Marijuana Driving Limits in Colorado Have No Scientific Basis, AAA Study Finds

The cover image for a new AAA report about marijuana driving limits. The document and more below.
The cover image for a new AAA report about marijuana driving limits. The document and more below.

Even before its 2013 passage (after two failed attempts), Colorado's marijuana driving limit, which establishes THC intoxication at five nanograms per milliliter of blood, has been controversial.

Cannabis activists have long argued that the standard is based not on science but on politics — the desire of law enforcers and officials to look as if they're doing something about stoned driving by imposing an intoxication number of the sort that applies to alcohol, even though the substances react very differently in the bodies of users.

Previous studies have questioned Colorado's approach, which has been mirrored in states such as Washington and Montana. For instance, an October 2015 study by the University of Iowa suggested that the five-nanogram figure was far too low.

However, a new analysis is arguably the most persuasive, and the most damning, to date, in part because of its sponsor: the American Automobile Association (AAA), an organization whose entire existence is predicated on safe motoring.

Marijuana Driving Limits in Colorado Have No Scientific Basis, AAA Study Finds
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"An Evaluation of Data from Drivers Arrested for Driving Under the Influence in Relation to Per se Limits for Cannabis," on view below in its entirety, comes to a strong negative conclusion about laws like Colorado's. It states in part: "There is no evidence from the data collected...that any objective threshold exists that established impairment."

As we've reported, driving while stoned was illegal under Colorado law long before the passage of the 2013 measure. But unlike in the case of alcohol, there was no number at which a marijuana-using driver was considered to be officially impaired — and cannabis activists saw that as a good thing, since the science on the subject was infinitely less certain than it is in the case of booze. Nonetheless, the medical marijuana industry boom caused assorted legislators to believe one was needed anyhow.

Legislation from 2011 and 2012 would have established THC intoxication at five nanograms per milliliter of blood and made the standard per se — meaning that a test registering five nanograms or more would be seen as irrefutable proof of intoxication. In response, critics argued that because THC tends to linger in users for longer periods of time, it's next to impossible to determine actual impairment via a blood test, at least under presently available technology.

In 2013, the five-nanogram limit was still part of the legislation, originally known as HB 1114, but the per se language vanished from the measure, sponsored by Representative Rhonda Fields. Instead, the text referred to "permissible inference," which allowed people who registered at five nanograms or above to present other evidence in court to prove that they weren't actually impaired, rather than being considered guilty as a result of the test reading.

Marijuana attorney Rob Corry saw this change as only a slight improvement over the previous legislation, making the new proposal 95 percent bad as opposed to 100 percent.

Governor John Hickenlooper, who signed the Colorado marijuana limits bill, with original sponsor Representative Rhonda Fields and former Lieutenant Governor Joe Garcia.
Governor John Hickenlooper, who signed the Colorado marijuana limits bill, with original sponsor Representative Rhonda Fields and former Lieutenant Governor Joe Garcia.
File photo

Nonetheless, a version of Fields's original bill was approved and signed into law due in part to pressure from law enforcement groups, which said in a letter to Governor John Hickenlooper that passing marijuana laws without such a standard would be irresponsible.

Problem is, the AAA study showed almost no correlation between the five-nanogram standard and intoxication as measured by typical roadside tests. For instance, the document states,  "No differences were found in performance in the walk-and-turn, or one-leg-stand tests, according to whether subjects were in the higher (>5 ng/mL), or lower (<5 ng/mL) THC groups." Indeed, only the ability of an individual to touch a finger to his or her nose showed a higher failure rate among those above the limit.

The report's conclusion reads:

There is no evidence from the data collected, particularly from the subjects assessed through the DRE exam, that any objective threshold exists that established impairment, based on THC concentrations measured in specimens collected from cannabis-positive subjects placed under arrest for impaired driving. An association between the presence and degree of indicators of impairment or effect from cannabis use were evident when comparing data from cannabis-positive and cannabis-negative subjects. However, when examining differences in performance in these parameters between subjects with high (>5 ng/mL) and low (<5 ng/mL) THC concentrations, minimal differences were found. There was no correlation between blood THC concentration and scores on the individual indicators, and performance on the indicators could not reliably assign a subject to the high or low blood THC categories. Analysis of the sensitivity, specificity, and accuracy of various per se thresholds suggested the highest sensitivity was found at 1 ng/mL: 80 percent of drivers who demonstrated impairment on the SFST had THC concentrations of 1 ng/mL or greater. However, 30 percent of drivers who did not demonstrate impairment on the SFST also had THC concentrations of 1 ng/mL or greater. Finally, among both samples of drivers who came into contact with law enforcement and were subsequently placed under arrest for DUI, only 30-49 percent would have been considered impaired under a per se standard set at 5 ng/mL, depending on whether alcohol or other drugs are detected and taken into consideration.

Based on this analysis, a quantitative threshold for per se laws for THC following cannabis use cannot be scientifically supported. 

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As noted above, Colorado's law isn't per se — but that doesn't mean the authors of the AAA study support its language. "This approach can be cumbersome and lead to lengthy trials," the study maintains, adding, "It also raises inevitable objections that such laws are shifting the burden to the defendant to prove their innocence."

Whether these findings will convince Colorado legislators to revisit the current law is doubtful, at least in the immediate future. After all, the current legislative session is almost over — and arguing in favor of removing the limits could be considered politically risky.

Meanwhile, though, the evidence is mounting that Colorado's THC limit is arbitrary nonsense.

Here's the AAA report.



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