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
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.
Brian Nichol MD