This seems to be a pretty thorough, wide-spread study. The rate of traffic deaths declined more in the states with legal marijuana laws than in other states, and over a ten-year period.
The study also shows that in those states, beer sales declined five percent overall.
Does it conclusively prove the reduction in traffic deaths in those states was due to the reduced beer consumption in young people, or increased marijuana use? No, but it is good correlation, and doesn't appear to be "mere coincidence."
It appears likely to be associated with the reduction in traffic fatalities.
Either young drivers drink less, and or drive less in states where legal medical marijuana laws have been passed.
It was interesting that hard liquor consumption did not change, just beer consumption, the favorite beverage of younger drinkers.
Sure, it would be better if no one drove after drinking, smoking pot, or taking any medication, but that is not going to happen.