A study published on January 29 in the American Journal of Epidemiology purports to find that marijuana use has played an increasing role in fatal car accidents over the past decade. But are the study’s conclusions scientifically sound?
The study surveyed toxicology results of drivers involved in fatal car accidents in California, Hawaii, Illinois, New Hampshire, Rhode Island and West Virginia, the six states that routinely perform toxicology tests on all drivers in fatal car accidents. According to the study, marijuana played a role in 12 percent of all fatal traffic accidents in 2010, a drastic increase from 4 percent in 1999. Alcohol’s role in fatal accidents did not change significantly over the decade and remained constant at about 40 percent. In addition, the study found that the combination of marijuana and alcohol increases the risk that a person will be in a fatal car accident by a factor of 24 when compared to a sober person. Alcohol alone increases the risk by a factor of 13.
These statistics are compelling at first glance, but are they scientifically sound? Did the study simply note whether evidence of marijuana usage was found in the blood or did it determine whether the person was under the influence to a degree which would interfere with safe driving at the time of the crash? The biggest problem with determining whether a driver was under the influence of marijuana when he was driving is determining when the driver smoked. Depending on how frequently and how much a person smokes, THC metabolites can show up in a person’s system more than 30 days later. A person’s positive result for marijuana on a toxicology screen really has no bearing on whether or not that person was high and driving at the same time – all it means is that the person smoked marijuana at some point in the weeks prior to the crash. In addition, the study is silent on the issue of prescription drug combinations. Can you really attribute a crash to marijuana usage if the person was also under the influence of alcohol, cocaine, or some other drug that interferes with safe driving?
With marijuana legalization becoming increasingly prevalent, how big of a role do you think it will play in fatal car accidents? Let us know in the comment section.