In 2009 out of 3,952 fatally injured drivers, 18% tested positive for marijuana. But, studies analyzing the effect of driving under the influence of pot have contradictory research and imprecise measurement. National Highway Traffic Safety Administration (NHTSA) studies shed some light since 24 states legalized marijuana.
The first report confirmed predictions that legalization would lead to more stoned drivers. In a 2013-14 roadside survey of more than 9,000 drivers, 12.6 percent tested positive for THC (principal psychoactive ingredient in marijuana,) compared to 8.6 percent in 2007 – a 47% increase. The percentage of drivers at or above the legal limit for alcohol was 8.3 percent, down 80% since a 1973 NHTSA roadside survey.
But the story takes an unexpected turn. The second NHTSA report focused on crash risk in what the agency described as the “first large-scale study in the United States to include drugs other than alcohol.” Data was collected from more than 3,000 crash-involved drivers and compared to 6,000 drivers not involved in crashes. While drivers who tested positive for THC were 25 percent more likely to be involved in a crash, other factors like age, gender, ethnicity and blood alcohol concentration were considered more likely the culprit than the presence of drugs. NHTSA researchers wrote: “Caution should be exercised in assuming that drug presence implies driver impairment.”
Meanwhile, the number of traffic fatalities involving marijuana-stoned drivers has increased in Washington and Colorado since both states legalized the recreational use of the drug, according to two recent reports. The percentage of drivers who used pot within hours of a fatal crash in Washington nearly doubled from 2013 to 2014, according to a just-published AAA study. A similar increase was recorded in Colorado with 10 percent of drivers in fatal accidents testing positive for marijuana in 2011 versus 5.9 percent in 2009.
OK, So What’s “Under the Influence”?
Measuring impairment in a person using marijuana isn’t comparable to blood alcohol concentration. “Most psychoactive drugs are chemically complex molecules, whose absorption, action and elimination from the body are difficult to predict,” NHTSA writes, “and considerable differences exist between individuals with regard to the rates at which these processes occur. Alcohol, in comparison, is more predictable.” Some marijuana users can have measurable amounts of THC in their bodies days or even weeks after using the drug, long after any psychoactive effects remain.
The uncertainty surrounding the intoxicating effects of marijuana is reflected in the patchwork of state laws defining driving “under the influence” of drugs. Some states follow a zero tolerance standard, making it illegal to have any presence of THC or other illegal drugs in your body while driving. Others set a legal THC limit expressed in nanograms (one-billionth of a gram) per milliliter of blood. In others, impairment is inferred based on the circumstances rather than defined by blood THC levels.
Of course, no one is suggesting that people should “blaze one and go for a joyride. There is plenty of evidence to show that marijuana use impairs driving skills. And as all these studies make clear, we need more research on the effects of marijuana and driving, along with better equipment for detecting and measuring marijuana-related impairment. It is also clear that the science of intoxication surrounding marijuana is different than that of alcohol and may demand a more nuanced response by policymakers, law enforcement and court officers. In other words, stay tuned for more on this evolving topic. http://letamericaknow.com/view_feature_ysk.php?memberid=21699&orderid=511&issueid=1606#