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Blood Testing To Keep Stoned Drivers Off The Roads Is Scientifically Invalid, AAA Says

Colorado Department of Transportation official Robin Rocke, left, trains a state trooper in how to conduct field sobriety tests for marijuana intoxication. CREDIT: AP PHOTO/BRENNAN LINSLEY
Colorado Department of Transportation official Robin Rocke, left, trains a state trooper in how to conduct field sobriety tests for marijuana intoxication. CREDIT: AP PHOTO/BRENNAN LINSLEY

The thorniest issue with state marijuana legalization just got a lot harder to navigate. Most state laws designed to catch drugged drivers are based on junk science, according to new analysis by the AAA Foundation for Traffic safety.

There is no blood test that can accurately gauge whether or not a person is actively stoned and the most common legal standard for drugged driving “cannot be scientifically supported,” the traffic safety analysts found.

It’s not that the current blood-level thresholds are set in the wrong place, either. “The data do not support science-based per se limits for THC” in drugged driving laws at all, AAA found.

Previous research on blood tests for drugged driving have arrived at the same conclusion, though well-known flaws with the tests have not stopped people from smearing the victims of police violence based on toxicology reports.

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But with the latest debunk of current stoned driving laws coming from inside the most prominent travelers’ organization in the United States, a shake-up seems likely.

Many states currently define drugged driving as operating a vehicle with a certain concentration of THC in one’s bloodstream. These per-se definitions of criminal drugged driving range from as low as 2 nanograms per mililiter in Ohio and Nevada to 5 ng/ml in Washington, where recreational marijuana is legal.

Such specific limits based on blood tests fail in both directions, according to AAA’s analysis. THC may remain in a person’s blood long after they have ceased feeling any effects of the drug, while a person who is still actively stoned might test well below their state’s threshold for drugged driving.

The data AAA analyzed suggests that a driver’s ability to touch their nose and to focus their eyes are well correlated with the level of THC in their bloodstream — but again, there is no scientifically valid blood level of that chemical that can serve as an automatic proxy for driver impairment. And blood tests will register a higher level of THC among people who use marijuana frequently even if they have not ingested the drug in the past few hours. That means even drivers who are careful to avoid getting behind the wheel while high could face arrest for a serious crime they didn’t commit, if the crime is based strictly on a blood test threshold.

But AAA’s rejection of blood-level test thresholds isn’t really about being kinder to the conscientious stoners among us. States that rely on strict blood-level limits to determine what constitutes criminal drugged driving are probably letting many people off the hook who police officers believe were driving dangerously, the analysis found. At the 5 ng/ml threshold used in Colorado, Washington, and Montana, “approximately 70 percent of all cannabis using drivers, whose actions led to them being arrested, will escape prosecution” under such a rule because a blood test will find they were below that legal limit.

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Instead of relying on such specific, quantitative definitions of stoned driving, the report suggests states would be better served training more officers to conduct field sobriety tests. A qualitative approach to assessing drivers for THC intoxication is inherently murkier because it relies on the judgment of individual police officers. But compared to a per-se blood-level definition of criminal behavior based on junk science, that judgment-based approach is both fairer to drivers and better able to protect public safety, the analysis shows.

The findings are likely to bring disarray to existing drugged-driving laws in various states, nearly all of which rely on some form of the blood-level standards debunked by AAA’s analysis. That includes states that have yet to legalize recreational marijuana, several of which have zero-tolerance laws based on blood tests. “A law against driving with THC in your bloodstream is not a law you can know you are obeying except by never smoking marijuana or never driving,” New York University drug policy expert Mark Kleiman told the Associated Press.

Beyond the obvious public safety concern, legalization states riding the economic boom that sensible marijuana policy brings have another reason to worry. Federal law enforcement is cautiously tolerating these legalization experiments for now. But the Department of Justice specifically lists “preventing drugged driving” as one of just eight conditions that legalization states must meet in order to continue exploring a post-prohibition weedscape.

The easiest way to satisfy the feds’ drugged-driving emphasis and make the public feel safe from stoners on the motorway is to come up with a pot version of the breathalyzer. But there is no such thing, and real responses to the threat will not be able to replicate the quantifiable certitude of the blood-alcohol limits that officials around the country use to establish how drunk is too drunk to responsibly operate a car.