Driving under the influence (of marijuana)

February 19, 2014, 12:06 am

How safe is driving under the influence of marijuana? For forensic purposes, are there tests that can establish impairment from THC? If so, how can they be used fairly and accurately. These are important questions now — especially in the states of Washington and Colorado — and they are explored in a very interesting article by Maggie Koerth-Baker in this morning’s New York Times science section.

One can argue about the use of roadside field sobriety tests and Breathalyzer results to demonstrate driving under the influence of alcohol — lawyers hash out these issues in coatrooms every day — but the science behind these tests has evolved over many decades, and is well accepted.

Not so with marijuana. One researcher quoted in the article says of the existing science examining cannabis and driving: “It’s a mishmash.”

Although studies to date have yielded inconsistent results, most scientists Koerth-Baker interviewed believe that THC does indeed cause degradation of driving skills, but not nearly to the extent that alcohol does. Whereas even an alcohol level at the legal limit can increase the risk of a fatal accident by a factor of 10 or 20:

[S]everal researchers, working independently of one another, have come up with the same estimate: a twofold increase in the risk of an accident if there is nay measurable amount of THC in the bloodstream.

One problem with this is that THC, being lipophilic, is taken up by fatty tissues and slowly released. Therefore, it may be detected in the blood for a day or more after last use. This makes it hard to establish a legal limit that would reliably indicate impairment but not capture previous users not under the influence at the time the sample was drawn. In Washington State and Colorado the limit is 5 ng/ml. Some experts think this cutoff is set way too high.

THC seems to affect drivers in a different way than alcohol does:

Drunken drivers tend to drive faster than normal and to overestimate their skills, studies have shown; the opposite is true for stoned drivers

My opinion is that THC must surely impair driving skills in important and quantifiable ways. Dr. Marilyn A. Huetis of the National Institute on Drug Abuse has studied this question:

The deficits of being stoned really began to show up, she said, when people had to handle multiple tasks at once and were confronted with something unexpected.

It seems to me that multiple tasking and dealing with the unexpected are among the basic skills required for driving any automobile safely.



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