As Virginia weighs whether to move forward with a legal retail cannabis market as early as 2027, officials and researchers said one question remains unsettled: how legalization may affect impaired driving — and whether current data can offer answers.
A 2024 survey conducted by the Virginia Cannabis Control Authority found that roughly 30% of drivers believe they are safer behind the wheel after using cannabis.
“That statistic is alarming,” said Jamie Patton, the agency’s chief administrative officer. “It just further reinforces the need for a safe driving campaign… they are not a safer driver while under the influence.”
Patton said the agency is focusing on public education to discourage people from driving after using cannabis, including campaigns at festivals, college campuses and community events.
But researchers said how cannabis affects driving, and how often it contributes to crashes, is still difficult to measure.
JM Pedini, executive director at the Virginia chapter of the National Organization for the Reform of Marijuana Laws, said the state is taking “appropriate steps” to ensure Virginians understand that driving while impaired remains illegal, regardless of whether cannabis is legalized for adult use.
But unlike alcohol, enforcement relies on a different set of tools.
“We don't have a special prosecutorial tool for every substance in the United States. That breathalyzer exists for alcohol, not for everything else,” Pedini said.
That leaves officers mostly reliant on observation, field tests and occasional blood samples to build a case.
Pedini said cannabis alone accounts for a relatively small share of impaired driving cases in Virginia. Many incidents involve multiple substances, especially alcohol, which remains the dominant factor in DUI arrests.
According to state crime statistics, there were 105 marijuana-related DUI arrests in 2024, compared to more than 17,000 alcohol-related cases.
Kevin Caldwell, southeast legislative manager for the Marijuana Policy Project, said the difficulty in measuring cannabis impairment is not unique, and should not hold up legalization.
He said the bigger focus should be prevention, not just enforcement.
“Robust public education has severely dropped the number of people that drive under the influence of alcohol,” Caldwell said, noting similar campaigns like what Virginia is undertaking now could help reduce cannabis-impaired driving as more states enable greater access.
Data from states with longer histories of legalization, such as Colorado, shows mixed trends.
Ashley Brooks-Russell, professor at University of Colorado Anschutz, said the number of drivers testing positive for THC increased in the state in the years after legalization, but those numbers have largely leveled off over time.
“It’s been somewhat flat in the last several years – no dramatic increases, but also no declines,” Brooks-Russell said.
She said testing positive for THC does not necessarily mean a driver was impaired at the time of a crash. THC can remain in the body long after its effects wear off, making it difficult to draw direct conclusions.
“If you have a policy where you're deciding impairment based on a blood level, it is not a good policy,” Brooks-Russell said. “It’s going to overidentify someone as impaired when they might not be … and underidentify someone who doesn’t use it often and could be impaired.”
Brooks-Russell’s research is now exploring alternative methods, including cognitive and eye-based tests, as well as cannabis breathalyzers that could better detect recent use within the few hours when impairment is most likely.
