A recent New York Times article presents a number of interesting findings about driving under the influence of marijuana, including public perception, the actual risks posed and the methods used to determine the level of impairment.
With regard to public perception, an alarming number of Colorado residents contacted in a phone survey by the Department of Transportation were surprised to learn that DUI laws apply to driving while under the influence of marijuana. Indeed, some people felt that their driving abilities were improved by marijuana consumption.
While researchers don’t agree, there is a growing body of contradictory studies, some of which suggest that drivers under the influence of marijuana are not nearly as prone to accidents as those impaired or intoxicated by alcohol. A majority of studies, according to the Times article, find roughly a twofold increase in the risk of accidents for persons on a marijuana high, a number that is still five to ten times less than that for drunk drivers. One researcher, Eduardo Romano, a scientist with the Pacific Institute for Research and Evaluation, concluded that once adjusted for demographics and the presence of alcohol, marijuana did not statistically increase the risk of a crash, possibly because “stoned” drivers tended to drive slower than normal, or because they tend to travel less when using the drug.
Yet another problem is how to measure marijuana impairment. A 2012 study showed that while certain types of roadside sobriety maneuvers were effective in identifying nearly 90 percent of persons intoxicated by alcohol, less than a third of stoned drivers failed the same test and that those who were most accustomed to being stoned found it easiest to pass.
In 2013, Colorado and Washington, reacting to legalization of limited amounts of recreational marijuana, passed legislation creating a “permissive inference” that anyone having five or more nanograms of THC in their system, per milliliter of blood, was under the influence. Some studies suggest that this “limit” is far too generous, and that as many as 90 percent of those impaired by marijuana would have less than that amount in their system. But other studies show that frequent users of marijuana can retain that same amount of THC in their system an entire day after they last lit up.
The debate will no doubt continue until more studies are completed. A growing number of experts do seem to be in agreement that drunken driving remains a more dangerous practice and that limited safety enforcement resources would best be directed there.