Opponents to any legalization of cannabis have always insisted that allowing its use would lead to a increase of traffic accidents and avoidable deaths. With more states permitting cannabis use for medical as well as adult use, the worries over cannabis impaired drivers has drawn the interest of legislators and law enforcement. Some states have legislation which specifies a specific blood level of THC at which a person is considered impaired, just like the law does with blood alcohol levels. In some states which have a “zero tolerance” law, the presence of any trace amount of THC or even the non-psychoactive metabolites will be considered “pro se” evidence for driving under the influence. Its presence means you are guilty. There is no legal defense. Five states have pro se blood THC limits from 1 nanogram/ml to 5 nanograms/ml as the legal threshold of impairment.
Now have some studies examining this issue of impairment and highway deaths. Recent studies in the USA compared motor vehicle fatalities in the states of Colorado and Washington which have essentially legalized cannabis to a “synthetic control group” from records of fatal accidents from states without legalization. Both groups showed the same changes in overall fatality rates, the conclusion being cannabis legalization did not increase highway accident fatalities. A recent study from British Columbia, Canada looked at over 3000 injured drivers blood samples by performing extensive toxicology testing. They found no increase in accident risk for drivers with less than 5 ng/ml THC levels but a small but statistically insignificant increased risk in drivers with greater than 5 ng/ml.
The Congressional Research Service provided a report to congress titled ” Marijuana Use and Highway Safety” on May 14, 2019 addressing some of these issues. They, as well as the National Highway Traffic Safety Administration, also found that using a measure of THC as evidence of a drivers impairment is unsupported by the scientific evidence. So THC is not at all like alcohol as far as drawing conclusions about driver impairment based on blood levels detected. They also note that there is no demonstrable difference in highway deaths in states which have legalized cannabis for any use compared to states with total prohibition.
There have been laboratory studies done which have shown decreased reaction times in subjects consuming THC, so one would expect the rate of crashes to increase; but this is not the case. When the NHTSA reviewed the scientific literature they found interesting differences between the driving of subjects who were dosed with alcohol vs those dosed with cannabis. Cannabis dosed patients in driving simulators or monitored automobiles on a closed track seemed to drive below the speed limit, allow greater distance between themselves and the vehicles ahead of them and take fewer risks than when they were not under the influence of cannabis. Alcohol dosed subjects, on the other hand, tended to drive faster than the speed limit, follow leading vehicles more closely and generally drove in a much riskier fashion than when they were sober. They proposed that perhaps subjects under the influence of cannabis “felt “the intoxicating effects and were consciously altering their driving behavior to compensate. On the other side of the argument, the National Academy of Sciences and National Institutes of Health came to a very different conclusion. They stated that “There is substantial evidence of a statistical association between cannabis use and increased risk of motor vehicle crashes.” How can this be? Well, there is a big difference between correlation in which things occur together and causation in which one thing causes the other. Just because cannabis is present in drivers involved in a crash doesn’t mean it caused the crash. As an example, young male drivers are and have always been the group with the highest rate of motor vehicle crashes, as any insurance agent will tell you. They are also the population with the highest rate of cannabis use. Finding THC in this group and saying it caused the accidents is disingenuous by not considering the differences between correlation and causation.
Why is this important to medical cannabis patients? Well you are subject to the law. If you live in a state with strict zero tolerance for THC and/or metabolites or pro se limits of THC in the bloodstream, every time you get behind the wheel, even if not impaired, you are legally guilty of driving under the influence. Consequences of a conviction can be devastating. Lobby your legislators to change the law and keep your status as a medical cannabis patient to yourself at traffic stops. If you are in a state without these pro se laws, pay attention to what the states legislators are doing and make your voice heard before they pass such laws. The prohibitionists are pushing for such legislation in many states. [Currently, Arkansas has no such laws]
A final word of advice. Never drive after consuming cannabis if you feel the effects. Patients taking oral cannabis should be tolerant to its effects before using and again should not drive if feeling the psychological effects. That is just common sense. Medical cannabis is medicine. Let us all treat it as such with the respect it and the patients who rely on it deserve.