States that set a THC-level standard confront weak technology and limited science. THC testing is imprecise at best, since the chemical can stay in someone’s bloodstream for weeks after it was ingested. Someone could legally smoke a joint and still have THC appear in blood or urine samples long after the high passes.
There’s general agreement that driving while high is bad, but there’s no linear relationship between THC levels and degree of impairment. States that have picked a number to reflect when THC in the bloodstream becomes a hazard have “made it up,” argued Dr. Humphreys.
“The ones who wrote [a number] into legislation felt they had to say something,” he said. But “we don’t know what would be the analogy. Is the legal amount [of THC] equal to a beer? Is that how impaired you are? Is it a six-pack?”
Roadside testing for THC is also logistically difficult. Blood, for instance, needs to be analyzed in a lab, and collecting urine gets ... complicated.
In Canada, which legalized recreational pot just this year, law enforcement will test drivers with a saliva test called the Dräger DrugTest 5000, but that isn’t perfect, either.
Some private companies are trying to develop a sort of breathalyzer for marijuana. But Jonathan Caulkins, a drug policy researcher at Carnegie Mellon University, Pittsburgh, said, “There are fundamental issues with the chemistry and pharmacokinetics. It’s really hard to have an objective, easy-to-administer roadside test.”
Some states rely on law enforcement to assess whether someone’s driving appears impaired and ascertain after the fact if marijuana was involved.
In California, every highway patrol member learns to administer “field sobriety tests” – undergoing an extra 16 hours of training to recognize the influence of different drugs, including marijuana. Because medical marijuana has been legal there since 1996, officers are “very used” to recognizing its influence, said Glenn Glazer, the state’s coordinator for its drug recognition expert training program.
That kind of training is taking off in other states, too, Ms. Kitch said. Lobbying groups such as Mothers Against Drunk Driving are pushing to bump up law enforcement training and rely on officers to assess whether a driver is impaired.
These tests, though, risk their own kind of error.
“They are subjective,” Mr. Davenport warned.
For one thing, officer-administered tests can be influenced by racial bias. Someone who has previously had poor experiences with law enforcement may also perform worse, not because of greater impairment but because of nervousness.
Indeed, relying on more subjective testing is in some ways the direct opposite of conventional wisdom.
“A general pattern of the last ... 40 years is to try to take human judgment out of decision making processes when possible. Because we fear exactly these issues,” Mr. Caulkins said. “The idea that you could come up with a completely objective test of performance ... is ambitious.”
Researchers like Dr. Marcotte are trying to devise some kind of test that can, in fact, gauge whether someone is showing signs of marijuana impairment. But that could take years.
In the meantime, the public health threat is real. States with legalized pot do appear to experience more car crashes, though the relationship is muddled. “This is going to be a headache of an issue for a decade,” Mr. Caulkins said.
Kaiser Health News is a nonprofit national health policy news service. It is an editorially independent program of the Henry J. Kaiser Family Foundation that is not affiliated with Kaiser Permanente.