Thresholds for stoned-driving charges remain hazy

Greg Campbell Contributor
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Now that Colorado has legalized marijuana, it has sparked a debate over how much THC in the bloodstream should trigger a DUI arrest.

Consensus may be a long way off, but Boulder District Attorney Stan Garnett says lawmakers should just “pick a number and let’s go with it,” adjusting the stoned-driving threshold later if need be.

Garnett was one of three panelists speaking at CU-Boulder Thursday night about one of the more complicated aspects of marijuana legalization: Establishing a blood-THC level for determining when a driver is too impaired to be on the road.

THC, or delta-9 tetrahyrocannabinol, is the main psychoactive ingredient in marijuana. But unlike alcohol, which leaves the bloodstream relatively quickly, THC can persist in the blood long after its intoxicating effect has worn off.

In fact, consensus on the panel was that the amount of THC in one’s system — which is measured in nanograms per milliliter — is a poor indicator of a person’s level of impairment.

Even the National Highway Traffic Safety Administration says it’s “inadvisable” to try to judge impairment based on THC blood levels.

Still, state lawmakers have tried without success to establish a blood-THC level of 5 ng/mL, a level that critics say is far too low and which could result in DUI convictions even if a driver was performing perfectly behind the wheel.

In 2011, pseudonymous Denver pot critic William Breathes — who uses marijuana daily for a medical condition — illustrated the concern.

He had his blood tested 15 hours after he last smoked, and his physician, Dr. Alan Shackelford, confirmed that he was “in no way incapacitated” when his blood was drawn.

Results showed that Breathes had 13.5 ng/mL in his bloodstream, nearly three times the proposed limit that would trigger a DUI arrest.

Shackelford, who was also on the discussion panel, said he preferred to see a performance-based determination of impairment, such as a roadside sobriety test, rather than one that relies too heavily on the results of a blood test.

“I also don’t want people convicted when the conviction is the result of a mere number,” he said.

Garnett agreed, but said that it’s important to start somewhere. DUI thresholds for alcohol have been adjusted repeatedly over the years, he said, and the same should happen with blood-THC levels.

“Nobody wants to convict an innocent person,” he said, “but the reality is that bad driving is a very significant public safety issue. … We have to draw the line somewhere.”

According to the NHTSA, stoned drivers have delayed reaction times and difficulty estimating time and distance. Other studies have shown them to overcompensate for their impairment, often driving too cautiously.

Garnett said one of the frustrations about the federal prohibition of marijuana is that it has prevented rigorous scientific study of the drug that would be useful in this discussion.

“We don’t have anything to work with,” he said.

The panel discussion, held before a small audience of fewer than 20 people, including two state lawmakers, comes as a state-appointed task force is working on a variety of issues related to Colorado’s legalization of marijuana. Its main goal is to recommend a system for retail sales, but it will also consider the question of marijuana-impaired drivers.

The task force has until the end of February to present its recommendations to the governor and the state legislature.

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