June 22 (UPI) -- Traffic fatalities have increased in Colorado since the state legalized the use of recreational marijuana, an analysis published Monday by JAMA Internal Medicine has found.
Since the state opted to legalize recreational use of the drug in 2014, there have been an additional 75 deaths resulting from traffic accidents, on average, annually, the researchers estimated.
Conversely, Washington state, which lifted prohibitions on marijuana the same year, has seen traffic-related deaths remain relatively stable, they said.
"We observed that recreational cannabis laws were associated with increases in traffic fatalities in Colorado but not in Washington state, [perhaps due to] the size of the marijuana industry in Colorado, evidence of cannabis tourism in Colorado and other local aspects," study co-author Julian Santaella-Tenorio told UPI.
According to Santaella-Tenorio, things such as purchasing limits, sales taxes, ability to grow cannabis at home and density of retail story could be among the difference makers.
"Any legal or illegal medication/drug that impacts neuro-cognitive and neuro-motor skills can increase the risk of traffic events [and] higher rates of traffic events are likely if more drivers are driving under the influence of cannabis in combination with alcohol and/or other drugs," said Santaella-Tenorio, an epidemiologist at Columbia University.
To date, 11 states and Washington, D.C., have legalized marijuana. These states have changed the way they define -- and prosecute -- impaired driving as a result, according to research.
Earlier studies have suggested that traffic accidents in general have increased in many of these states since the drug was legalized.
For the new analysis, Santaella-Tenorio and colleagues reviewed data on traffic fatalities in both states from the Fatality Analysis Reporting System. Their review covered a 12-year period from 2005 through 20017.
To estimate differences, the researchers created "synthetic controls" for both states, an approach that uses an algorithm to identify a combination of states from a pool of control states with similar traffic fatality rates.
Compared to its synthetic control, Colorado since legalization has seen 1.46 traffic deaths per 1 billion vehicle miles traveled, or VMT, per year, they found. Washington state, however, has had 0.08 deaths per 1 billion VMT per year since legalization, they said.
"People are concerned with the harmful unintended consequences [legalization] legislation may pose on communities," Santaella-Tenorio said. "It is important to conduct policy evaluations studies that can shed light on these effects."