The United States is currently in the middle of a massive wave of marijuana legalization. Despite federal law still classifying the drug as a Schedule 1 substance, 12 states and districts have passed recreational use laws, and another 14 have decriminalized possession and use. Alongside Canada and Uruguay's nation-wide legalizations, it is clear the progressive wave of acceptance is growing.

However, there are plenty of questions surrounding the broader societal consequences of legalization that have yet to be clearly answered. One of the big issues, frequently raised by anti-legalization advocates, is the association between marijuana and traffic accidents.

As more time passes, scientists are able to gather larger sets of data, and more accurately understand the impact of marijuana legalization on traffic accidents and fatalities. A new study from two Australian researchers has collected seven years of data from Colorado, Washington, and Oregon – three of the earliest US states to legalize marijuana.

The study revealed that traffic fatalities did slightly rise in the immediate months following marijuana legalization. The researchers calculated there was about one extra traffic fatality per million citizens, per month, in the states that legalized marijuana and the neighboring jurisdictions. These spill-over effects into neighboring states were most prominent in large populations closer to the border.

"The results suggest that legalizing the sale of cannabis for recreational use can lead to a temporary increase in traffic fatalities in legalizing states," says Tyler Lane, one of the authors of the study. "This spills over into neighboring jurisdictions through cross-border sales, trafficking, or cannabis tourists driving back to their state of residence while impaired. "

The most interesting revelation in the study, however, was the finding that these traffic fatality increases seem to only be temporary. The rise in fatal incidents seemed to peak a few months into legalization, and then drop back to normal after about one year. The researchers hypothesize this rise and fall is possibly related to both a celebratory response resulting in riskier behaviors, and the increased use of marijuana by more inexperienced users who are less accustomed to the impairment effected by the drug.

Of course, this study does present some significant limitations, the biggest being that it is only taking into account overall traffic fatality numbers. There is no detail or distinction in the cause of individual fatalities. So, realistically, the increase in fatality numbers may have very little to do with marijuana use, although even if every individual extra fatal accident did have an explicit relationship with marijuana use, these are still relatively small, and most importantly, temporary increases.

Another recent study trying to determine a relationship between marijuana legalization and traffic fatalities came to a similar conclusion. That study constructed what is referred to as a "synthetic control", allowing for a relatively similar control group to be created to compare to the legalized marijuana states. That research concluded, "… since legalizing marijuana, Colorado and Washington have not experienced significantly different rates of marijuana- or alcohol-related traffic fatalities relative to their synthetic controls."

Of course, traffic fatalities are just one metric by which to examine the effects of marijuana legalization on road safety. While fatalities may not notably increase, any effect on the rates of car collisions in general and physical injuries is a different story yet to be clearly studied. More studies are certain to come soon.

The new research was published in the journal Addiction.