In November, 1996, California voters approved Proposition 215, which legalized the use of marijuana for medical purposes. Campaigns for and against the Proposition focused largely on the medical need for and effectiveness of marijuana. Although the debate emphasized cancer, AIDS, and spinal cord injuries, the Proposition’s language included virtually all chronic conditions, thereby effectively decriminalizing most marijuana use (Vitiello, 1998). Cultural values played a smaller role in the campaign. Opponents argued that legalization would “send the wrong message” to youth, for example, and this fear was realized to some extent. Public opinion surveys conducted before and after the election found increased acceptance of marijuana by Californians but no change in actual use (Khatapoush and Hallfors, 2004). The potential collateral consequences of legalization played no role in the public debate over Proposition 215. Would easier access to marijuana lead to an increase in driving under the influence, for example, and if so, would the higher prevalence of driving under the influence lead to an increase in traffic fatalities?
To test this hypothesis, Anderson et al. (2013) compared traffic fatality rates before and after legalization of medical marijuana in fourteen states and the District of Columbia. Their surprise finding: traffic fatality rates decreased in medical marijuana states. How is that possible? Anderson et al. speculate that, unlike alcohol which is consumed in public places, marijuana is consumed at home. Though one should view all post facto explanations with some skepticism, this one makes some sense. Another possibility, of course, is that Anderson et al. failed to control plausible threats to internal validity. To investigate this possibility, we devise a time series experiment for California’s Proposition 215 using a synthetic California time series to control the common threats to internal validity. Our finding, that motor vehicle fatalities in California dropped after medical marijuana was legalized with Proposition 215, supports Anderson et al.’s (2013) finding,