Over 40% of Drivers Killed in Crashes Had THC in Their Blood — What the New Study Reveals About Cannabis and Driving

A new study presented at the American College of Surgeons (ACS) Clinical Congress 2025 has uncovered a deeply concerning pattern: more than 40% of drivers who died in car crashes in a single Ohio county had active THC — the main psychoactive component in cannabis — in their bloodstream at the time of death. The findings have reignited serious discussions about marijuana use and road safety, especially in the wake of widespread legalization across the United States.

The Study and Its Scope

Researchers from Wright State University and the Miami Valley Hospital in Dayton, Ohio, examined coroner records from Montgomery County covering the period January 2019 through September 2024. Their goal was straightforward: to analyze toxicology data from 246 drivers who were killed in motor vehicle crashes and determine how many had active THC in their blood at the time of death.

This six-year window also happened to include a major policy shift — the legalization of recreational marijuana in Ohio in 2023. This allowed the researchers to compare THC prevalence in fatal crashes before and after legalization to see whether changing laws influenced behavior on the roads.

Startling Findings

The numbers were striking. Out of the 246 deceased drivers studied, 41.9% tested positive for THC. That’s nearly half of all fatal crash victims showing evidence of recent cannabis use.

Even more concerning were the THC levels themselves. The average blood concentration was 30.7 nanograms per milliliter (ng/mL) — a figure that far exceeds the legal impairment threshold in most states, which ranges from 2 to 5 ng/mL.

To put that into perspective, someone at 30 ng/mL has likely consumed cannabis very recently, possibly within an hour or two before driving. These aren’t leftover traces from smoking days ago; they represent active intoxication levels.

When looking year by year, THC positivity rates fluctuated between 25.7% and 48.9%, showing consistent prevalence throughout the study. Crucially, the researchers found no significant difference in THC positivity before and after legalization — 42.1% versus 45.2%. In other words, legalization didn’t appear to increase or decrease the number of drivers under the influence. The problem simply persisted.

What the Numbers Mean

This data paints a sobering picture. Despite growing public acceptance of cannabis and the increasing number of states where it’s legal, the risk of driving while impaired remains both real and widespread.

According to the lead researcher, Dr. Akpofure P. Ekeh, a trauma surgeon and professor at Wright State University, the consistently high THC levels suggest many of these drivers had recently consumed cannabis — not hours or days earlier. Blood samples were drawn by coroners within hours of death, providing a close snapshot of impairment at the time of the crash.

Importantly, THC affects cognitive and motor functions essential for safe driving — such as reaction time, attention, and coordination. Even modest doses can impair judgment and slow reflexes. Studies have repeatedly shown that driving under the influence of cannabis increases crash risk, especially when combined with other substances like alcohol.

Legalization Didn’t Change the Behavior

One of the study’s most noteworthy findings is that legalization had no measurable effect on the proportion of fatal crash drivers testing positive for THC.

This challenges both sides of the legalization debate. Supporters often argue that regulation reduces illegal use and encourages safer behavior, while critics fear it leads to more drugged driving. Yet this research suggests the reality is more complex — the rates remained stable, regardless of whether cannabis was legal or not.

This could mean that legalization alone doesn’t automatically alter risky habits. People who choose to drive high may not be influenced by legality; instead, it might require stronger public awareness and behavioral change campaigns.

Beyond the Numbers: A Public Health Concern

The researchers didn’t stop at statistics. They emphasized that while marijuana use is becoming normalized, public education has not kept pace with legalization. Campaigns around cannabis have focused heavily on its medical and economic benefits, but not nearly enough on its dangers when combined with driving.

In states where alcohol is legal, decades of education and strict enforcement have helped shape attitudes toward drunk driving. However, similar anti-drugged-driving messaging has lagged behind for cannabis. The public tends to underestimate its impairing effects — often assuming it’s “safer than alcohol” — but science tells a different story.

How THC Affects Driving Ability

THC, or delta-9-tetrahydrocannabinol, interacts with the brain’s cannabinoid receptors, altering perception, coordination, and memory. Depending on the dose, method of consumption, and user tolerance, these effects can last from two to six hours or longer.

