States have put forth a bevy of approaches. At least five have what’s called a “per se” law, which outlaws driving if someone’s blood level of tetrahydrocannabinol, or THC, exceeds a set amount. THC is marijuana’s main intoxicant.
Colorado, where voters approved legalization of recreational marijuana in 2012, has this type of driving law on the books. It took three years to pass amid fiery debate and deems “intoxicated” any driver who tests higher than 5 nanograms of THC per milliliter of blood.
Rhode Island, Pennsylvania and Indiana are among states that forbid driving at any THC level. Still others say drivers should be penalized only if they are impaired by the chemical — a standard that sounds reasonable but quickly gets difficult to measure or even define.
None of these approaches offers an ideal solution, experts said.
“We’re still definitely evaluating which policies are the most effective,” said Ann Kitch, who tracks the marijuana and driving issue for the National Conference of State Legislatures.
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 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 last 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, 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, 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,” Rand Corp.’s 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 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,” Caulkins said. “The idea that you could come up with a completely objective test of performance … is ambitious.”
Researchers like 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,” Caulkins said.
This story was produced by Kaiser Health News, which publishes California Healthline, an editorially independent service of the California Health Care Foundation.
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