A High Cost to Smoking and Driving?

Posted by Coleen D. St. Clair | Feb 24, 2016 | 0 Comments

More Washingtonians involved in DUIs are testing positive for marijuana, according to a recent NPR article. But whether this means that marijuana is a culpable factor in these cases remains contested.

There has been a 14% increase since 2012 in Washington DUI drivers testing positive for THC, brining the number up to a total of 33%. A recent study by the Washington Traffic Safety Commission found that currently 85% of fatal DUI cases involve active levels of THC, not just the carboxy THC that stays in the system long after someone has imbibed, meaning that more people are smoking and driving.

The Washington State Legislature has set the THC blood limit at 5 ng/mL, and any amount over can warrant an immediate DUI regardless of implication of impaired driving. If you get pulled over for even a minor infraction, like a broken taillight, and the police find reason to test your THC levels, you could be in danger of a DUI. Possession of a medical marijuana card has no effect on THC blood level limits.

For scale, a 5 ng/mL blood amount is about what you would test three hours after imbibing. A single puff of marijuana can result in 7 to 18 ng/mL THC levels in the blood stream. Part of the reason why marijuana or THC levels are hard to limit is because THC affects everybody in different ways, both in blood content levels and in psychological effects.

According to the National Highway Safety Traffic Association, smoking 8.8mg of THC resulted in blood levels that range from 46- 188 ng/mL in 6 subjects. Psychological factors are harder to test for, making it difficult to set any kind of consistent standard. Studies have shown that some marijuana users can adapt to some of its effects and compensate behavior accordingly, while others may become significantly impaired when they are within the legal THC blood level limit for driving.

The National Institute on Drug Abuse (NIDA) reports that, “marijuana significantly impairs judgment, motor coordination, and reaction time, and studies have found a direct relationship between blood THC concentration and impaired driving ability,” but that direct relationship could be minimal.

In a recent study the NIDA tested THC's effect on driving ability, specifically three factors of driving ability: weaving within the lane, the number of times the car left the lane, and the speed of the weaving. Marijuana did not effect driving the same way alcohol did, but drivers under the influence of marijuana were shown to be more prone to weaving. Levels of 13.1 ng/mL (ug/L) of THC in the blood resulted increased weaving while driving that mirrored a .08 concentration of breath alcohol.

R. Andrew Sewell, MD with the VA Connecticut Healthcare/Yale University School of Medicine, did a meta-analysis of over 210 studies and found that, “in general, the higher the estimated concentration of THC in blood, the greater the driving impairment,” but that “the effects between individuals vary more than they do with alcohol because of tolerance, differences in smoking technique, and different absorptions of THC.” D. Mark Anderson with Montana State University found similar results and further reported that there “is little evidence that marijuana use leads to an increased risk of collision.”

So while some officials are finding the increased prevalence of THC in Washington DUI cases alarming, the evidence remains unclear.

About the Author

Coleen D. St. Clair

Ms. St. Clair is a former senior prosecuting attorney with over 25 years experience handling criminal cases. She has successfully handled tens of thousands of criminal and DUI cases with remarkable results.


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