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A study conducted by the Insurance Institute for Highway Safety reviewed insurance claims for vehicle crashes from 2012 to 2016 and found that in states which had legalized marijuana, there was a three percent increase in collision rates relative to neighboring states who did not legalize marijuana. However, a different study published in the American Journal of Public Health found no increase in car collision fatality rates in Colorado and Washington. The two competing studies released, what initially appeared to be, conflicting results regarding the effect of legalized marijuana on car collisions.

Interpreting Results: Increased or No Increase?

These studies, while appearing to contradict one-another, actually measure slightly different issues. The first study examines car collisions overall. Conversely, the second study is limited to examining fatalities from car collisions. It is possible that marijuana could increase car accidents but not fatalities. Marijuana is linked to an impairment in driving capability but not nearly to the same extent as alcohol. It is, therefore, possible that marijuana could result in increased car crash rates but not necessarily an increase in fatalities.

Marijuana Resulting in Higher Crash Rates

Colorado and Washington were the first states to legalize marijuana in November 2012 with retail sales beginning in January 2014 in Colorado and July 2014 for Washington. Oregon approved legalized marijuana in November 2014 with recreational sales starting October 2015. The study examined Utah, Wyoming, Montana, Nevada, and Idaho as control states to compare against the three legalized states. The study found a three percent increase in car accident rates in states that legalized marijuana versus those that did not. The study stressed that three percent is a slight increase in crashes and could be attributed to other factors.

The study is crucial because, so far, researcher’s haven’t been able to determine the effect, if any, that marijuana has on driving rates. Most studies have found no link between marijuana use and car crashes; even a large-scale federal study failed to find any link. Some medical studies found that marijuana use does slightly degrade driving capability but does not necessarily result in a link to increased car crash rates. It is believed that additional studies will be necessary to improve highway safety to address widespread legalized marijuana.