Evidence suggests that legalizing marijuana could make the roads safer, reducing traffic fatalities by encouraging the substitution of marijuana for alcohol.
A major reason to doubt the premise that more pot smoking means more deadly crashes: Total traffic fatalities have fallen as cannabis consumption has risen; there were about 20 percent fewer in 2012 than in 2002. Perhaps fatalities would have fallen faster if it weren’t for all those new pot smokers. But there is reason to believe the opposite may be true, that there would have been more fatalities if marijuana consumption had remained level or declined.
While marijuana can impair driving ability, it has a less dramatic impact than alcohol does. A 1993 report from the National Highway Traffic Safety Administration, for example, concluded:
The impairment [from cannabis] manifests itself mainly in the ability to maintain a lateral position on the road, but its magnitude is not exceptional in comparison with changes produced by many medicinal drugs and alcohol.
Drivers under the influence of marijuana retain insight in their performance and will compensate when they can, for example, by slowing down or increasing effort. As a consequence, THC’s adverse effects on driving performance appear relatively small.
Cannabis, the Safer Alternative
Given these differences, it stands to reason that if more pot smoking is accompanied by less drinking, the upshot could be fewer traffic fatalities. Consistent with that hypothesis, a study published last year in the Journal of Law and Economics found that legalization of medical marijuana is associated with an 8-to-11-percent drop in traffic fatalities, beyond what would be expected based on national trends. Montana State University economist D. Mark Anderson and his colleagues found that the reduction in alcohol-related accidents was especially clear, as you would expect if loosening restrictions on marijuana led to less drinking. They also cite evidence that alcohol consumption declined in states with medical marijuana laws.
A study published last month by the online journal PLOS One suggests that the substitution of marijuana for alcohol, assuming it happens, could affect crime rates as well as car crashes. Robert G. Morris and three other University of Texas at Dallas criminologists looked at trends in homicide, rape, robbery, assault, burglary, larceny, and auto theft in the 11 states that legalized marijuana for medical use between 1990 and 2006.
While crime fell nationwide during this period, it fell more sharply in the medical marijuana states, even after the researchers adjusted for various other differences between states. Morris and his colleagues conclude that legalization of medical marijuana “may be related to reductions in rates of homicide and assault,” possibly because of a decline in drinking, although they caution that the extra drop in crime could be due to a variable they did not consider.
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