Scientific Evidence in a Marijuana DUI Case

California prosecutors routinely offer scientific evidence against defendants in criminal court. But the law requires the trial court to first screen the evidence to make sure there is a sound basis for the testimony. That foundational requirement is important because such expert testimony may carry greater weight with a jury simply because of the witness’s impressive credentials.

In Sargon Enterprises, Inc. v. University of Southern California (2012) 55 Cal.4th 747, the California Supreme Court held that as the gatekeeper of the evidence, the trial court should first discern whether there is a scientifically reasonable basis for expert testimony before allowing the admission of the evidence. The Court held:

“Thus, under Evidence Code sections 801, subdivision (b), and 802, the trial court acts as a gatekeeper to exclude expert opinion testimony that is (1) based on matter of a type on which an expert may not reasonably rely, (2) based on reasons unsupported by the material on which the expert relies, or (3) speculative. Other provisions of law, including decisional law, may also provide reasons for excluding expert opinion testimony.” Id. at 771-772.

The Court noted that this does not require a trial court to choose between competing expert opinions. The Court wrote: “Rather, [the trial court] conducts a ‘circumscribed inquiry’ to ‘determine whether, as a matter of logic, the studies and other information cited by experts adequately support the conclusion that the expert’s general theory or technique is valid.’” Sargon, supra, at 772 (quoting Imwinkelried & Faigman, Evidence Code Section 802: The Neglected Key to Rationalizing the California Law of Expert Testimony (2009) 42 Loyola L.A. L.Rev. 427). Expert testimony with no scientifically sound support should be excluded.

One area of increasing interest with respect to the admissibility of scientific evidence in criminal cases involves THC impairment. Since the legalization of marijuana in California, it has become even more important to understand the scientific principles behind the prosecution of a marijuana driving under the influence case.

The government has conducted scientific studies to determine whether THC levels can indicate driving impairment. The National Highway Traffic Safety Administration (NHTSA) submitted a report to Congress delineating its findings. NHTSA concluded that levels of THC in the blood are not indicative of impairment for purposes of driving. The report states: “THC level in blood (or oral fluid) does not appear to be an accurate and reliable predictor of impairment from THC.” See U.S. Department of Transportation, National Highway Traffic Safety Administration, Marijuana Impaired Driving, A Report to Congress, page 8. NHTSA also reported that “the presence of THC in the driver’s body has not been shown to be a reliable measure of marijuana impairment of driving.” Id. at 14. In discussing the tests used to detect THC levels, NHTSA stated: “These toxicological tests confirm the presence of THC but they do not indicate driver impairment ….” Ibid.

Other scientific studies have conveyed similar results. In 1995, the authors of one such study concluded that “it is not possible to conclude anything about a driver’s impairment on the basis of his/her plasma concentrations of THC and THC-COOH determined in a single sample.” Robbe, H.W.J. 1995, Marijuana’s effects on actual driving performance. IN: Proceedings of the 13th International Conference on Alcohol, Drugs and Traffic Safety. And a decade later, another study reported the same findings. See Ramaekers, J.G.; Moeller, M.R.; van Ruitenbeck, P.; Theunissen, E.L.; Schneider, E.; and Kauert, G. 2006. Cognition and motor control as a function of delta 9-THC concentration in serum and oral fluid: Limits of impairment, Drug and Alcohol Dependence 85:114-122.

Studies have also shown that THC levels do not show recent use. The authors of one such study wrote that the “results of this study argue against the utility of detectable THC or 11-OH-THC in the plasma of chronic frequent cannabis smokers as a reliable marker for recent cannabis use.” See Karschner, E., et al. 2015, Extended plasma cannabinoid excretion in chronic frequent cannabis smokers during sustained abstinence and correlation with psychomotor performance, page 5.

A defendant in a criminal case has a constitutional right to due process and a constitutional right to a fair trial. The admission of scientific evidence by prosecution witnesses that does not meet the legal standards for reliability violates those constitutional rights. And under Sargon, the trial court should exclude evidence that does not meet the Evidence Code’s standards for admissibility. Based on NHTSA’s findings, it appears there is no scientifically sound basis for an expert to testify that levels of THC or Hydroxy THC in the defendant’s blood indicate anything about their alleged impairment at the time of driving. But yet in courtrooms across the state, this type of evidence is admitted. An effective criminal defense attorney handling a marijuana DUI trial must challenge such evidence in order to protect a defendant’s right to due process and a fair trial under the Fifth, Sixth, and Fourteenth Amendments to the United States Constitution.

About the Author

Lara J. Gressley

Lara Gressley

Lara Gressley has been an attorney for over two decades. She focuses her practice on criminal defense, DUI defense, and writs and appeals in state and federal court, and has worked on cases before the United States Supreme Court. Lara also lectures lawyers across California on various criminal defense topics.

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