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Why It's So Hard to Figure Out the Real Dangers of Stoned Driving

Photo by Anthony Camera

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Colorado Division of Criminal Justice
Detection and Law Enforcement Training

The main ways of detecting marijuana impairment, the report states, "are behavioral and toxicological. The former comes in the form of observations by law enforcement officers during psychophysical roadside tests, and the latter comes in the form of chemical tests of breath and bodily fluids."

When in the behavior-assessment mode, officers in Colorado are likely to use one of three main approaches: Advanced Roadside Impaired Driving Enforcement (ARIDE), Drug Recognition Expert Training (DRE) or Standardized Field Sobriety Testing (SFST). Trainings in this last methodology are supplemented by so-called wet labs, at which "law enforcement can participate in mock contact with a volunteer who has or has not consumed alcohol. The consumption is concealed and occurs in a separate setting from officers. Law enforcement interacts with these volunteers as though they are suspected of impaired driving and implement the battery of tests to detect and assess impairment."

In contrast, there's only one marijuana-focused training lab in the entire state: "The Green Lab," set up in September 2015. As of July 2018, only 410 law enforcers in the state had undergone training at the facility, as compared to 5,674 active SFST operators.

The report maintains that "training to detect drugged driving impairment is critical for peace officers because there is no equivalent to the preliminary breath alcohol test (PBAT) for other drugs." But it's simply not happening at a level comparable to educational efforts pertaining to alcohol intoxication.

Field Sobriety Efficacy and Delta-THC Levels

Police officers frequently try to employ the same field tests used to determine alcohol impairment on drivers they think may be stoned. Some work okay, the authors of the report state; others, not so much.

Cited is a 2016 study that found the Horizontal Gaze Nystagmus analysis that's a staple of roadside testing for suspected drunk drivers "is not exhibited or expected in cannabis consumers." In contrast, the Lack of Convergence test was "a strong indicator of cannabis presence," as were the One Leg Stand and the Walk and Turn. Yet "despite individuals exhibiting clues of impairment during these standardized roadside tests, no correlation was found between the tests and Delta-9 THC concentration in whole-blood samples," the report acknowledges.

Another study found that pupil dilation, elevated pulse, the Lack of Convergence test and "the exhibition of impairment clues in two other psychophysical tasks were best at indicating impairment. However, the latter results were only for exams administered by Drug Recognition Experts. Again, there was no correlation in this study between test performance and whole-blood THC."

In recent years, manufacturers have attempted to come up with a cannabis equivalent for preliminary breath alcohol tests, which aren't admissible in court but are considered to be good screening tools. But a National Highway Traffic and Safety Admission study referenced in the report admits that the oral-fluid roadside tests developed to date "have not been shown to be completely reliable and accurate" when it comes to making distinctions between occasional and heavy users or the method of consumption.

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Colorado Division of Criminal Justice
Marijuana Impairment

A Colorado Department of Public Health and Environment review from last year found that "recent marijuana use increases a driver’s risk of a motor vehicle crash, less-than-weekly marijuana users exhibit meaningful driving impairment with THC levels of 2-5 ng/mL or ingestion of 10 mg or more of THC, combining marijuana and alcohol increases impairment and motor vehicle crash risk more than each alone" and "delaying driving for a minimum of six hours after smoking allows THC-induced impairment to resolve for less-than-weekly users at 18 mg of THC."

Still, the report stresses that plenty of uncertainties remain. One study cited by the authors determined that THC didn't have an effect on a driver's "time and distance perception," while another one found the opposite.  Likewise, a separate analysis showed that reaction time increased with THC impairment for both occasional and frequent users, while another one didn't register similar delays.

Such differences pop up again and again in the report. Take this excerpt: "Some studies find that, in comparison to drivers with no cannabis, there is no significant crash risk associated with cannabis impaired driving. However, other studies find that there is a higher crash risk associated with cannabis consumption."

So what's the truth? Researchers continue to seek it out — but as of now, consensus is hard to come by.

Click to read the Colorado Division of Criminal Justice's "Driving Under the Influence of Drugs and Alcohol" report.
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Michael Roberts has written for Westword since October 1990, serving stints as music editor and media columnist. He currently covers everything from breaking news and politics to sports and stories that defy categorization.
Contact: Michael Roberts