If Medical Marijuana Laws Cause a 'Surge in Drugged Driving Deaths,' Why Are Fatalities Falling?

A
study published by the American Journal of
Epidemiology last month found that 12.2 percent of drivers
killed by car crashes in six states tested positive for cannabinol,
a marijuana metabolite, in 2010, up from 4.2 percent in 1999. Here
is how NBC News translated that finding in the headline over a

story posted on Saturday: “Pot Fuels Surge in Drugged
Driving Deaths.” The article, which begins by describing the deaths
of a Colorado woman and her infant son in a crash caused by “a
driver who admitted he smoked pot that day,” links the purported
surge in marijuana-related traffic fatalities to laws allowing
medical use of cannabis. “As medical marijuana sales expanded into
20 states,” writes health reporter Bill Briggs, “legal weed was
detected in the bodies of dead drivers three times more often
during 2010 when compared to those who died behind the wheel in
1999.” There are several problems with reading the trend described
by this study as evidence that legalizing medical marijuana causes
an increase in fatal car crashes:

1. The fact that cannabinol was detected in a driver’s blood
does not mean he was under the influence at the time of the crash,
let alone that marijuana caused the crash. “It is possible for a
driver to test positive for cannabinol in the blood up to 1 week
after use,” the researchers note. “Thus, the prevalence of
nonalcohol drugs reported in this study should be interpreted as an
indicator of drug use, not necessarily a measurement of drug
impairment.”

2. Only three of the six states included in the study (which
were chosen because they routinely do drug testing on drivers
killed in crashes) have medical marijuana laws: California, Hawaii,
and Rhode Island. 

3. Traffic fatalities
fell by more than 20 percent nationwide during the study
period, even as “medical marijuana sales expanded.” Between
enactment of its medical marijuana law in 1996 and
2010, California saw a 31
percent drop in traffic fatalities. The number of traffic
fatalities also fell in
Hawaii and
Rhode Island after they legalized medical marijuana—by 14
percent and 21 percent, respectively.

4.  A
study published last year by the Journal of Law &
Economics found that adoption of medical marijuana laws
is associated with a decline in traffic fatalities,
possibly because people in those states are substituting marijuana
for alcohol, which has a more dramatic impact on driving ability.
Briggs mentions that study in the 17th paragraph of his
article.

It is important to keep these points in mind as more states
liberalize their marijuana laws, especially since “preventing
drugged driving” is one of the “enforcement
priorities” that the Justice Department says might justify
federal interference with legalization in Colorado and Washington.
If “drugged driving” means operating a motor vehicle with any
detectable amount of cannabinol in your blood, “drugged driving”
inevitably will rise after legalization as consumption rises. But
having cannabinol in your blood is not the same as being
intoxicated. And even if the share or absolute number of traffic
fatalities caused by marijuana-related impairment rises, the total
number of fatal accidents could still drop thanks to substitution
effects. Regardless of what happens with traffic fatalities, the
possibility of marijuana-related accidents is a reason for
discouraging people from driving while impaired, not a reason for
prohibiting the drug altogether.