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Legal Cannabis Linked to Traffic Deaths, Studies Find

By Leo Bear-McGuinness

Published: Jun 23, 2020   
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Several US states have seen an increase in traffic deaths since they gained legal access to recreational cannabis, two new studies have found.

One study found that Colorado has experienced an average of 75 excess road deaths per year since legalizing recreational cannabis in 2014.

Neither piece of research can explain how legal cannabis is linked to the increases in car-related deaths; as observational studies, they only illustrate a trend in the data.

Colorado, Washington, and two more

To get their findings, the researchers of one study went through the National Highway Traffic Safety Administration’s list of reported traffic fatalities from 2005 to 2017.

To act as a comparison, the research team created a “synthetic control” state using data drawn from several eligible states that best resembled the fatality rates of Washington and Colorado pre-legalization.

After running this statistical simulation, the team found that Colorado differed by 1.46 deaths per 1 billion from its synthetic control. This difference translated into an estimated 37 excess deaths from 488 traffic fatalities in 2014, 63 more deaths in 2015, 78 more in 2016, and 123 more in 2017. On average, these figures come to 75 excess car-related fatalities per year.

Traffic deaths in Washington, on the other hand, didn’t differ significantly from the state’s synthetic control.

“This study provides evidence that implementation of legal commercial retail sales of cannabis was associated with increased traffic fatalities in Colorado but not in Washington state,” the researchers wrote in their paper, which was published in JAMA Internal Medicine.

The authors think this difference might come down to the number of retail stores each state has.

“In December 2018, Colorado had 549 retail stores and 474 medical marijuana centers compared with 433 retail stores in Washington State,” they wrote in their discussion section.

“These factors may have contributed to higher cannabis availability and driving under the influence in Colorado than in Washington state.”

Colorado’s cannabis tourism, they say, may also play a part in its higher traffic fatality rate.

In a separate research letter, a different research group used the National Highway Traffic Safety Administration’s fatality list to investigate two more states, Alaska and Oregon.

After combing through data sourced as recently as 2018, the researchers found an average increase of 2.1 traffic fatalities per billion vehicle miles traveled in the four states relative to a generated control.

“We have provided additional data that legalization of recreational marijuana is associated with increased traffic fatality rates,” the wrote in their research letter, which was also published in JAMA Internal Medicine.

“Applying these results to national driving statistics, nationwide legalization would be associated with 6800 excess roadway deaths each year.”

From speed limits to study limits

As both studies were conducted from traffic data, the researchers didn’t know what factors influenced each fatality. And so, it’s possible that cannabis may never have been involved in each traffic death.

“FARS [Fatality Analysis Reporting System, provided by the National Highway Traffic Safety Administration] data do not indicate whether the driver was driving under the influence of cannabis when the event occurred,” the Colorado-Washington researchers wrote in their conclusion.

Both research groups therefore acknowledge that other factors, entirely separate from marijuana legalization, could be the actual cause of the increase in roadway deaths.

Nonetheless, the authors believe their findings can be helpful in informing better public health policies for cannabis commercialization.

Regarding the relationship between marijuana and the ability to drive, one extensive review of 60 studies found that cannabis affected all areas relevant to safe driving, including psycho-motor skills, continued attention, visual function, and reaction time.

But another study found that, when compared to drunk drivers, high drivers had an increased awareness that they were impaired and were often able to compensate for their intoxication.

“Marijuana smokers tend to compensate effectively for their impairment by utilizing a variety of behavioral strategies such as driving more slowly, passing less, and leaving more space between themselves and cars in front of them,” the authors summarized in their conclusion.


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