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    The Confusing Science of Stoned Driving

    Written by

    David Bienenstock

    Photo via Flickr / CC

    Let's start with a simple, unassailable premise:

    Operating a 3,000+ pound motor vehicle on public roads while dangerously impaired on any psychoactive substance—from opium to Oxycontin—is not only illegal, it's immoral.

    The trick, when it comes to marijuana and driving, involves determining what exactly “dangerously impaired” means. Ask a few average all-American pot smoking teenagers, and they'll likely say there's no such thing. A recent survey conducted by insurance giant Liberty Mutual, for example, found that among teens who admitted to driving after consuming cannabis, more than 70 percent self-reported no negative effects whatsoever on their competence behind the wheel, including 34 percent who believed, however dubiously, that getting blazed was actually performance enhancing.

    Meanwhile, at least ten states mandate severe penalties for any trace of THC in a roadside drug test, even inactive metabolites that remain detectable up to a month after use. In February, the Arizona State Court of Appeals went so far as to reinstate the conviction (overturned by a lower court) of a man charged with a marijuana DUI despite a blood test that proved he wasn't under the influence. An Orwellian decision that confirmed Arizona's zero-tolerance “legislative ban extends to all substances, whether capable of causing impairment or not."

    The Michigan Supreme Court, on the other hand, just last week ruled that registered pot patients do enjoy a limited exemption from zero tolerance prosecutions. Although driving while impaired remains illegal in all circumstances, the court decided that the mere presence of THC in a patient's blood or urine should be viewed as a legal form of “internal possession”. Since state law allows patients to carry up to 2.5 ounces of cannabis, and currently offers no clear guidelines to determine impairment, the justices urged lawmakers to move quickly towards creating a universal standard similar to the one used for alcohol.

    Which sounds imminently reasonable, until you start to look at the science of stoned driving, including a significant number of peer reviewed studies with results that range from confusing to contradictory. For example, according to a widely-reported meta-analysis published in the British Medical Journal, marijuana use within one hour of driving doubles the risk of a serious automobile accident. But a later review in the journal Accident Analysis and Prevention claimed those findings were overblown, likening increased risk to the use of antihistamines or penicillin. While a 2009 report from the U.S. National Library of Medicine concluded that though “cognitive studies suggest that cannabis use may lead to unsafe driving, experimental studies have suggested that it can have the opposite effect.”

    And if all that isn't confounding enough, wrap your mind around one last study that shows widespread use of medical marijuana actually produces a major improvement in public safety. Mostly because legal access to cannabis leads a significant number of drivers to smoke buds instead of drink Bud, a relatively safer choice that ends up saving a lot of lives.

    A few years back, a journalist with the BBC put herself through a guantlet of impaired driving

    “Specifically, we find that traffic fatalities fall by nearly 9 percent after the legalization of medical marijuana,” concluded University of Colorado Professor Daniel Rees and Montana State University Assistant Professor D. Mark Anderson. Which may sound counterintuitive at first, but actually reflects the one data point that holds steady across all the research: Drunk drivers are far more dangerous than stoned drivers, with alcohol use increasing accident risk seventeen-fold, according to the US National Highway Safety and Transportation Agency.

    The same federally-funded NHSTA study also conceded that blood tests measuring THC—even “active” THC—don't accurately assess impairment in all cases, finding that it's “difficult to establish a relationship between a person's THC blood or plasma concentration and performance impairing effects … Drivers with high concentrations showed substantial [impairment], but also no impairment, or even some improvement.”

    The main reason for such a wide variation seems to be that regular cannabis users don't experience the drug's effects nearly the same way as infrequent or naïve users. In 2010, the journal Psychopharmacology published a study assessing motor skills, cognition and dual task processing, in which the authors concluded that “heavy cannabis users develop tolerance to the impairing effects of THC on neurocognitive task performance.” While a 2012 study in the Journal of Analytical Toxicology confirmed “minimal impairment in driving-related psychomotor tasks in chronic daily cannabis users.”

    So will the “smoke weed every day” defense hold up in court?

    Legal access to cannabis leads a significant number of drivers to smoke buds instead of drink Bud, a relatively safer choice that ends up saving a lot of lives

    “A lot of long-term, heavy-use medical marijuana patients feel like they can never legally drive again,” Colorado criminal defense attorney Sean McAllister informs me. “And unfortunately I have to tell them that's possibly accurate.”

    As founder of Sensible Colorado, and co-chair of the state legislature's official “Driving Under the Influence of Drugs” subcommittee, McAllister has been fighting proposed per se DUID limits for years. His subcommittee, appointed by the governor, met for six months to review the science, including interviewing the world's leading experts on the subject, before deadlocking in 2011 without making an official recommendation.

    But despite already winning 90 percent of their DUID cases, Colorado's district attorneys continued to push for a per se standard that McAllister finds grossly unfair.

    “It basically changes the presumption of innocence to a presumption of guilt. To do that, in a country where you're supposed to be innocent until proven guilty beyond a reasonable doubt, the science must also be beyond a reasonable doubt. And it's just not.”


    With marijuana legalization rapidly taking root in two states, and set to spread like a grass fire in the near future, the question of how to effectively determine impairment has never been more urgent, or controversial. In Washington state, the legalization voter initiative that passed last November proactively established 5 nanograms of active THC per milliliter of blood as the limit beyond which drivers now face a de-facto DUI charge. Colorado's legalization initiative contained no such language, but state legislators there recently passed a similar 5 nanogram law, though their version at least gives defendants the right to argue in court that they weren't impaired.

