THC Per Se Limits are Unreliable Predictors of DUI

Responsible cannabis consumers refrains from driving while impaired from significant cannabis use, nobody wants a DUI.

That point is apparently lost on cannabis opponents, who argue cannabis reform will inevitably and immediately be followed by a ‘stoned driver’ epidemic.

The ‘terror on the roadways’ argument is one of the most popular talking points for cannabis opponents, even though it is not well-founded.

No state or country on the planet that has legalized cannabis for adult use has also legalized impaired driving, nor is that expected to ever happen.

Impaired driving due to cannabis use was, is, and likely always will be illegal and related laws will be enforced against those that violate the law.

With that being said, not all cannabis DUI policies are created equal, as demonstrated by a recent study.

THC Per Se Laws

In some jurisdictions, cannabis DUI policies are based on per se THC limits. Per se THC limits establish a testing threshold.

If someone has enough THC in their system to meet or exceed the threshold, they automatically get a DUI, regardless of if they are actually impaired or not.

A team of researchers in Australia recently explored the relationship between THC levels in a person’s system and their driving performance.

“The blood and oral fluid per se limits examined often failed to discriminate between impaired and unimpaired drivers,” the researchers stated. 

“Moreover, blood and oral fluid THC concentrations were poorly correlated with driving impairment… It is almost impossible to infer how much cannabis was consumed, or when it was consumed, based solely on a given concentration of THC in any biological matrix,” the researchers concluded.

DUI Laws Need To Be Based On Science

Everyone wants public roadways to be safe, and that includes responsible cannabis consumers. That fact should be obvious to any reasonable person.

Public policy needs to be effective, logical, and based on science, which is why per se cannabis laws should be avoided by lawmakers.

Per se limits may work for alcohol, however, they clearly do not differentiate from someone that has consumed cannabis weeks ago versus someone that is impaired at the time.

The cannabis plant interacts with people’s biology in different ways, which is why a frequent user can consume a lot of cannabis and not actually be impaired, whereas a rookie consumer will be inebriated after consuming very little cannabis.

That is problematic from an impairment testing standpoint.

A much better approach to impaired cannabis driving policy is to rely on a series of field sobriety tests and a totality of circumstances. 

Hopefully, jurisdictions adopt that approach instead of per se policies that are based on junk science.


This article was provided to Cannabis And Tech Today by the International Cannabis Business Conference.


  • Johnny Green is the Media and Content Director for the International Cannabis Business Conference and has blogged about cannabis since January 2010.

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