What Effect Does Marijuana Have On A Person’s Driving Ability


As the laws and law enforcement scramble to keep up with the rapidly changing laws on marijuana use, researchers are asking the question: How does marijuana affect driving ability? The answer to this question has important implications for the establishment of driving under the influence of marijuana thresholds such as those already in place for driving under the influence of alcohol.

As to be expected, law enforcement organizations with an anti-drug agenda, say that any amount of marijuana use significantly impairs driving ability but studies, perhaps with a less biased point of view, have not yet definitively answered the question. The primary psychoactive ingredient in marijuana, THC, affects a person’s coordination, sensory and time perceptions (according to the National Institute on Drug Abuse (NIDA)). These brain activities are important in driving so it would stand to reason that marijuana use would affect a person’s ability to drive safely.

The NIDA conducted a rigorous study on the effects of driving while under the influence of marijuana. The study involved participants driving in simulated tests who were under various amounts of marijuana influence. Two-hundred and fifty driving parameters were measured but the study paid close attention to the most common exhibition of driving the under the influence: weaving. Those drivers under the influence of marijuana up to 13.1 ug/L THC concentrations did not exhibit increased weaving, while those with 0.08% BAC did. This is an important reference point as those states that have established THC driving thresholds (Washington and Colorado), have set that limit at 5 ug/L. This study confirms other studies that have found that driving is impaired when the driver is under the influence of significant levels of THC but at lower THC levels, drivers tend to overcompensate by being more cautious drivers.

One thing researchers are sure of is that marijuana can affect reaction times and decision making abilities. And furthermore, studies, including the aforementioned NIDA study, have found that when combined with alcohol, even in small amounts, the effects of driving under the influence of marijuana are amplified.

To complicate matters more, studies show that regular marijuana users do not experience the same influence on their driving abilities as does the occasional user and marijuana affects different individuals in different way, not to mention, that the different marijuana strains can have significantly different effects on the brain. These factors, individually, cannot be accounted for in any particular threshold law but they do make it more difficult to design a reliable study on the effects of marijuana on driving.

Threshold limits become even more problematic because THC can remain in a person’s system for days and even weeks, long after the effect has worn off. The effects of marijuana on driving, at least as the science stands thus far, are much more difficult to measure than driving under the influence of alcohol. However, now that marijuana is a legal recreational drug in California, this issue must be addressed, particularly because THC can remain in a person’s system after the effects are gone.

William Weinberg has almost 25 years of experience in defending drivers charged with driving under the influence. You may consult with him about your DUI matter by contacting him at his Irvine office at 949-474-8008 or emailing him at Bill@williamweinberg.com

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