No, A Study Didn’t Say Cannabis Impairs Driving Once You’re Sober

A man undergoes a sobriety test at a LAPD police DUI checkpoint in Reseda, Los Angeles, California ... [+]AFP VIA GETTY IMAGES

By Janet Burns

In today’s climate, confusion and misinformation about cannabis can move quickly, like other aspects of the growing ‘legal industry.’ But scientific study is a necessarily slow process, and media outlets must learn to treat it as such if our society ever hopes to get its cannabis regulations, stigma, and health risks sorted out.

This week, for example, dozens of media outlets referenced a recent small study on cannabis and driving under various headlines, suggesting the study found that long-term, early, or even any cannabis use may cause impaired driving in the days or years to come.

It found none of these things.

If you only read certain news articles about it, or (to be fair) the potentially misleading title and top points of the study — or if you’re not used to decoding scientific studies generally — that fact is easy to miss.

For cannabis medical patients who, for example, carefully monitor their use and were reasonably planning to drive the kids to soccer practice tomorrow, the popular but erroneous interpretation of this study’s results may also be deeply disturbing.

The study in question, prepared by researchers at Harvard’s Marijuana Investigations for Neuroscientific Discovery (MIND) Program, aimed to explore (in a preliminary way) whether heavy cannabis users who began using the plant earlier in life display poorer driving habits than people who started later on.

To test this hypothesis, they recruited 28 people who they defined as heavy cannabis users, and 16 ‘healthy control’ (HC) participants, meaning they don’t use cannabis nor have any known health problems. Cannabis users couldn’t have used the drug for at least 12 hours, and researchers administered triage-quality rapid drug tests (such as police or EMTs might carry) to ensure their cannabis-using subjects really use it, and HCs really don’t. Cannabis users’ reported intake ranged from ~1 to 11 grams a week.

These 44 people who live near Boston, ranging in age from about 18 to 30, were then asked to complete a driving simulation — complete with fake chair, fake wheel and pedals, and a 19-inch monitor showing a simulated roadway — so that their performance could be measured in terms of lane use, speeding, hitting pedestrians, observing road signals, and a few other metrics.

Results showed that the heavy cannabis users were slightly more likely than HCs to miss stop signs, speed, and hit the animated pedestrians; other driving metrics were about the same between groups. Cannabis users also completed the simulation more quickly. Apparently none of the participants made any illegal turns.

As part of their fairly extensive analysis of the fairly limited data they gathered, researchers also divided heavy cannabis users into two groups: those who started using it at the age of 16 or younger, and those who started after that. Broadly speaking, the early-use group performed the worst in those areas where driving habits diverged.

Researchers had also asked participants to complete a separate behavioral survey tied to impulsiveness, which they then compared to subjects’ cannabis histories. As it happens, cannabis users self-reported that they had somewhat lower attention spans and higher non-planning tendencies (i.e. impulsiveness) than non-users.

At best, this study could be seen as a pilot for further investigation down the road, according to cannabis and research experts who’ve weighed in this week — in part, perhaps, because of certain questionable methods or decisions therein, which researchers would do well to avoid in the future if accuracy is what they’re after.

For one thing, it’s important to recognize what the study did and didn’t seek to examine, and exactly what that study found, or did not.

Paul Armentano, Deputy Director for the cannabis reform nonprofit NORML, commented by email, “To be clear, this paper identifies differences in driving behaviors between non-users and those who acknowledge being heavy users of cannabis and having begun their use prior to the age of 16. But, as acknowledged by one of the paper’s authors, differences in driving style should not be conflated with driving impairment.”

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