Unlike alcohol, the relationship between blood concentration and impairment isn’t perfectly linear — meaning a specific THC number doesn’t correspond neatly to a certain level of intoxication. However, the 30.7 ng/mL average found in this study is well above any threshold considered safe.

Even at lower doses, THC can:

• Reduce reaction time, making it harder to respond to sudden hazards.
• Distort perception of time and speed, leading drivers to misjudge distances.
• Decrease coordination, affecting steering and lane control.
• Induce drowsiness or distraction, particularly with prolonged use or edibles.

When combined with alcohol or other drugs, the effects multiply — producing severe impairment that’s much greater than either substance alone.

The Enforcement Challenge

One of the most difficult aspects of combating drugged driving is enforcement. Unlike alcohol, where a breathalyzer provides immediate results, THC testing requires blood or saliva analysis, often in a lab.

Moreover, THC can linger in the bloodstream long after impairment has faded, complicating the legal process. This makes it challenging for police to establish clear, real-time evidence of intoxication.

Some states have implemented per se limits — meaning any driver above a certain THC concentration is considered impaired, similar to blood alcohol concentration laws. But not all states agree on these limits, and the lack of uniformity can lead to confusion in courts and among drivers.

The Ohio Study in a National Context

This study, although limited to one county in Ohio, reflects broader national concerns. Across the United States, fatal crashes involving drug-positive drivers have been steadily increasing.

According to previous research by the National Highway Traffic Safety Administration (NHTSA), the number of drivers testing positive for drugs (including cannabis, opioids, and stimulants) in fatal crashes has more than doubled over the past two decades. Cannabis in particular has become the most frequently detected drug after alcohol.

What makes the Ohio study stand out is its detailed measurement of THC concentration levels, not just presence. Many previous studies only reported whether THC was detected at all — not how much. By quantifying the average levels, the researchers highlight how many victims were far beyond any plausible “residual” level, underscoring the seriousness of active impairment.

Limitations of the Study

As with any research, this one has limitations worth noting.

• Geographic Scope: The study only covered Montgomery County, so its findings may not apply to other regions with different demographics or traffic patterns.
• Population Bias: Only deceased drivers were included — it doesn’t reflect how many impaired drivers survive crashes or avoid accidents entirely.
• Causation vs. Correlation: The presence of THC doesn’t definitively prove that cannabis caused the crash. Other factors, such as fatigue, distraction, weather, or additional substances, may have played roles.
• Peer Review Status: The research was presented as a conference abstract, not yet a peer-reviewed journal publication, meaning further validation is needed.

Still, the data are alarming enough to warrant national attention and policy discussion.

The Road Ahead: Education and Responsibility

The takeaway is clear: Driving under the influence of cannabis remains dangerous, regardless of its legal status. As marijuana use becomes more common, both recreationally and medically, public health authorities need to double down on education and prevention.

Just as campaigns like “Don’t Drink and Drive” reshaped public attitudes toward alcohol, similar efforts are urgently needed for drugged driving. This includes public service announcements, stricter enforcement, and more consistent messaging from health professionals.

Ultimately, legalization and safety must go hand in hand. Cannabis may now be legal in many places, but that doesn’t make it harmless — especially when you’re behind the wheel.

Understanding THC Testing and Limits

For readers curious about the numbers: most U.S. states set legal THC driving limits between 2 and 5 ng/mL, although there’s ongoing debate about how accurate these thresholds are. Some experts argue that a single standard is misleading because THC’s effects vary by individual and method of use (smoking, vaping, edibles, etc.).

Still, when toxicology results show averages above 30 ng/mL, as seen in this Ohio study, there’s little doubt about active impairment. That concentration level suggests recent, heavy use — often within an hour or two of the crash.

Broader Public Discussion

The study has sparked renewed conversation about how society balances personal freedom and public safety in the age of cannabis legalization. It challenges both policymakers and consumers to acknowledge that “legal” doesn’t mean “safe in all contexts.”

While cannabis has legitimate therapeutic benefits for many users, its impairing effects on driving are undeniable. Recognizing this doesn’t mean opposing legalization — it means promoting responsible use and protecting others on the road.

Research Reference:
American College of Surgeons Clinical Congress 2025 – “Cannabis Prevalence in Drivers Involved in Motor Vehicle Crash Fatalities over a 6-Year Period”