Determining DUI threshold for pot hinges on impairment

Colorado voters’ legalization of marijuana for medical uses has created problems for courts in resolving issues related to drug-impaired driving.

Contrary to what may be a common belief, Colorado law does not prohibit driving when any quantity of a drug is found in a person’s system. Instead, to convict a party of “driving under the influence” of drugs, prosecutors must show that the person consumed enough of the drug to have actually affected him such that he was “substantially incapable” of exercising “clear judgment, sufficient physical control or due care in the safe operation of a vehicle.”

To convict of “driving while ability is impaired,” prosecutors must show that the defendant consumed enough that he was “less able than the person ordinarily would have been” in operating the vehicle. In theory, a person could have a level of THC (the impairing substance in marijuana) in his or her system that does not actually impair or affect the individual’s ability to drive. That person would not be guilty of DUID.

As a society, we have accepted this premise when it comes to driving after the consumption of alcohol; it is widely agreed that a person can consume a little bit of alcohol and still be capable of driving safely.

But determining how much is too much is difficult.

Even more difficult for prosecutors is proving that driving ability has been affected. In response, the legislature has created benchmark figures that are presumed to affect a person’s ability to drive.

If one has a blood-alcohol content of .05 or above, he or she is presumed to be impaired, and can be charged with “driving while ability impaired.” Above .08, and an individual is presumed to be under the influence and can face “driving under the influence” charges. If these numbers are proved, prosecutors do not need to show that a person’s driving ability actually was affected; the law presumes it

The dirty secret, however, is that even below those levels, a driver could still be affected by alcohol, but police and prosecutors rarely pursue such cases. This is because the designation of the BAC numbers by the Legislature has also created a de-facto baseline whereby drivers are typically presumed not to be affected unless they cross the .05 threshold.

So what of marijuana? Prosecutors frequently show blood test results for drivers accused of driving under the influence of drugs to show the presence of THC in their systems, but the practice raises the question of how much is too much.

In recent years, the Legislature has toyed with the idea of creating some kind of threshold for THC similar to that for alcohol – a figure that would create a presumption of impaired ability. In some states that threshold is zero; it is assumed that any level of THC in a person’s system is unsafe. But as pointed out above,

Colorado law requires a level that actually affects driving. In 2012, a bill was considered that would have set a threshold of 5 nanograms of THC in a person’s blood, but that bill died by a single vote. Some legislators feel that the measurement of THC in a body is too imprecise to accurately reflect a person’s ability, while others fear creating a “safe harbor” below which a person could be considered clean.

As it stands now, prosecutors will presumably have to continue to show actual impairment to prove a person is driving under the influence of drugs. But it is also likely that juries will still be shown results of blood tests that show what may be trace amounts of THC.

Faced with such evidence, it is uncertain how willing most juries will be to accept the premise that a person could legally have marijuana in their system but not be impaired.

Douglas Reynolds practices civil litigation and criminal defense at the Bopp Reynolds Law Group in Durango. Reach him at 259-0661 or