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This paper reports a quasi-experimental evaluation of California’s 1996 medical marijuana law (MML), known as Proposition 215, on statewide motor vehicle fatalities between 1996 and 2015.

To infer the causal impact of California’s MML enactment on statewide motor vehicle fatalities, we construct a synthetic control group for California (i.e., California had it NOT enacted MMLs) as a weighted sum of annual traffic fatality time series from a donor pool of untreated (no MML) states. The post-MML difference between California and its constructed counterfactual reflects the net effect of MMLs on statewide traffic fatalities. The synthetic control group design avoids the problematic homogeneity assumptions intrinsic to panel regression models, which have been employed in prominent studies of this topic.

California’s 1996 MML appears to have produced a large, sustained decrease in …