A new study from the National Highway Traffic Safety Administration finds that drivers who use marijuana are at a significantly lower risk for a crash than drivers who use alcohol. And after adjusting for age, gender, race and alcohol use, drivers who tested positive for marijuana were no more likely to crash than who had not used any drugs or alcohol prior to driving.
The chart above tells the story. For marijuana, and for a number of other legal and illegal drugs including antidepressants, painkillers, stimulants and the like, there is no statistically significant change in the risk of a crash associated with using that drug prior to driving. But overall alcohol use, measured at a blood alcohol concentration (BAC) threshold of 0.05 or above, increases your odds of a wreck nearly seven-fold.
The study’s findings underscore an important point: that the measurable presence of THC (marijuana’s primary active ingredient) in a person’s system doesn’t correlate with impairment in the same way that blood alcohol concentration does. The NHTSA doesn’t mince words: “At the current time, specific drug concentration levels cannot be reliably equated with a specific degree of driver impairment.”
There are a whole host of factors why detectable drug presence doesn’t indicate impairment the way it does with alcohol. “Most psychoactive drugs are chemically complex molecules, whose absorption, action, and elimination from the body are difficult to predict,” the report authors write, “and considerable differences exist between individuals with regard to the rates with which these processes occur. Alcohol, in comparison, is more predictable.” In heavy marijuana users, measurable amounts of THC can be detectable in the body days or even weeks after the last use, and long after any psychoactive effects remain.