Chemical tests to determine whether marijuana impairs driving are problematic, experts say
With marijuana use increasingly legal and more socially acceptable in the U.S., states are struggling with how to determine whether drivers are high and a danger on the roads.
Every state and the District of Columbia has laws prohibiting driving under the influence of alcohol and drugs, including marijuana. There are blood, urine and saliva tests to determine marijuana use, but critics say they are neither effective nor accurate in determining a user’s ability to drive safely.
One issue is that while it’s relatively easy to determine whether a driver is under the influence of alcohol, it’s much more challenging to conclude if someone is driving stoned, according to Andrea Roth, a professor specializing in criminal justice at the University of California at Berkeley School of Law.
And laws relying mostly or exclusively on chemical testing on bodily fluids are misguided, says Jake Nelson, director of traffic safety, advocacy and research at the auto club organization AAA.
“We are attempting to effectively legislate” driving while high, but “we still don’t know enough about cannabis impairment,” Nelson says. “We are nowhere near understanding it the way we understand alcohol.”
He emphasizes that, as a society, “we are painting the plane as we fly it.”
Researchers are working on creating better tools for detecting marijuana impairment. States increasingly experiment with roadside saliva tests, and scientists are studying breath alcohol testing devices.
But Paul Armentano, deputy director of the National Organization for the Reform of Marijuana Laws, says that law enforcement agencies shouldn’t be investing in new types of chemical testing. Instead, they should consider performance-based technology that can evaluate motor skills or short-term memory.
One app, called Druid, tests for hand-eye coordination and can be used to determine if someone is able to drive, says Armentano. AlertMeter, another app, provides a 60- to 90-second cognitive alertness assessment.
Law enforcement officers also can test balance, estimation of time and other physical manifestations to determine someone isn’t fit to drive, Armentano says. “Police need to look at the signs and symptoms that someone is driving while drugged. That’s how they win cases and discourage driving under the influence of marijuana.”
Lack of consensus
The landscape for marijuana use in the U.S. is rapidly changing. The recreational use of marijuana has been legalized in 24 states and Washington, D.C. In addition, 40 states and Washington, D.C., have sanctioned its use for medical reasons. About 15% of U.S. adults say they smoke marijuana, according to a 2024 Gallup survey.
A 2022 National Transportation Safety Board report found that alcohol and marijuana were the two drugs most detected during arrests for impaired driving.
In March, the AAA Foundation for Traffic Safety published a report that involved 2,000 cannabis users. It showed 53% of them consumed cannabis an hour or less before driving. The respondents were from eight states.
Roadside and laboratory tests examine levels of THC, the main ingredient in marijuana responsible for its intoxicating effects. The tests are inherently problematic, experts say, because not only is the technology limited in what it can assess, but there’s a lack of scientific consensus on the level of THC after which every driver is impaired to the point they shouldn’t be driving.
Certainly, there’s evidence that marijuana use affects coordination, memory and judgment, along with slowing reaction time and distorting perception, according to the Centers for Disease Control and Prevention. However, there’s no predictable relationship between the level of THC in the blood and the level of intoxication in the brain, says Roth, in contrast to alcohol, which has a nationally recognized level of impairment for drunk driving.
A variety of factors, such as whether an individual is a chronic user, affect an individual’s THC levels at any given time.
‘Unjust results’
The most common methods of detecting THC are through blood or urine. However, some states, like Alabama, have launched saliva testing programs.
Karen O’Keefe is the director of state policies for the Marijuana Policy Project, a nonprofit dedicated to nonpunitive cannabis laws and cannabis legalization. She says saliva tests have been known to have false positives with a “high degree of variability.”
She says it’s important to keep the roads as free as possible from people who can’t drive safely by “developing tests that focus on actual impairment versus what’s in a person’s body.”
Some states—such as Ohio and Montana—have set an amount of THC in a driver’s body that is deemed impermissible for driving, according to the National Conference of State Legislatures. Other states have what are called “per se zero tolerance” laws, which means that no THC in a driver’s system is permissible.
Additionally, states have laws that criminalize driving under the influence of drugs but rely on a variety of tests and observations, including evidence of erratic driving.
Roderick Kennedy, a retired chief judge of the New Mexico Court of Appeals who has taught forensic evidence, emphasizes that zero tolerance laws can produce “unjust results.”
Testing for THC in states with zero tolerance laws “may end up penalizing prior use but not whether someone is actually a danger to the public,” says Kennedy, a past co-chair of the ABA Section of Science & Technology Law’s Scientific Evidence Committee.
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