Does Legalized Marijuana Result in Increased Auto Accidents?

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After more than a solid decade of continuous decline in auto accidents, the number is gradually going back up in several states.

There have been new studies that have associated marijuana usage with these accidents. With medical marijuana recently legal for various purposes in 28 U.S. states ,as well as the District of Columbia, it seems easy to draw this conclusion. There is a lot of data suggesting the correlation between marijuana use and car collisions.

But could there be other factors at play?

Auto Accidents Appear to Have Increased in States Which Have Legalized Marijuana

A September 2015 report created by the Rocky Mountain High Intensity Drug Traffic Area (RMHIDTA)— a collaboration of federal, state and local drug enforcement agencies—found that marijuana legalization could have resulted in, among other things, an increase in impaired driving.

In Colorado alone, traffic deaths which were linked to marijuana usage increased by 154 percent between 2006 and 2014. Colorado emergency room hospital visits which were “likely related” to marijuana usage increased by 77 percent from 2011 to 2014.

But further research by the U.S. Department of Transportation strongly suggests that marijuana is not the reason for these auto accidents.

In Washington, a survey was conducted to put together research involving drugs and alcohol inhibiting driving skills. The report, released July 2016,  found that of approximately 2,400 participants who voluntarily provided an oral fluid or blood sample, between 15 percent of drivers and 21 percent of drivers tested positive for THC (the active ingredient in marijuana).

The study found that there was a statistically significant increase in daytime prevalence of THC-positive drivers between the time before marijuana legalization and after legalization. However, the study did not find a statistically significant increase in THC-positive nighttime drivers. Of critical importance, this study did not attempt to measure—nor did it find—an increased correlation between auto accidents due to impairment by marijuana.

In short, the study simply found an increase in marijuana use among drivers in Colorado after legalization.

Following the RMHIDTA study, the Highway Data Loss Institute (HDLI) released a study last week which found that collision claims in Colorado, Washington and Oregon went up 2.7 percent in the years since legal recreational marijuana sales began, when compared with surrounding states.

The research the HDLI conducted reveals marijuana use is something that drivers are more often divulging, but it’s not the only component that could be at fault. High-traffic areas, blood alcohol content and age of the driver, amongst other elements have all been factored into the research.

In short, the study found that more studies are needed.

Of note, the Insurance Institute for Highway Safety (IIHS) has begun a large-scale case-control study in Oregon to examine how legalized marijuana use may be changing the risk of crashes with injuries. Preliminary results are expected in 2020.

In a graph shown on The Weed Blog, it was thoroughly explained how marijuana has not seemed to impact the car insurance rates in the states where the drug is legal.

While two cities (Seattle and Denver) do have high insurance costs, it can be expected as such, since they are large cities. When insurance companies put together rates, they look at the traffic area as a whole. If the city is prone to having few traffic accidents, that means the price for insurance could be relatively low. But in high-accident areas, car insurance could be predicted to spike in order to compensate for this.

So while marijuana may not be the cause of car insurance rates increasing, the rates could still go up if states where the drug is legal continue having car accidents.


While the research attempted to lay out the connections in black and white, the full data is relatively inconclusive. It cannot be for certain whether marijuana is the culprit of these increased auto accidents until further research has been completed.

The fact remains that the study found a correlation between legalized marijuana use and increased auto accidents in the aforementioned states between January 2012 and October 2016.

However, there is a distinct difference between correlation and causation.

The former suggests a series of connections between two things, while the latter suggests that an incident is a result of something else occurring. Until we know with more certainty that marijuana is the ultimate cause for the rise in auto accidents, there is little credible support for criticism of marijuana legalization for increase in auto insurance rates in states that have adopted marijuana legalization.

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