An "Open" sign sits outside a marijuana dispensary in Springfield, OR. Photo: Joshua Rainey Photography

Two studies published last week reached conflicting conclusions on the connection between legalized recreational marijuana and its impact on vehicular accident rates.

One study, published in the American Journal of Public Health on June 22, found that car accident fatality rates did not significantly increase in Washington or Colorado in the first three years after recreational marijuana was legalized in these states.

Researchers from the University of Texas at Austin analyzed fatality data from car crashes that occurred between 2009 and 2015 in Washington and Colorado –  two of the first states that legalized marijuana.

Washington and Colorado both legalized marijuana for adults 21 years and older in November 2012. Oregon voters followed suit in November 2014.

They compared the data with eight control states – Alabama, Indiana, Kentucky, Missouri, South Carolina, Tennessee, Texas and Wisconsin. The control group states were chosen based on having similar traffic, roadway and population characteristics as Washington and Colorado, but none from the control group legally allow recreational marijuana use.

The data used for the study was obtained from the U.S. Fatality Analysis Reporting System. Year-over-year changes in fatality rates were calculated before and after recreational marijuana legalization was passed on a basis of per billion vehicle miles traveled.

According to the study, annual changes in crash fatality rates for Washington and Colorado prior to 2012 – pre-recreational marijuana legalization – were similar to those for the control states. After marijuana was legalized, the team found an increase of .2 fatalities per billion vehicle miles traveled compared to the control states, which the team determined was not “statistically significant."

“Post–recreational marijuana legalization changes in motor vehicle crash fatality rates for Washington and Colorado also did not significantly differ from those for the control states,” they concluded.

But a separate study released by the Highway Loss Data Institute on the same day as the American Journal of Public Health research found that legalized marijuana use in Colorado, Oregon and Washington were linked to a three percent increase in collision claim frequencies. The analysis suggests the three percent higher rate of collision claim frequency is more than what would have been expected if marijuana wasn’t legalized.

Control states for this study included Idaho, Montana, Nevada, Utah and Wyoming. The researchers also considered data from Colorado, Washington and Oregon prior to legalization.

During the study timeframe, Nevada and Montana permitted medical use of marijuana, Wyoming and Utah allowed limited medical use, and Idaho did not have any type of legalization legislation.

The researchers looked through collision claims filed between January 2012 and October 2016 for 1981 to 2017 model vehicles. The team took a few factors into account, such as rated driver population, insured vehicle fleet, mix of urban vs rural exposure, unemployment, weather and seasonality.

"The combined-state analysis shows that the first three states to legalize recreational marijuana have experienced more crashes," said Matt Moore, senior vice president of HLDI. "The individual state analyses suggest that the size of the effect varies by state."

Colorado saw the biggest estimated increase in claim frequency compared with its control states. After retail marijuana sales began in Colorado, the increase in collision claim frequency was 14 percent higher than in nearby Nebraska, Utah and Wyoming. Washington's estimated increase in claim frequency was six percent higher than in Montana and Idaho, and Oregon's estimated increase in claim frequency was four percent higher than in Idaho, Montana and Nevada, according to HLDI.

But there were key differences between the two studies that may have led to the conflicting results. The American Journal of Public Health focused on crash fatalities, whereas the HLDI study was a bit broader and tallied total collision claims. There were also differences in the two studies’ methodologies and control group states.

However, both teams of authors from each study did agree that further research conducted over a longer period of time is warranted to get a better grasp on the potential effects of legalized recreational marijuana on drivers.

HLDI has already initiated a large-scale case study in Oregon to assess how legalized marijuana use may impact the risk of crashes with injuries. Preliminary results are expected in 2020.