While numerous substances, from prescription medications to illicit drugs, can impair driving performance, alcohol remains far and away the substance that is most likely to increase one’s risk of experiencing a fatal accident behind the wheel.

So concludes the findings of a major new study, entitled “Drugs and Alcohol: Their Relative Crash Risk,” published this week in the Journal of Studies on Alcohol and Drugs.

Investigators at the Pacific Research Institute in Maryland and the University of Puerto Rico assessed whether alcohol, licit or illicit drugs, or a combination of substances is most likely to contribute to fatal rash risk. The answer: “[T]he contribution of alcohol to crash risk is much larger than that by other drugs,” researchers concluded.

In fact, even the presence of alcohol in the blood at permissible limits (below 0.08% in the United States) elevated drivers’ risk of accident in a more significant manner than did the presence of barbituates, benzodiazepenes, sleep aids, opiates, amphetamines, cocaine, PCP, or marijuana. The authors wrote, “[O]ur finding that the risk of involvement in a fatal crash at a BAC of 0.05% is significantly higher than that for being positive for drugs other than alcohol.”

As for the crash risk associated with the presence of marijuana, authors determined — much to their surprise — that there existed little association at all. They wrote: “Although drugs other than alcohol do contribute to crash risk, we found that such a contribution depends on the type of drug under consideration. Somewhat unexpected was the finding that although marijuana’s crude OR (odds ratios) indicated a significant contribution to fatal crash risk, once it was adjusted by the presence of alcohol and drivers’ demographics, marijuana’s OR was no longer significant among either sober or drinking drivers.”

Overall, authors concluded, “Alcohol was not only found to be an important contributor to fatal crash risk, … it was associated with fatal crash risk levels significantly higher than those for other drugs. … The much higher crash risk of alcohol compared with that of other drugs suggests that in times of limited resources, efforts to curb drugged driving should not reduce our efforts to pass and implement effective alcohol- elated laws and policies.”

Unfortunately, lawmakers in a growing number of states are rejecting this advice — instead opting to enact unscientific per se laws or zero tolerance per laws that criminalize those who operate a vehicle with the trace presence of THC (or in some cases the presence of the inert THC metabolite carboxy THC) in one’s blood or urine, regardless of whether or not there exists demonstrable evidence that the driver was actually impaired. Further, recent analyses regarding the imposition of such laws found that even when strictly enforced, such policies do not actually reduce incidences of drug-impaired driving or traffic fatalities.

Predictably, the new study in JSAD is far from the only recent paper to affirm alcohol’s primary role in car crashes. A Danish study published in the October issue of the journal Accident Analysis and Prevention determined: "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." Authors concluded, “[A]mong psychoactive substances, alcohol still poses the largest problem in terms of driver risk of getting injured.”

Likewise, a study published in November in the Scandinavian Journal of Public Health determined, “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 identified 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.

A recent study by US researchers also reported similar findings in regards to the presence of drugs and crash risk. Investigators at Columbia University conducted a case-control study to assess the association between drug use and fatal crash risk. Of the substances identified by researchers, authors reported that depressants were most likely to be associated with elevated car crash risk (estimated odds ratio = 4.83). Estimated odds ratios for other specific drug categories were 3.57 for stimulants, 3.41 for polydrug use (excluding alcohol), and 3.03 for narcotics. Marijuana (1.83) possessed the lowest odds ratios of the substances identified.

Similarly, a comprehensive 2013 meta-analysis of 66 separate studies assessing the risk of road accident associated with the presence of various licit and illicit drugs estimated that marijuana was associated with only a nominally increased risk of fatal accident (estimated odds ratio = 1.26) or injury (1.10). In that study, only anti-histamines (1.12), penicillin (1.12), and analgesics (1.02) were associated with comparable odds ratios to that of cannabis.

Of course, highlighting these scientific findings is not intended to imply that driving under the acute influence of cannabis is not without risk or that such behavior should not be discouraged, both socially and legally. (The act itself is a criminal offense in all 50 states.) However, as more states contemplate plans to liberalize their pot possession policies, general (and sometimes unsubstantiated) concerns regarding the substance’s potential impact on driving performance ought not to serve as a justification for continuing arrest of millions of people who consume cannabis responsibly.

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