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Drugged Driving Simulator Shows Legal Limit for Marijuana Too Low

In the evolving landscape of marijuana legalization, one of the most contentious issues revolves around establishing a legal limit for marijuana impairment in drivers. While Colorado has set a threshold of five nanograms of THC per milliliter of blood, this arbitrary number has faced criticism for its lack of correlation with actual impairment. Recent research conducted by the University of Iowa challenges the existing limit. It proposes a new standard that aligns more closely with the legal benchmark for drunk driving: 13.1 nanograms per milliliter.

The journey toward establishing a meaningful limit has been fraught with challenges, primarily due to the complex nature of measuring impairment from marijuana. Unlike alcohol, where a blood alcohol concentration (BAC) of 0.08% has become the standard for determining legal impairment, marijuana presents a more intricate scenario. The five-nanogram limit in Colorado has been a point of contention, with critics arguing that it merely indicates the presence of marijuana in the system without effectively gauging impairment.

To delve deeper into this issue, researchers at the University of Iowa designed a groundbreaking study aimed at reevaluating the current THC limit. Picture this: participants are seated in a sedan within a 24-foot diameter dome, surrounded by a simulated city street – a virtual reality drugged driving chamber. The subjects were administered a combination of alcohol, vaporized marijuana, and placebos before being placed behind the wheel of the simulation.

The study measured various parameters to assess impairment, including in-lane weaving, frequency of lane departures, and the speed at which participants exhibited weaving behavior. The data gathered from this immersive experiment led the researchers to propose a new THC limit of 13.1 nanograms per milliliter as a more accurate reflection of impairment comparable to the legal standard for drunk driving.

The significance of this study lies not only in challenging the existing THC limit but also in providing experimental evidence to support an alternative threshold. While the five-nanogram limit has been criticized for its lack of connection to impairment levels, the 13.1-nanogram proposal is a groundbreaking step toward establishing a more scientifically backed standard for drugged driving laws.

Acknowledging that this research represents just one milestone in the ongoing efforts to formulate sensible drugged driving laws is crucial. The proposed THC limit needs thorough consideration by lawmakers, who will undoubtedly be influenced by the experimental evidence presented by the University of Iowa researchers. As the marijuana legalization movement progresses, the intersection of science and policy will continue to shape the landscape of drugged driving regulations.

This study sparks a vital conversation about the need for evidence-based approaches in establishing impairment limits for marijuana. As we await lawmakers’ response, whether this proposed limit will gain traction and lead to further revisions in drugged driving laws remains to be seen. In the ever-evolving world of marijuana legislation, the University of Iowa’s research guides us toward more informed and equitable policies.


In conclusion, the journey to establish meaningful and fair marijuana impairment limits is ongoing, and the proposed 13.1 nanogram limit presents a promising alternative backed by experimental evidence. As we navigate the intersection of science and policy, the University of Iowa’s research opens new avenues for discussion. It prompts a reevaluation of existing drugged driving laws. The road ahead may be long, but with each milestone, we move closer to a comprehensive and evidence-based framework for addressing marijuana impairment in drivers. For more insights into Colorado’s drugged driving laws, refer to our marijuana FAQ.

Do you have further questions or concerns? Call us or contact the attorneys at Thomas & Ahnell, LLC, and we will be happy to help.

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