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.
“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.”
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.
“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.
excerpted from: http://motherboard.vice.com/blog/the-confusing-science-of-stoned-driving