We find that, since legalizing marijuana, Colorado and Washington have not experienced significantly different rates of marijuana‐ or alcohol‐related traffic fatalities relative to their synthetic controls. To ensure that our results are not driven by an idiosyncratic selection of control weights, we show that we obtain similar results across reasonable variations in the preperiod specification used to create the weights that generate the synthetic control counterfactual. In addition to examining fatalities identified by states as drug‐ or alcohol‐related, we also look for changes in the overall fatality rate to avoid state‐level differences in classification (as opposed to state‐level differences in testing) and find a similar null result.
Several mechanisms may be driving these results. The amount of marijuana sold in recreational stores has grown dramatically, increasing from 3,991 pounds in Washington in 2014 to 179,301 pounds in 2017, while in Colorado it grew from 36,031 pounds in 2014 to 102,871 pounds in 2016. However, it is difficult to discern how much of that growth in legal recreational marijuana came at the expense of sales in blackmarket or medical marijuana. Indeed, recreational marijuana can be viewed as a close substitute to black‐market or medical marijuana, with differences in price, quality, and ease of access. The relatively small effects we estimate are consistent with a story of the legal market crowding out the illegal market, and they could explain why we don’t observe spillover effects on alcohol‐related traffic accidents as other studies have found. It is also possible that marijuana impairment may be associated more with nonfatal accidents and property damage than, for example, alcohol impairment is.
Although our results suggest that the marijuana legalization in Colorado and Washington did not cause discernible increases in traffic fatalities, estimating the externalities of marijuana abuse and driving while high is still crucial in determining future policy. Indeed, Colorado has recently chosen to allow the consumption of marijuana in public spaces, which may increase the potential for negative externalities. Whereas Colorado and Washington have set the legal limit for driving while high at 5 nanograms of THC per milliliter of blood, we don’t yet know if the sanctions for driving while high will be effective in discouraging that behavior given the local population of drivers affected by that threshold. Furthermore, there is still ample debate about what the right legal threshold would be and whether the threshold should even be based on THC. Although the use of blood alcohol content is common today for measuring impairment in drunk driving, it took decades of research and innovation from the passage of the first drunk driving laws to the creation of the first Breathalyzers. Science and policy alike are playing catchup in measuring both the incidence and the relative risks of driving while high.
This research brief is based on Benjamin Hansen, Keaton S. Miller, and Caroline Weber, “Early Evidence on Recreational Marijuana Legalization and Traffic Fatalities,” NBER Working Paper no. 24417, March 2018, http://www.nber.org/papers/w24417.