A team of researchers found no association between adult-use cannabis legalization and an increase in the number of motor vehicle crash victims who test positive for THC.

The findings come courtesy of researchers at the Hackensack Medical Center in New Jersey who wanted to determine whether state-level cannabis laws appear to have an impact on the detection of cannabis, as well as alcohol, in people who’ve been hospitalized as a result of a motor vehicle crash. Opponents of cannabis legalization frequently cite increased instances of impaired driving as a reason to maintain marijuana prohibition.

To arrive at their conclusions, published in the journal The American Surgeon, the researchers analyzed trauma center data in six states over a 12 year period from 2006 to 2018. Two of those states – California and Oregon – legalized the use and sales of cannabis in 2016 and 2014 respectively. The other four states – Arizona, Ohio, New Jersey and Texas – did not allow for recreational marijuana use or sales over the study period.

The researchers reported the incidences of motor vehicle crash patients testing positive for marijuana increased in all states over time but the largest increase was found in Texas (15.3 percent), where adult-use cannabis remains prohibited. Alongside New Jersey (2.3 percent), both California (5.4 percent) and Oregon (3 percent) had the smallest increase of all the states under investigation.

“There did not appear to be a relationship between the legalization of marijuana and the likelihood of finding THC in patients admitted after MVC (a motor vehicle crash). There was no apparent increase in the incidence of driving under the influence of marijuana after legalization,” the study’s authors wrote in their conclusion.

Prohibitionist claims that marijuana legalization leads to more impaired driving and thus more road fatalities have not been validated by research to date. Some studies reported a small increase in marijuana-related vehicle crashes several years after a state legalized cannabis, while others have found no such association. This inconsistency is reflected in a congressional report that said claims marijuana use impairs driving are inconclusive. That said, driving under the influence of marijuana is strictly prohibited in all states and is not to be encouraged.

There also remains the problem of measuring a driver’s impairment due to marijuana consumption. THC, the psychoactive component of cannabis, remains in a user’s system long after its intoxicating effects have worn off, meaning roadside tests could produce false positives. Oklahoma launched a pilot roadside breathalyzer program last year using technology from a company that claims its devices can give an instant and accurate measurement of an individual’s THC levels. What constitutes an impaired level of THC intoxication, however, remains to be seen. Meanwhile, federal lawmakers are working on a bipartisan bill to prevent drug-impaired driving with the input of NORML, the country’s leading marijuana reform advocacy organization.