In a recent study, the Governors Highway Safety Association noted that pedestrian fatalities in states with legal marijuana were higher in 2017 than they were in 2016. What they don't note is that the "increase" was virtually statistically insignificant. Colorado was the only state with a significant increase in pedestrian deaths at 12 percent. But that's not very significant when you realize that means Colorado's pedestrian deaths went from 33 in the first half of 2016 to 37 in 2017.
The GHSA hasn't made any conclusions about marijuana and driving. While they note that states with legalized cannabis have three percent more auto insurance claims than their neighbors, they haven't studied how legalization has affected auto fatalities.
Meanwhile, there are several other factors that explain why traffic fatalities have increased greatly over the past decade. First of all, there are simply more people driving. When more people drive and for longer distances, there are going to be more accidents. They also note that smartphone usage has tripled from 2010 to 2016, which they ay is a much more likely factor for the increase in deaths.
It should also be noted that states with legalized marijuana were not the worst offenders when it comes to pedestrian deaths. California, Texas, Florida, New York and Arizona accounted for 43 percent of all pedestrian deaths (the data is from 2017, before California legalized marijuana). And the states with the highest rates of pedestrian deaths were Arizona, New Mexico, Delaware, Louisiana and Florida. None of those are states that had legalized marijuana in 2017.
In short, anyone who claims that marijuana legalization leads to more traffic fatalities simply doesn't have the data to back it up.
(h/t New York Times)