The National Highway Traffic Safety Administration has reported a decrease in alcohol consumption an increase in drug use by drivers in 2014. The agency has cited several studies in its latest report, including the Roadside Survey of Alcohol and Drug Use by Drivers (PDF) and another research paper, titled Drug and Alcohol Crash Risk (PDF), that is claimed to be the most precisely controlled study of its kind yet conducted.
"The rising prevalence of marijuana and other drugs is a challenge to everyone who is dedicated to saving lives and reducing crashes," said NHTSA Administrator Mark Rosekind.
Notably, the crash study appears to contradict previous claims related to marijuana use by drivers and combinations of alcohol and drug use. The authors did find a statistically significant link between marijuana use and crash risk, however the relationship disappeared once the data was corrected for driver age and other variables.
"This analysis shows that the significant increased risk of crash involvement associated with THC and illegal drugs ... is not found after adjusting for these demographic variables," the authors write. "This finding suggests that these demographic variables may have co-varied with drug use and accounted for most of the increased crash risk. For example, if the THC-positive drivers were predominantly young males, their apparent crash risk may have been related to age and gender rather than use of THC."
Some reports have claimed that marijuana or other drugs bring exponential increases in crash risk when combined with alcohol, but the latest study finds no such relationship.
"No statistically significant interaction effect on crash risk was found between any drug class or drug category and BrAC level," the study notes.
The NHTSA plans to release results of additional studies to further understand the risks of drugged driving. Of particular interest is the Washington State Roadside Survey, conducted following marijuana legalization.
