It’s impossible to accurately measure THC impairment. Now what?


In a recent appellate case in another state, a DUI defense attorney brought forward some pretty compelling evidence that the usual methods police use to determine whether someone is under the influence simply don’t apply to marijuana. No level of THC, marijuana’s psychoactive compound, has been shown to cause more impairment than someone would experience at a 0.05 blood alcohol level. Moreover, there’s currently no Breathalyzer-style test for THC impairment because no “red line” level for illegal driving has been established.

Florida, like about half of the states and the District of Columbia, has legalized marijuana for at least some purposes. Here, marijuana is allowed for certain medical purposes. Even someone with a medical marijuana card, however, is prohibited from driving under the influence of marijuana — or any drug. But what does it mean to be under the influence?

What is prohibited is impairment, not simply being affected

In Florida, the prosecution has to prove you were actually impaired by the drug in order to convict you of DUID. It’s not enough to prove there was a drug in your system, or that you were merely affected, but not impaired. With cannabis, this is critical because the period of impairment, if any, is caused by THC. THC only causes a “high” for a couple of hours, and this is when the driver would be most impaired. However, THC remains in the bloodstream for far longer than it causes the “high” — often days or weeks. So, a blood test showing some level of THC in your system wouldn’t be enough for a DUID.

Is law enforcement able to recognize THC intoxication?

Probably not, according to the ABA Journal. Courts in at least three states have already ruled that recognizing impairment by marijuana is not something most police are trained to do. “Unlike alcohol intoxication,” wrote one supreme court, “no … general awareness exists as yet with regard to the signs and symptoms of the condition described as being ‘high’ on marijuana.”

Even drug recognition experts have trouble. DREs offer opinions about whether someone is using drugs and what kind — after an intense, 12-step process that takes at least an hour to complete. Even so, many courts won’t even allow DRE opinions as evidence because they aren’t reliable enough.

State after state is going to have to confront the fact that we don’t have valid, scientific standards for marijuana intoxication and may not have them for a while. Is it fair to subject people to criminal penalties when the proof they’ve done something wrong is of limited value?

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