What is a “marijuana DUI”?  What does it mean to be “under the influence” of marijuana?  What is the safe level, and how is it measured?  How are marijuana DUI’s defended?

In Washington State, police officers have increased their arrests and prosecutions for Marijuana DUI’s.

Law enforcement in the last few years has really been pushing investigations and arrests over so-called “marijuana DUI’s.”  In this two-part blog post we will look at how marijuana affects a person’s ability to drive, and more importantly, how it does not.  Today we will discuss the scientific studies, and then we will come back in a couple of weeks to discuss more law, and how marijuana DUI court cases work for a defendant and the criminal defense lawyer.  It is illegal under Washington State  law RCW 46.61.502(1) to drive “under the influence” of any drug.  “Under the influence” is typically defined as when the person’s ability to drive “is affected to an appreciable degree.”  There is no .08 equivalent for marijuana.  In each case, a police officer must prove “under the influence.”   Let’s turn to the subject of the scientific studies.

Professor Harry Klonoff Experiments in 1973

In 1973, Professor Harry Klonoff of the University of British Columbia decided to run experiments on the effects of marijuana on a person’s ability to drive a motor vehicle. Professor Klonoff provided marijuana cigarettes that contained .7 grams to his subjects, and he instructed them to smoke the marijuana by inhaling for 3 seconds, and holding it in their lungs for 15 seconds, and then exhaling and then resting for 15 seconds until the cigarette was completed.  Since the test was “double blind,” he had half the group use a placebo form of marijuana.  He then sent the test subjects out on a driving course, and then later sent the drivers out into rush hour traffic in the city of Vancouver, B.C.  (Yeah, I know, it is hard to imagine the city of Spokane allowing such a test today).  The only apparent safety precaution was that the vehicles used were “dual control,” so the cars had driver’s-education style brakes on the right-hand side where a professional observer sat.

Above is the diagram of Professor Klonoff’s driving course where he experimented with the effects of marijuana consumption on driving in 1973.

The study was extremely complex with a number of different variables, methods of scoring, and scientific controls. Professor Klonoff concluded that for some drivers the use of marijuana hurt the individual’s driving ability.  However, the study also found that some drivers performed better after smoking marijuana.   Dr. Klonoff wrote:

It is evident that the smoking of marijuana by human subjects does have a detrimental effect on their driving skills and performance in a restricted driving area, and that this effect is even greater under normal conditions of driving on city streets.  The effect of marijuana on driving is not uniform for all subjects, however, but it is in fact bidirectional; whether or not a significant decline occurs in driving ability is dependent both on the subject’s capacity to compensate and on the dose of marijuana.  For those subjects who improved their performance, the explanation may lie in overcompensation and possibly the sedative effect of the drug.

The biggest takeaway from Dr. Klonoff’s study is that marijuana does not have a predictable negative effect on a person’s ability to drive. Consequently, it is very difficult for any principled expert to testify in a court of law that a person was affected by the marijuana in any certain way. If you want to look up the article at your local medical school library, the citation is: Klonoff H. Marijuana and Driving in Real-Life Situations. Science 1974;186(4161);317-24. Klonoff’s test pretty much carried the day in the scientific community until a similar test was run in Maastricht, Holland two decades later.

The Maastricht Studies of 1993

In 1993, the U.S. Department of Transportation sponsored a study done by the Institute of Human Psycho-pharmacology at the University of Limburg in Maastricht, Netherlands.

In a study in Maastricht, Netherlands, scientists concluded it was not possible to detect driving impairment from THC levels in the blood. The study was sponsored by the U.S. Department of Transportation.

Like the Klonoff study, the Maastricht study was done by providing marijuana for the test subjects to smoke, and then monitoring their ability to drive.  Doses were provided in the THC amounts of 0 (control group), 110, 200, and 300 ug/kg.  The subjects then drove through a closed roadway and then a roadway with traffic present. As with the Klonoff study, vehicles with “redundant controls” were used as a safety precaution. In the executive summary of the published study, the scientists wrote:

This program of research has shown that marijuana, when taken alone, produces a moderate degree of driving impairment which is related to the consumed THC dose.  The impairment manifests itself mainly in the ability to maintain a steady lateral position on the road, but its magnitude is not exceptional in comparison with changes produced by many medicinal drugs and alcohol.  Drivers under the influence of marijuana retain insight in their performance, and will compensate where they can (for example) by slowing down or increasing effort.  As a consequence, THC’s adverse effects on driving performance appear relatively small.

The scientists had hoped to determine whether it was possible to predict driving impairment by the levels of THC in the drivers’ blood. However, they concluded that the answer was “very clear” that this was not possible. They explained that some drivers were impaired  with both high and low levels of THC, and conversely some drivers with high levels of THC in their blood performed quite well. For more details see: Robbe HW, O’Hanlon JF. Marijuana and actual driving performance. US Department of Transportation/National Highway Traffic Safety Administration November: 1-133 (1993). DOT HS 808 078.  The study failed to provide clear guidance to the U.S. government on marijuana DUI policy.

The Grotenhermen Study of 2007

The last study to look at is the research report entitled “Developing Limits for Driving Under Cannabis” published in 2007 in the journal for the Society for the Study of Addiction.  That study again tried to determine a limit for THC levels in a driver’s blood that would be similar to the .08 limit for alcohol DUI charges.  The scientists compared a THC blood level of 4.2 ng/ml to driving with a blood alcohol level of .04.   The study explains that the margin of error in testing is great with a confidence interval  of 3.1 to 7.7.  Due to such a large margin of error, the study suggests that a legal limit be set in the amount of 7 to 10mg/ml for blood.   Rather than base the study on driving tests, this study based its conclusions on studies done on individuals who had had their blood tested after a crash.

The Challenge to Defense Lawyers

Many jurors have difficulty understanding the way marijuana affects the human driver.  Jurors’ knowledge and experience with marijuana will vary significantly, and it can be awkward to ask about marijuana use during jury selection.  Juror views on marijuana can vary sharply between metropolitan areas like Spokane, and more rural areas like Okanogan or Colville, Washington.  Jurors usually try to analogize other drugs to alcohol, which jurors are more familiar with.  However, the effects of marijuana are very different from alcohol.  Criminal defense lawyers also have a steep learning curve in understanding the science of marijuana intoxication.  A defense attorney should consider hiring or consulting with an independent toxicologist.  Due to the complexity of the science of marijuana use, it is a challenge to all the participants of the criminal justice system.  Because the science is so vague, the representatives from the Washington State Toxicology Lab usually testify about the ways THC is “likely” to affect an individual.  The Washington state toxicologists typically base their testimony on the National Highway Traffic Safety Administration’s manual entitled “Drugs and Human Performance Fact Sheets.”  That manual cautions “It is difficult to establish a relationship between a person’s THC blood or plasma concentration and performance impairing effects. Concentrations of parent drug and metabolite are very dependent on pattern of use as well as dose.”  In light of the difficulty in interpreting blood test results, marijuana DUI charges often come down to officer testimony and the field sobriety tests.  We will discuss this aspect in part two of this blog post.

(The author Steve Graham is a criminal defense lawyer practicing in eastern Washington.  See his further information on the laws of Marijuana and DUI in the State of Washington.)

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