Despite Fears, No Clear Connection Between Pot and Driving Fatalities

By Paul Gaita 09/08/14

Fears of THC-caused traffic accidents are either inconclusive or exaggerated.

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Paramount Pictures

As more states debate the issue of legalizing marijuana, the lingering issue of pot’s impact upon driving ability continues to generate concern for both sides of the argument.

While Colorado and Washington have allowed retail sales of marijuana, and 23 states approved its use for medical purposes, it remains illegal to drive under the influence of pot in all 50 states. However, reports of increased numbers of drivers who tested positive for THC have given rise to fears that more users are getting behind the wheel and boosting the risk of traffic accidents or fatalities.

In 2013, nearly 25% more drivers tested positive for marijuana use than in the previous year. But in terms of traffic fatalities, the numbers remain inconclusive. A National Safety Council study looked at statistics in 12 states for drivers who tested positive for marijuana and were involved in deadly crashes between 1992 and 2009. Of that group, only California, Hawaii, and Washington showed an increase, but that was apparently due to differences in testing between states.

That variable between tests has prevented researchers from drawing any significant conclusions from data about pot and driving. Dr. Mehmet Sofuoglu of the Yale University Medical School, noted that such studies remain “highly inconclusive,” as evidenced by certain studies that show the risk of crash increasing two to three times after using marijuana, while others show no increase or even a decrease in the risk among users.

A pair of studies conducted by Columbia University and the Pacific Institute for Research and Evaluation (PIRE) best exemplified the inconclusive nature of these tests. The Columbia University research compared drivers who tested positive for THC with state drug and alcohol tests for drivers killed in crashes. In doing so, they saw that marijuana could increase the likelihood of a fatal crash by 80%, a jarring fact until the whole picture is considered.

The study was conducted in six states, some of which do not test drivers for drugs and alcohol, including those who died in fatal crashes. In excluding these drivers, the tests may not portray an entirely accurate picture of the test’s focus. The use of urine tests to determine THC levels instead of blood tests may also contribute to skewed results, as the former will frequently give higher levels of impairment.

Information for the PIRE study was culled from a roadside survey by the National Highway Traffic Safety Administration from 2007, as well as data from nine states that test more than 80% of drivers killed in crashes. What researchers discovered was that drivers who tested for marijuana were less likely to have been involved in a crash than those who posted negative test results for drugs. The extreme variance between these two tests are due to a number of factors, from user tolerance levels to methods of testing.

With so much conflicting information, states are left with few options to contend with driver impairment until more conclusive studies are made. Currently, Colorado, Washington, and Montana have an intoxication threshold of 5 parts per billion of THC in blood levels. Most states have not set specific levels, which has prompted concern from the National Transportation Safety Board (NTSB).

“If states legalize marijuana, they must set clear limits for impairment behind the wheel,” said former NTSB chairwoman Deborah Hersman. “Right now, we have a patchwork system across the nation.”

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Paul Gaita lives in Los Angeles. He has contributed to the Los Angeles Times, Variety, LA Weekly, and The Los Angeles Beat, among many other publications and websites.