    The Colorado legislature actually took up a version of that same marijuana DUI legislation six times over the last three years, before finally pushing it through at the urging of Governor John Hickenlooper. Most dramatically, in 2011, the bill appeared poised for passage when local medical marijuana reviewer William Breathes, from Denver's alt-weekly Westword newspaper, arranged to test his own blood “after a night of sleep and not smoking for fifteen hours”. When the results showed his level of active THC at 13.5 nanograms, nearly three times the proposed limit, the bill was temporarily put on hold. Since then, other heavy users have come forward with similar accounts.

    Here I am with a fiery nug of Red Congolese, a sought-after sativa-dominant hybrid

    A 2009 study published in the journal Addiction, however, examined 25 chronic pot users during a week of monitored abstinence, and found that only a single subject exceeded 5 nanograms of active THC per milliliter of blood at any time, and that was on admission day. Presumably, she could have smoked a blunt (one of four she reported typically enjoying each day) in the parking lot right before checking in.

    According to the pro-legalization Marijuana Policy Project, which opposes per se laws, “Depending on the dose, THC typically reaches peak concentrations of more than 100 nanograms per milliliter five to ten minutes after inhalation and then rapidly decreases to between one and four nanograms per milliliter within three to four hours.”

    Which doesn't make me doubt William Breathes's results in the least, so much as get curious about how much THC runs in my own veins.


    After scheduling a blood test of both active THC and inactive metabolites, I settled on a fourteen hour period of total pot abstinence prior to facing the needle. Then I carefully considered a far more important decision. What, and how much, to puff immediately prior to this self imposed cannabis curfew. Obviously, I should go for something well beyond irie. After all, I've smoked some seriously strong weed in my time, and plenty of it, and nothing's ever left me feeling impaired the next day. Not in the slightest.

    So, from deep in my personal stash archive, I selected a magnificent bud of Red Congolese, a sought-after sativa-dominant hybrid mixing African, Mexican and Afghani genetics that lab tested at 19.96 percent THC. Between 8:30 PM and my official “last toke” at 1:30 AM, I twisted up five joints roughly the size of the one I'm holding in the photo and smoked about three-quarters of each, sharing the rest with my wife.

    At 3:38 PM the next day, after a good night's sleep (okay a great night's sleep), a raw juice breakfast, and an hour of basketball, I had the surreal experience of taking a drug test without really worrying about the results. Aside from a slight prick, and the sight of my own blood, it was all pretty painless.

    But during the five days I waited for the results, something awful began to dawn on me. If I can't smoke a bunch of herb with my wife at home on a Tuesday night to accompany an Arrested Development marathon, and then legally drive a car the next afternoon, that means America's millions of “chronic daily cannabis smokers” (like me!) must continue to live in fear of persecution for a victimless crime, even after we “legalize it” once and for all.

    Here I am again, this time getting my blood drawn

    “When you have .08 blood alcohol level, you are absolutely under the influence and impaired, I don't care if you're an alcoholic or a first time user,” McAllister says. “The difference with marijuana is that somebody could be above 5 nanograms twenty-four hours after use. And no expert in the world would say that person's still high. Which means this political agenda to drive the nanogram level lower in the name of public safety is moving forward at the sacrifice of innocent people. And that's not the way our justice system works.”

    Right now, more than 800,000 otherwise law-abiding American marijuana users face arrest each year, most for simple possession. With the sweet, skunky smell of change blowing in the wind, it's not hard to imagine that number dropping significantly in the next decade. But after seventy-five years of the federal War on Weed, it's also not hard to look across the chess board and anticipate the pot prohibitionists' next move.

    The White House's Office of National Drug Control Policy currently “encourages states to pursue enhanced legal responses” to drugged driving, “such as per se 'zero tolerance' laws,” despite the fact that ONDCP head Gil Kerlikowske once admitted, “I’ll be dead—and so will lots of other people—from old age, before we know the impairment levels’’ for marijuana. Already, the race is on to develop a faster, less invasive roadside THC test, possibly a saliva swab that gives a reading in just minutes. Which would be a blessing if the legal standard behind it fairly assesses impairment, and a nightmare if it doesn't.

    According to Paul Armentano, Deputy Director of the National Organization for the Reform of Marijuana Laws, despite “lip service” on the issue from the Obama administration, the real battle over stoned driving will be fought state-by-state as legalization spreads.

    “State lawmakers recognize that the public desires serious discourse regarding how best to regulate cannabis and its consumption," Armentano tell me. "But proposed per se laws for cannabinoids are an attempt to misuse traffic safety standards as a way to punish legally protected behavior by those who pose no actionable traffic safety threat.”


    When my doctor at last calls with the test results, he sounds duly impressed that I'm rocking 35 nanograms of Delta 9 Carboxy THC in each and every milliliter of my blood. That's the metabolite, the one that sticks around for a month. And needless to say, I'm about 35 nanograms over the line in any zero-tolerance state. Also, I'm not about to land a job at Blockbuster Video.

    But the good news is that at 2 nanograms of “active” Delta 9 THC, I'm fine to drive in Colorado or Washington. I just don't know at what point in the course of those fourteen hours of abstinence I fell below the 5 nanogram threshold. And even if I did, I can't reliably expect the same results in the future.

    Which gives me an idea for a new business. A pizza delivery joint that picks you up at work with two large pies and then drives you home.

    David Bienenstock is a former High Times journalist and the author of Legalized It!