how cannabis works in the human body. Someone could test at 10 or even 30 ng/mL long after their last use and still be perfectly sober. Meanwhile, another person with 2 ng/mL could be visibly impaired.

That’s why relying solely on nanogram limits is both scientifically inaccurate and legally unfair.

For medical patients, veterans, and individuals who use cannabis regularly, these limits create a two-tiered system of justice. One where biological tolerance is ignored, and guilt is assumed by number alone.

As cannabis policy continues to evolve, experts and advocates alike are calling for impairment testing that measures function, not residual chemistry.

Smarter Cannabis DUI Policy

Advocates argue it’s time to rethink how we approach cannabis and driving. Instead of relying on arbitrary numbers, we should:

• Develop real impairment assessments, such as eye-tracking, cognitive testing, or reaction-based evaluations to measure actual ability.
• Acknowledge tolerance differences as medical and frequent users can test at much higher than the legal limit and are not impaired.
• Invest in more research, including large-scale driving studies that analyze both THC levels and performance metrics with a wide variety of consumers, not just blood concentration.

Understanding Cannabis Data Responsibly

It’s also important to remember that studies like this one involve people who have lost their lives. To assume they were “high” or “impaired” based solely on a number is not only scientifically unsound but deeply unfair.

Every person’s relationship with cannabis is unique. The law and researchers must evolve to reflect that complexity, not punish it.

Conclusion: Evidence Over Assumptions

The Ohio researchers are right to call for stronger education around driving while impaired by cannabis. But it’s equally important that we do not conflate presence with impairment.

Cannabis policy should be guided by evidence, not assumptions. Research continuously shows that THC levels alone cannot define who is impaired and who is not until DUI laws reflect that reality, patients and responsible consumers may continue facing legal risks for THC levels that science shows do not reliably indicate impairment.

Proper safety and justice demand more than arbitrary limits; they demand understanding. Because when it comes to cannabis and driving, the question is not how much THC is in your blood; it’s if you are impaired while driving.

References:

^{1. Ekeh A, et al. Cannabis Prevalence in Drivers Involved in Motor Vehicle Crash Fatalities over a 6-Year Period, Scientific Forum, American College of Surgeons (ACS) Clinical Congress 2025.}