Drugged Driving: The State of the Research 4/21/06

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14. Drugged Driving: The State of the Research

With the Office of National Drug Control Policy pushing states across the country to adopt zero-tolerance Driving Under the Influence of Drugs (DUID) laws that would criminalize drivers who have any detectable amount of cannabis or its metabolites in their systems, research on the effect of cannabis on driving has been a growth industry in recent years. The emerging consensus among scientists is that cannabis use slightly impairs driving ability for a short period of time immediately after ingestion, while cannabis stays in the system for a much longer period of time, thus suggesting that zero-tolerance DUID laws will snare unimpaired drivers who smoked hours or even days before being tested.

nighttimedriving checkpoint
Two recent "metanalyses," or reviews of the literature, one from the Canadian Parliament's committee on cannabis in 2002 and one by a group of 11 researchers from around the world in 2005, as well as 2004 Dutch comparative analysis of cannabis, alcohol, and other drugs and their impact on driving make the case for a more nuanced policy.

The 2004 study, by the SWOV Institute for Road Safety Research looked at the link between drug use and traffic accidents requiring hospitalization by studying actual accident victims. It found "high relative risks" of accident for people using multiple substances (including alcohol) and, somewhat surprisingly, increased but "not statistically significant" risk for users of amphetamines, cocaine, and opiates. And marijuana? "No increased risk for road trauma was found for drivers exposed to cannabis," the study concluded.

The Canadian Senate Special Committee on Illegal Drugs, chaired by Sen. Pierre Nolin, examined a wide variety of issues related to marijuana use. In the section of the report devoted to driving under the influence of cannabis, which reviewed a number of previous studies, the committee noted that while cannabis use slightly impairs driving ability, that impairment takes an unusual form. "Cannabis makes users more cautious," the report found. But "cannabis alone, particularly in low doses, has little effect on the skills involved in automobile driving."

In the most recent overview of the research on marijuana and driving, "Developing Science-Based Per Se Limits for Driving under the Influence of Cannabis (DUIC) Findings and Recommendations by an Expert Panel," a team of researchers agreed with the Canadians that the weed "has a negative impact on decision time and trajectory" even though "cannabis leads to a more cautious style of driving." Thus, unlike alcohol users, marijuana users realize they are impaired and compensate accordingly.

With a mandate to address the question of per se limits (laws that assume that more than a certain quantity of a substance in one's system constitutes proof of impairment, like blood alcohol content laws), the expert team rejected the zero-tolerance laws recommended by drug czar John Walters and passed by more states each year. Because marijuana metabolites are detectable for days or weeks after smoking, "zero-tolerance laws will classify many cannabis users as impaired drivers even if they separate drug use and driving by many hours," the study concluded. "The same applies to the increasing number of individuals who legally use cannabis for medicinal purposes and, while not acutely impaired, may present with measurable THC concentrations at all times."

The study instead recommended setting a per se limit of 3.5 to 5 nanograms of THC per milliliter of whole blood, an amount that "would clearly separate unimpaired drivers with residual THC concentrations from drivers who consumed cannabis within the last hour or so." But the study seems a bit conservative on the per se level. Those most likely to be impaired, the study reported, typically showed THC levels greater than 10 nanograms, at least double the recommended per se limit.

Zero-tolerance is not a workable policy, the researchers concluded. "Extended or complete abstinence, as implicitly stipulated by a zero THC limit in blood, is not a realistic option, particularly for many younger drivers, who are the primary target for DUIC control. A zero limit also interferes with the concept of using designated drivers. A driver who used cannabis more than 12 hours before an event may still present with measurable THC concentrations, and would therefore be in violation of a zero limit whether he or she abstained from use at the event or not."

Finally, the researchers warned against using DUID laws as a means of attempting to control marijuana use. "DUIC control should focus on improving traffic safety and not on achieving other societal goals. Completely eliminating the (illegal) use of cannabis through a zero-tolerance law for DUIC seems unrealistic and counterproductive."

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Issue #432 -- 4/21/06

Drug War Chronicle, recent top items


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