To date, 16 states [in the US] have passed medical marijuana laws, yet very little is known about their effects. Using state-level data, we examine the relationship between medical marijuana laws and a variety of outcomes. Legalization of medical marijuana is associated with increased use of marijuana among adults, but not among minors. In addition, legalization is associated with a nearly 9 percent decrease in traffic fatalities, most likely to due to its impact on alcohol consumption. Our estimates provide strong evidence that marijuana and alcohol are substitutes.
That is from this new paper by Anderson and Rees.
The authors conclude:
Using data from the Fatality Analysis Reporting System (FARS) for the period 1990- 2009, we find that traffic fatalities fall by nearly 9 percent after the legalization of medical marijuana. This effect is comparable in magnitude to those found by economists using the FARS data to examine other policies. For instance, Dee (1999) found that increasing the minimum legal drinking age to 21 reduces fatalities by approximately 9 percent; Carpenter and Stehr (2008) found that mandatory seatbelt laws decrease traffic fatalities among 14- through 18-year- olds by approximately 8 percent.
Why does legalizing medical marijuana reduce traffic fatalities? Alcohol consumption appears to play a key role. The legalization of medical marijuana is associated with a 6.4 percent decrease in fatal crashes that did not involve alcohol, but this estimate is not statistically significant at conventional levels. In comparison, the legalization of medical marijuana is associated with an almost 12 percent decrease in any-BAC fatal crashes per 100,000 licensed drivers, and an almost 14 percent decrease in high-BAC fatal crashes per 100,000 licensed drivers.
The negative relationship between legalization of medical marijuana and traffic fatalities involving alcohol is consistent with the hypothesis that marijuana and alcohol are substitutes. In order to explore this hypothesis further, we examine the relationship between medical marijuana laws and alcohol consumption using data from the Behavioral Risk Factor Surveillance System and The Brewer’s Almanac. We find that the legalization of medical marijuana is associated with decreased alcohol consumption, especially by 20- through 29-year-olds. In addition, we find that legalization is associated with decreased beer sales, the most popular alcoholic beverage among young adults (Jones 2008).
However, because other mechanisms cannot be ruled out, the negative relationship between medical marijuana laws and alcohol-related traffic fatalities does not necessarily imply that driving under the influence of marijuana is safer than driving under the influence of alcohol.