Alcohol remains far and away the most prevalent drug present in the blood of drivers involved in severe or fatal traffic accidents, according to a pair of recently published European studies.

In one study, published in the October issue of the journal Accident Analysis and Prevention, Danish researchers assessed the overall risk of a driver being severely injured in an accident after having consumed alcohol, illicit substances, or various types of pharmaceutical drugs, including opioids, benzodiazepenes, and so-called 'Z-drugs' (sedatives/sleep aids) such as Ambien. Case samples (N = 2490) were collected from severely injured drivers in selected hospitals in six European countries. Cases were matched against nearly 16,000 randomly stopped controls. Odds rations were adjusted for age, gender and country.

Investigators reported: "The highest risk of the driver being severely injured was associated with driving positive for high concentrations of alcohol (≥0.8 g/L), alone or in combination with other psychoactive substances. ...The second most risky category contained various drug-drug combinations, amphetamines and medicinal opioids. Medium increased risk was associated with medium sized BACs (at or above 0.5 g/L, below 0.8 g/L) and benzoylecgonine. The least risky drug seemed to be cannabis and benzodiazepines and Z-drugs."

Specifically, drivers with concentrations of alcohol in their blood possessed an elevated risk of accident that was nearly 10 times more than that of sober drivers. Subjects who tested positive for blood/alcohol concentrations between .08 and 1.2 possessed an elevated risk that was more than 16 times that of sober drivers, while drivers with concentrations above 1.2 possessed an elevated risk that was nearly 80 times higher. By contrast, “The driver injury risk estimated with driving positive for cannabis was just above one.” Drivers who tested positive for the presence of legal medicines possessed an elevated risk that was nearly twice that of cannabis.

“It is concluded that among psychoactive substances alcohol still poses the largest problem in terms of driver risk of getting injured,” the authors determined.

They are not alone in their finding. Writing this month in the Scandinavian Journal of Public Health, Swedish researchers similarly concluded that alcohol is far more likely to be detected in the blood of fatally injured drivers than is the presence of either illicit substances or prescription drugs.

Investigators evaluated the concentrations of alcohol and other drugs in blood samples from Swedish drivers killed in road-traffic crashes over a four-year period (2008-2011). They concluded: “Not surprisingly, the legal drug alcohol topped the list of psychoactive substances identified in blood samples from fatally injured drivers, which confirms results and surveys done in other nations…Indeed, in 76 percent of fatalities the autopsy BAC was over 1.0 g/L, which gives convincing evidence that these drivers were impaired at the time of the crash.”

By contrast, investigators acknowledged that the presence of an illicit drug alone was only present in 2.5 percent of all fatal crashes. THC specifically was identified in the blood of 3 percent of all drivers, though in many of these cases other substances were also identified. The presence of prescription drugs was documented in nearly 8 percent of all fatal traffic accidents. 

Other recent studies and reviews have similarly reported that cannabis is typically less likely to be associated with traffic injury compared to other controlled substances, including various prescription medications.

These findings call into question politicians’ and law enforcements’ supposed justification for the enactment of per se or so-called zero tolerant per se limits for the presence of cannabinoids. Such proposals forbid drivers from operating a motor vehicle if they have a detectable level of THC or its metabolite present in their bodily fluids above a specific, state-imposed threshold. To date, eleven US states -- Arizona, Delaware, Georgia, Illinois, Indiana, Iowa, Michigan, Oklahoma, Rhode Island, Utah, and Wisconsin -- impose zero tolerance per se thresholds for the presence of cannabinoids and/or their metabolites. Five states impose non-zero-tolerant per se thresholds for cannabinoids in blood: Montana (5ng/ml), Pennsylvania (1ng/ml), Ohio (2ng/ml), Nevada (2ng/ml) and Washington (5ng/ml). In Colorado, the presence of THC/blood levels above 5ng/ml “gives rise to permissible inference that the defendant was under the influence.” Nonetheless, despite the growing popularity of these laws, a recent white paper analyzing their impact found “no evidence that per se drugged driving laws reduce traffic fatalities.”

Paul Armentano is deputy director of NORML.