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Drugged Driving: Not as Easy to Prove as You Might Think

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cartoon of a man smoking a bong while driving

Despite the fact that recreational marijuana is now legal in Colorado, driving while under the influence is not. However, determining whether or not a driver is high on pot is definitely not as easy as testing someone for alcohol impairment.

Sobriety Tests

When a driver is pulled over on suspicion of drunk driving, he will typically be asked to complete three standard field sobriety tasks:

  • Horizontal gaze nystagmus (HGN), or following a pen with his eyes while it is moved back and forth
  • Walk-and-turn, which requires him to get out of the car and walk nine steps, heel to toe, turn on one foot and go back
  • One-leg-stand, or standing on one leg for 30 seconds

If he passes these tests, he’s likely not drunk, as these standard field sobriety tests catch an estimated 88 percent of drivers who are impaired by alcohol.

But it’s a different story with someone driving under the influence of marijuana.

Proving Intoxication

There is much debate as to how to most accurately prove that drivers under the influence of THC are intoxicated and should not drive. According to a 2012 study published in the medical journal Psychopharmacology, only an estimated 30 percent of those under the influence of THC, the active ingredient in marijuana, will fail a field sobriety test. The ability of the tests to identify a stoned driver actually appears to depend upon whether or not the driver is a habitual smoker, not how recently he smoked, because THC builds up in the fatty tissue of the body and is released slowly over time.

A Yale University study found that although both marijuana and alcohol impair driving ability, the impairment effects of marijuana depend upon the person, since smoking pot affects different people in different ways. If law enforcement has a reason to suspect that a driver is high on marijuana, he will probably ask the suspect to take the same roadside sobriety test administered to someone suspected of being impaired by alcohol, and if the driver refuses, the officer will have to decide whether to arrest him and conduct a voluntary blood test.

While blood-alcohol content can be reliably tested during a traffic stop with a Breathalyzer, THC levels must be measured from blood or urine samples taken hours after an arrest. The results of a blood test could take two to six weeks to come back, and urine tests may still return a positive result days or even weeks after someone has actually smoked.

Legal Limits

Six states have set legal limits for THC concentration in the blood, including Colorado, where the limit is 5 nanograms per milliliter of blood. There are still a lot of questions in the state about whether this limit is an accurate measure of impairment. Some experts feel that this limit is too high and many impaired driving cases might be missed at that level, but lower limits might catch frequent users who might not actually be high. And then there’s the issue of individuals who may have stopped using marijuana weeks or even months ago, but still might test over five nanograms and end up being wrongly convicted.

But Coloradoans can take heart: a driver charged with driving under the influence of marijuana is free to rebut the charge and a jury can still find them not guilty despite the test, which is definitely more lenient than the law in the state of Washington, where a THC level of 5 nanograms or more automatically means that the driver was impaired.

Image by mikekline.


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Sally from Denver, Colorado

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