Drivers who test positive for the presence of THC in their blood are no more likely to be involved in motor vehicle crashes than are drug-free drivers, according to a case-control study released by the United States National Highway Transportation and Safety Administration (NHTSA).
Authors reported that drivers who tested positive for the presence of THC possessed an unadjusted, elevated risk of accident of 25 percent (Odds Ratio=1.25) compared to controls (drivers who tested negative for any drug or alcohol). However, this elevated risk became insignificant (OR=1.05) after investigators adjusted for demographic variables, such as the drivers' age and gender. After researchers controlled for both demographic variables and the presence of alcohol, THC-positive drivers' elevated risk of accident was zero (OR=1).
By contrast, researchers reported that drivers who tested positive for low levels of alcohol possessed a statistically significant risk of accident, even after controlling for demographic variables (e.g., Drivers with a BAC of 0.03 possessed a 20 percent greater risk of motor vehicle accident [OR=1.20] compared to controls). Drivers with BAC levels of 0.05 possessed a greater than two-fold risk of accident (OR=2.07) while motorists with BAC levels of 0.08 possessed a nearly four-fold risk of accident (OR=3.93).
Researchers did not analyze variations in drivers' THC levels to similarly estimate whether higher or lower THC levels may impact crash risk in a dose-dependent manner, as has been previously reported in some separate analyses of fatal crash data.
Authors concluded, "This finding indicates that these other variables (age, gender, ethnicity, and alcohol use) were highly correlated with drug use and account for much of the increased (crash) risk associated with the use of illegal drugs and THC. ... The results of this study are consistent with ... previous well-controlled studies."
The study, which involved some 9,000 participants, is the first large-scale case-control study ever conducted in the United States to assess the crash risk associated with both drugs and alcohol use by drivers.
The study's finding contradict allegations by NIDA and others that "marijuana use more than doubles a driver's risk of being in an accident," but are largely consistent with those of a 2013 literature review published in the journal Accident Analysis and Prevention which reported that cannabis-positive drivers did not possess a statistically significant risk of a either fatal accident or a motor vehicle accident causing injury.
A previous NHTSA-sponsored study assessing whether psychomotor motor impairment may be positively correlated with THC/blood levels similarly reported, "The answer is very clear; it is not."
An online NHTSA fact-sheet, "Drugs and Human Performance: Cannabis," acknowledges: "It is difficult to establish a relationship between a person's THC blood or plasma concentration and performance impairing effects. ... It is inadvisable to try and predict effects based on blood THC concentrations alone, and currently impossible to predict specific effects based on THC-COOH (the carboxy THC metabolite) concentrations."