The Problem with Marijuana DUI Investigations
As a criminal defense attorney, one of the most rewarding moments in my career is when I successfully defend a client against charges that lack a strong foundation in evidence. One recent case stands out to me—a case that was ultimately dismissed when the prosecutor admitted the difficulty in proving Marijuana impairment. This case, like so many others, highlighted troubling patterns I often see in DUI investigations, especially those involving Marijuana. While I cannot share the specifics due to confidentiality, the broader issues it exposed are critical for the public to understand.
Marijuana DUI cases bring unique challenges for both law enforcement and the legal system. Unlike alcohol, which has scientifically established thresholds for impairment, Marijuana operates in a much grayer area. THC, the active compound in marijuana, does not correlate to impairment in the same way blood alcohol content does. Levels of THC in the bloodstream can linger for days, sometimes weeks, after use, particularly in regular users. This creates a significant challenge: how do we distinguish between someone who has used Marijuana at some point and someone who is genuinely impaired?
This ambiguity leaves room for subjective observations to take center stage in Marijuana DUI investigations. Officers frequently rely on reports of odors, physical signs, and field sobriety tests to establish probable cause. While these tools can be useful in certain situations, they are also rife with opportunities for bias, misinterpretation, and even fabrication—intentional or not.
Take odors, for example. An officer’s claim that they smelled marijuana is often unchallengeable. It’s a subjective statement that cannot be verified or disproven after the fact. In the case I recently handled, the officer’s claim that they smelled Marijuana played a significant role in their probable cause determination. Yet, no Marijuana or paraphernalia was found, and subsequent blood tests revealed nothing to suggest recent use. Despite this, the odor of Marijuana became a cornerstone of the case—a claim that I firmly believe was exaggerated to bolster probable cause.
Other Field Sobriety Tests like the Horizontal Gaze Nystagmus (HGN), a field sobriety test used to observe involuntary jerking of the eyes in alcohol investigations, often go unrecorded because it is rarely captured on body-worn cameras. This lack of documentation makes it nearly impossible to verify whether the test was conducted properly or whether the results were interpreted correctly. Further, the HGN tests’ use in Marijuana DUI cases are useless, and yet sometimes even misrepresented as a sign of Marijuana use or impairment.
Then there are the driving behaviors that officers use to justify DUI stops in the first place. Claims of “drifting within a lane,” “driving too slowly,” or “straddling the line” are common. These observations can be highly subjective, influenced by an officer’s preconceived notion that a driver is impaired. These claims are nothing more than opinions without corroborating evidence, such as dashcam footage. In the case I recently handled, the initial stop was based on vague driving behaviors that could not be verified. Yet these claims were used to build a case for impairment that ultimately fell apart under scrutiny.
The reliance on unverifiable observations and pseudo-scientific methods is especially problematic when officers lack the training or expertise to assess impairment properly. In a recent case, the officer’s testimony revealed a fundamental misunderstanding of the signs and symptoms of Marijuana use versus impairment. For example, the officer noted constricted pupils as an indicator of Marijuana impairment, despite scientific evidence showing that THC typically causes dilated or normal pupils. This type of misinformation—whether due to a lack of training or a desire to strengthen the case—misleads prosecutors, the courts, and jurors.
This isn’t just a one-off issue. It’s part of a broader pattern in DUI enforcement where certain tools and observations are treated as conclusive despite their inherent limitations. In Marijuana DUI cases, the stakes are even higher because the science is still catching up to the legal framework. Blood THC levels, for example, are often presented as damning evidence, even though the scientific community widely agrees that these levels are not reliable indicators of impairment. Yet jurors and even judges may not be aware of this nuance, leading to unjust outcomes.
As a defense attorney, my role is to expose these flaws and ensure that my clients receive a fair trial. In the case I’ve been referencing, we challenged the officer’s observations, the reliability of the field sobriety tests, and the misinterpretation of the blood test results. Ultimately, the prosecutor recognized the weaknesses in the case and dismissed it. But not every defendant has the resources to mount a robust defense, and not every prosecutor is willing to reevaluate their case.
What can be done to address these systemic issues? First, law enforcement agencies need better training on the signs of Marijuana impairment. Officers must be educated on the limitations of field sobriety tests and the science behind THC metabolism. Without this knowledge, officers will continue to make presumptive judgments that can lead to wrongful arrests and prosecutions.
Second, there needs to be greater accountability in investigating and prosecuting DUI cases. Body-worn cameras should be used consistently and effectively to document every step of a DUI investigation, from the initial stop to the administration of field sobriety tests. This would provide an objective record that can be reviewed by defense attorneys, prosecutors, and the courts.
Third, we need to educate the public—including jurors—about the limitations of the evidence commonly used in DUI cases. Jurors should understand that claims of odors, HGN results, and blood THC levels are not definitive proof of impairment. Public awareness can help reduce the likelihood of wrongful convictions and encourage a more critical examination of evidence in these cases.
Finally, prosecutors are responsible for evaluating the strength of their cases before moving forward. It’s not enough to rely on the presumption that an officer’s testimony will carry the day. Prosecutors should scrutinize the evidence presented to them and consider whether it truly meets the burden of proof.
The case I’ve discussed is a powerful example of what can go wrong in a marijuana DUI investigation. It’s a reminder that justice requires vigilance, transparency, and a commitment to the truth. While I’m grateful for the outcome in this instance, I know there is still much work to be done to ensure that every defendant receives a fair and impartial evaluation of the evidence against them. As a defense attorney, I’ll continue to fight for that standard, one case at a time
