Anderson, Hansen and Rees look at Medical Marijuana Laws, Traffic Fatalities, and Alcohol Consumption:
To date, 17 states have passed medical marijuana laws, yet very little is known about their effects. The current study examines the relationship between the legalization of medical marijuana and traffic fatalities, the leading cause of death among Americans ages 5 through 34. The first full year after coming into effect, legalization is associated with an 8 to 11 percent decrease in traffic fatalities. The impact of legalization on traffic fatalities involving alcohol is larger and estimated with more precision than its impact on traffic fatalities that do not involve alcohol. Legalization is also associated with sharp decreases in the price of marijuana and alcohol consumption, suggesting that marijuana and alcohol are substitutes. Because alternative mechanisms cannot be ruled out, the negative relationship between legalization 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.
The decline in alcohol consumption is consistent with the fact that alcohol producers have opposed marijuana legalization. Hat tip: Scott Cunningham.