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.
determination, and the clarity makes enforcement of drunken driving laws easier. Setting a marijuana-related impairment level is a much murkier proposition. But states that have legalized pot have to figure it out, experts said. “You can’t legalize a substance and not have a coherent policy for controlling driving under the influence of that substance,” said Steven Davenport, an assistant policy researcher at the nonprofit Rand, who specializes in marijuana research. Marijuana, after all, weakens a driver’s ability to maintain focus, and it slows reflexes. But regulators are “playing catch-up,” suggested Thomas Marcotte, a psychiatry professor at the University of California-San Diego and one of a number of academics around the country who is researching driving while high. 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
There’s general agreement that driving while high is bad, but there’s no linear relationship between THC levels and degree of impairment. 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.
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