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- Two main questions arise in the law surrounding driving after having ingested cannabis: (1) whether cannabis actually impairs driving ability, and (2) whether the common practice of testing for THC (the main psychoactive substance in cannabis) is a reliable means to measure impairment. On the first question, studies are mixed. Several recent, extensive studies–including one conducted by the National Highway Traffic Safety Administration and one conducted by the American Automobile Association (AAA)–show that drivers with detectable THC in their blood are no more likely to cause car crashes than drivers with no amount of THC in their blood. Others show that cannabis can impair certain abilities important to safe driving (such as reaction time, divided attention, and cognitive functions)–but no studies have been able to show that this increases the actual risk of crashing, or that drivers with THC in their blood cause a disproportionate number of crashes. On the second question, the studies that have been conducted so far have consistently found that THC blood levels and degree of impairment are not closely related. No known relationship between blood levels of THC (the main psychoactive substance in cannabis) and increased relative crash risk, or THC blood levels and level of driving impairment, has been shown by single-crash or classic-control studies. Thus, even though it is possible that cannabis impairs driving ability to some extent, there are currently no reliable means to test or measure whether a driver was actually impaired. Nonetheless, policymakers in the United States have generally dealt with cannabis-and-driving criminalization by importing the alcohol DUI regime into the cannabis context. This has led to complications down the road when cannabis-driving cases land in criminal court because cannabis detection science differs vastly from alcohol detection science. For example, blood alcohol content (BAC) has similar rates of absorption, distribution, and elimination across all humans, and there is also a fairly good correlation between BAC and level of impairment (in other words, impairment increases when BAC increases, and impairment decreases when BAC decreases). This has allowed law enforcement to use tools like breathalyzers and blood tests in criminal court because alcohol concentration is a relatively reliable indicator of how recently and how much alcohol was consumed. In contrast, THC levels can vary widely depending on the means of ingestion, THC is metabolized at an exponentially declining rate (as opposed to the steady metabolization rate for alcohol), and there is very poor correlation of THC blood levels with impairment. As stated in a report to Congress produced by the U.S. Department of Transportation's National Highway Traffic Safety Administration, "[I]n contrast to the situation with alcohol, someone can show little or no impairment at a THC level at which someone else may show a greater degree of impairment." The report also noted that, in some studies, THC was detectable as late as 30 days after ingestion–even though the acute psychoactive effects of cannabis last only for a few hours. (en)
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