Drug warriors often argue that looser marijuana laws result in more stoned drivers on the road, leading to more traffic accidents. A recent study of states that allow medical use of marijuana suggests the opposite may be true: Wider use of marijuana could, on balance, make the roads safer.
According to research published in November by the Institute for the Study of Labor, a German think tank, medical marijuana laws in the United States have been associated with a 9 percent decline in traffic fatalities. That result is based on data from the Fatality Analysis Reporting System for 13 states that legalized medical use of cannabis between 1990 and 2009. The study's authors, Montana State University economist D. Mark Anderson and University of Colorado at Denver economist Daniel Rees, argue that the most plausible explanation is the substitution of marijuana for alcohol, since laboratory research indicates that smoking pot impairs driving ability substantially less than drinking does.
In support of that hypothesis, which implies that much ostensibly medical use of marijuana is actually recreational (as prohibitionists commonly complain), Anderson and Rees cite data from the National Survey on Drug Use and Health for three states that enacted medical marijuana laws during the last decade: Montana, Vermont, and Rhode Island. Marijuana use by adults (but not teenagers) rose more in those states after they adopted their laws than it did in neighboring states, a difference that was statistically significant in Montana and Rhode Island. Anderson and Rees also note that the drop in fatal car accidents associated with medical marijuana laws was statistically significant for crashes involving alcohol but not for other crashes, and they present evidence that alcohol consumption has declined in medical marijuana states.
If people are substituting marijuana for alcohol, any impact on traffic fatalities might be due to shifting locations of consumption rather than the difference between the two drugs' effects on driving ability. Since alcohol is more likely to be consumed in public places, Anderson and Rees write, drinkers are more likely than pot smokers to end up driving, meaning that "legalization could reduce traffic fatalities even if driving under the influence of marijuana is every bit as dangerous as driving under the influence of alcohol."