Flawed Cannabis Driving Study: Get the Facts

A new flawed cannabis driving study from AAA, the Foundation for Traffic Safety claims that fatal traffic accidents “involving” THC have more than doubled in Washington since legalization in 2012. However, there are numerous problems with this claim.

Here are the findings:

Between 2008 and 2012, before cannabis was legal in Washington state, an “estimated” 8.8 percent of drivers involved in fatal crashes positive for THC—either they tested positive in some way or researchers assumed they were positive. See more on this below.

Between 2012 and 2017, that percentage increased to 18. In 2017, the study authors estimate that 21.4 percent of fatal crash drivers in the state were THC positive—a 10-year peak.

This is not the first misleading “cannabis and driving” study we’ve seen recently. Here are the problems with this study.

Cannabis and driving impairment

Some research indicates that cannabis consumption can impair some aspects of driving, such as motor skills and response times, but the research in this area is limited. Conclusive evidence concerning THC levels and impaired driving simply does not yet exist.

Many factors weigh into the impairment equation with cannabis, which is in part why experts cannot agree on a uniform standard. Even some researchers and the National Highway and Traffic Safety Association (NHTSA) have observed that scientific evidence to date does not support using a measure of THC as evidence of a driver’s impairment. Moreover, studies have not consistently correlated THC levels with levels of impairment.

Impairment depends on how often and how a person consumes cannabis, how much they use, how much body fat they have, and the sensitivity of the drug test. THC is lipid soluble, so it binds to the fat.

For someone who smokes marijuana for the first time, tests may detect it for up to three days. For someone who smokes daily, it’s a month or more. Blood tests can only detect THC for three to four hours, but urine tests can detect signs of use for much longer.

Some research even seems to indicate that THC can have a performance enhancing effect for certain drivers. For all of these reasons, there is no such expert consensus on a meaningful connection between impairment and THC levels in the body.

There is simply no consensus on cannabis and impairment, because there are so many different variables involved. Consensus is unlikely to be coming.

Unfortunately, cannabis and alcohol act completely different in the human body. The human body absorbs and metabolizes alcohol quickly, where it acts as a depressant on the nervous system.

In contrast, cannabis interacts the endocannabinoid system, which every mammalian body possesses. Depending on the form of consumption and a host of other factors, users may experience rapid effects, or more of a delayed onset. THC potency varies substantially from product to product and even from strain to strain. And different people respond differently to various cannabis strains and products.

The cannabis driving study itself

Complicating all of this is a lack of research on cannabis and any public health issue, including driving, in the United States. Add in the dual issues of polydrug testing—including alcohol—and this study seems questionable.

Alcohol is itself impairing, period. The added presence of THC does not prove anything new.

Furthermore, the study is imputing THC presence. The study states that over the 10-year period, 6,721 drivers were involved in fatal crashes in the state. Of the drivers who died, 88 percent were tested for THC, but this wasn’t true for surviving drivers, of whom only 29 percent were tested.

Instead, the study’s team used an imputation technique. They didn’t know how many drivers were THC-positive, so they estimated. Those guesses make up 31 to 56 percent of the people they classified as THC-positive.

Even if you still see a spike in presence of THC, presence doesn’t prove impairment or causality. The research. It shows the ability of people in the state to get a legal substance. The drivers were not tested for other legal substances that have the potential to impair such as supplements or prescription drugs, or even nicotine.

Take this weak cannabis driving study with a tremendous grain of salt.

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