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Cannabis Legalization Associated with Increase in Driving Accidents

By Alexander Beadle

Published: Apr 09, 2021   
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New study of data from the United States has revealed a “small but significant increase” in fatal motor vehicle accidents and fatalities following the legalization of recreational cannabis in select states.

The study authors, from the McGill University-affiliated Lady Davis Institute, Quebec, also published an accompanying analysis piece which reflects on this trend and what this could mean in the Canadian context.

The authors estimate that Canada’s nationwide legalization of cannabis may result in as many as 308 additional deaths due to cannabis-impaired driving, but that the implementation of effective regulations and educational campaigns could help prevent this potential increase in accidents.

US legalization linked to 15 percent increase in the relative risk

The new study, published in the Canadian Medical Association journal, CMAJ Open, looked at data taken from the US Fatality Analysis Reporting System between 2007 and 2018. With this information, the researchers examined jurisdiction-specific rates of fatal motor vehicle collisions and associated deaths before and after recreational cannabis legalization in key states.

After adjustment, the researchers found that legalization of cannabis was associated with an increased relative risk of fatal motor vehicle collisions of 15 percent, and a relative increase in associated deaths of 16 percent.

This is not the first work to examine the impact of cannabis legalization on motor vehicle accidents, as the accompanying analysis points out. One 2019 study compared the traffic accident fatality statistics in three states with legal recreational cannabis to those of neighboring states without recreational legalization.

This study found a noticeable but temporary increase in traffic fatalities in the first year after legalization, which the authors believe were due to an increase in inexperienced cannabis users and/or a possible celebratory response to legalization. On a related note, this latest work also examined changes in the first 12 months after legalization, but in comparison to subsequent years, and found no conclusive evidence of any significant differences.

Taking the results from their own study together with others available in the literature, the Lady Davis Institute/McGill University researchers conclude that the legalization of recreational cannabis is likely associated with “a small but important relative increase in fatal motor vehicle collisions in the US.”

Authors raise concern about similar issues arising in Canada

Given Canada’s nationwide legalization of recreational cannabis and its recent rollout of Cannabis 2.0, the researchers were interested to see what these results could mean if the same relative risk increase held true for the Canadian market.

“Analyses of data suggest that legalization of recreational cannabis in United States jurisdictions may be associated with a small but significant increase in fatal motor vehicle collisions and fatalities, which, if extrapolated to the Canadian context, could result in as many as 308 additional driving fatalities annually,” said the study’s first author Sarah Windle in a statement.

However, a multi-disciplinary approach to furthering regulatory and public health measures that deter cannabis-impaired driving could help to curb such an increase being seen in Canada, the authors say.

Canada already employs the use of roadside psychomotor tests to screen for drug-impaired drivers and has officers specially trained in assessing potential impairment. However, since this roadside testing was initially intended just for alcohol impairment, it is not known how effective these tests are at screening for cannabis impairment. Additionally, while the expert officers may be more able to assess impairment, they are of a limited number and are not medically trained, which limits their usefulness.

While the practical application of saliva-based tests for THC is unclear – as there is no demonstrable correlation between certain THC levels in the body and impairment that could stand up in court – Canada’s CBill C-46 does allow for such roadside devices to be used by law enforcement if cannabis impairment is suspected. And while there is some uncertainty over test accuracy and what THC levels indicate impairment, messaging that makes the public aware of these devices and the penalties of resulting from cannabis-impaired driving are still likely to be effective as a deterrent.

Additionally, the authors believe that healthcare professionals could help to curb the risk of a rise in cannabis-impaired driving by informing their patients – both those who are receiving medical cannabis and those who are known to use recreationally – about Canada’s Lower-Risk Cannabis Use Guidelines. This guidance advises that cannabis users wait at least 6 hours after use before driving, choose lower-potency products, and to never combine the use of cannabis and alcohol.

“Health care professionals have an opportunity to educate patients about the safer use of cannabis products, including advising against cannabis use and driving (especially in combination with alcohol), with a suggested wait time of at least 6 hours before driving,” the authors say.

Cannabis use and driving – the wider context

With an increase in the number of territories that are legalizing medicinal and/or recreational cannabis use, it has become even more important that the effects of cannabis impairment on driving be studied.

Much of the research from industry has focused on the development of cannabis impairment detection, first through adapted breathalyzers, then through biosensor-based saliva tests. Recently researchers announced the development of a new type of breathalyzer which they claim can “detect THC at levels comparable to or better than mass spectrometry.”

But detection is only half the story. Research is also trying to assess how exactly cannabis affects driving skills. One extensive review covering 60 individual studies found that cannabis affected all skill sets related to safe driving, including psycho-motor skills, continued attention, visual function, and reaction time. Other studies have focused on comparisons to other drugs and the effects of individual cannabinoids. One study found cannabis-impaired drivers to be more aware of their impairment and more able to compensate for this than drunk drivers. Another indicated that pure CBD use likely does not pose any risks to drivers.

Studies have also shown that more than half of American cannabis consumers would feel safe if they themselves were riding in a car where the driver was under the influence of cannabis. By comparison, three-quarters of respondents who identified as non-users said that they would feel somewhat-to-very unsafe.


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