Marijuana's Effect on Driving Not Cut-and-Dried Issue, Study Shows

Whether you think driving while high is potentially deadly or no big deal, you're likely to find ammunition for your argument in the latest major federally funded study of drugged driving. The National Highway Traffic Safety Administration called its Drug and Alcohol Crash Risk Study, which was released this year, the "largest and most carefully controlled of its kind to date," examining the crash risk associated with alcohol and drug use by motorists. And the results only underscore the issue's complexity.

Related: Legalized Marijuana Makes Drugged-Driving Study High Priority

Put simply, the study — conducted over 20 months in Virginia Beach, Va., using data collected from more than 3,000 crash-involved drivers — concluded unsurprisingly that alcohol consumption has a direct correlation to car crashes. Drivers with a 0.08 blood alcohol content, the legal limit in all 50 states and the District of Columbia, quadrupled the likelihood of a crash, while those with a 0.15 BAC were a dozen times more likely and those with 0.20 or higher two-dozen times.

Users of marijuana — the most frequently detected substance next to alcohol — meanwhile demonstrated a 25 percent increased likelihood of being involved in a crash than a sober driver. Despite paling by comparison to the risks associated with alcohol, the correlation between THC, the active substance in marijuana, and crashes is still substantial: The risk is 5 percent greater than a driver with a 0.03 BAC. The marijuana risk factor, however, is not as cut-and-dried as it might sound.

"There is evidence that marijuana use impairs psychomotor skills, divided attention, lane tracking, and cognitive functions," researchers stated in their report. "However, its role in contributing to the occurrence of crashes remains less clear."

Despite what would seem like evidence that marijuana use was a contributing factor in the crashes, when researchers factored in the age and gender of drivers involved, the crash risk attributable to the drug essentially disappeared. The implication there, researchers say, is that people who are more likely to get in accidents — for example, aggressive-driving younger males — are also more likely to smoke marijuana, as opposed to the drug actually causing the crash. Moreover, because of the way THC is absorbed by the body, a regular user could have significant traces of the substance in his system without actually being high. In other words: correlation, not necessarily causation.

Still, previous studies have shown increased risk associated with marijuana, and researchers hardly consider it a closed case.

"The findings of this study notwithstanding, the established body of evidence on the subject of drug impairment indicates that in some situations, drugs other than alcohol can seriously impair driving," NHTSA said in the study. "This study provides further confirmation that driver impairment is a very serious safety concern and that it involves a very certain element of alcohol impairment and a less-certain element of drug impairment."

With the rising tide among U.S. states to legalize marijuana for medical and recreational use, getting solid answers on the potential risks of drugged driving is of increasing concern. While touted as the biggest, this latest study isn't the only one of its kind. NHTSA previously partnered with the National Institute on Drug Abuse on a three-year, $500,000 project to determine the impact of inhaled marijuana on driving performance by observing participants in a controlled environment — namely the University of Iowa's National Advanced Driving Simulator. The participants were given low and high doses of THC, as well as a placebo, to assess the effects on performance, decision-making, motor control, risk taking and divided-attention tasks. Researchers continue to examine the completed study's data.

Sober-driving advocates aren't waiting for lab results to dissuade people from driving under the influence of marijuana and other drugs. Just this year, Mothers Against Drunk Driving amended its mission statement to include the phrase, "help fight drugged driving," and it supports ongoing efforts to train law enforcement officers to detect drugged drivers and take them off the roads, as well as the adoption of "per se" laws making illegal the presence of even small traces of certain drugs in a driver's system, including over-the-counter, prescription, legal marijuana and illicit drugs.

"MADD hopes to bring awareness to the growing threat of drugged driving on our roadways, much in the same way we have with drunk driving since our founding in 1980," MADD said in a statement. "While substances are different, the results are the same — needless deaths and injuries."