AASHTO Journal - New Study Finds Cannabis Use Impacting Vehicle Crashes

Findings from a new study unveiled at the American College of Surgeons (ACS) Clinical Congress 2025 in Chicago, October 4–7, indicated that over 40 percent of drivers who died in motor vehicle collisions tested positive for active delta-9-tetrahydrocannabinol or THC – the psychoactive chemical ingredient in cannabis or marijuana – in their system, with average blood levels far exceeding those considered to cause impairment.

[Above photo via Wikipedia]

That finding highlights a significant and persistent public health risk that is unchanged by the legalization of recreational cannabis, according to researchers at Wright State University in Dayton, OH, who conducted the study.

“I was surprised to see that level,” said lead author Dr. Akpofure P. Ekeh, a professor of surgery at Wright State, in a statement. “An average level of 30.7 ng/mL [nanograms per milliliter] generally means those people must have consumed marijuana at some time close to driving. This isn’t about residual use; it’s about recent consumption.”

Researchers analyzed coroner records from Montgomery County in Ohio from January 2019 to September 2024, focusing on 246 deceased drivers who were tested for THC following a fatal crash. When autopsies are performed, drug screening is typically part of the process. Furthermore, the study period included the state’s legalization of recreational cannabis in 2023.

Key study findings include:

High prevalence: 103 drivers (41.9 percent) overall tested positive for THC, with yearly rates ranging from 25.7 percent to 48.9 percent.
No effect from legalization: The rate of drivers who tested positive for THC did not change significantly before or after legalization (42.1 percent vs. 45.2 percent), indicating that legal status did not influence the behavior of those who chose to drive after using cannabis.
Consistent over time: The high rate of THC positivity showed no significant change over the six-year study period.

The study notes that blood THC levels are typically drawn by the coroner within hours of death, providing an accurate snapshot of a driver’s state at the time of the crash. Most states that have set legal limits for driving range from 2 to 5 ng/mL – a threshold the average level in this study (30.7 ng/mL) far exceeded.

“The messaging over the last few years has been just the push towards recreational legalization,” Dr. Ekeh noted. “The problem is that from a public health standpoint, there has not been enough emphasis on some of the downsides and the dangers that can occur. People should treat smoking marijuana just like they treat alcohol: don’t smoke and drive.”