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For years now, U.S. police departments have employed officers who are trained to be experts in detecting "drugged driving." The problem is, however, that the methods those officers use are not based on science, according to a in the Journal of Studies on Alcohol and Drugs (JSAD).

With marijuana now legal in many U.S. states, the need for reliable tests for marijuana impairment is more pressing than ever. Police can evaluate alcohol-intoxicated drivers by using an objective measure of breath alcohol results. But there is no "breathalyzer" equivalent for marijuana. The drug is metabolized differently from alcohol, and a person's blood levels of THC (the main intoxicating chemical in marijuana) do not correlate with impairment.

So law enforcement relies on subjective tactics—roadside tests and additional evaluations by police officers specially trained to be so-called drug recognition experts (DREs). These officers follow a standardized protocol that is said to detect drug impairment and is said to even determine the specific drug type, including marijuana.

The process involves numerous steps, including tests of physical coordination; checking the driver's blood pressure and pulse; squeezing the driver's limbs to determine if the muscle tone is "normal" or not; and examining pupil size and eye movements.

But while the protocol has the trappings of a scientific approach, it is not actually based on evidence that it works, said perspective author William J. McNichol, J.D., an adjunct professor at Rutgers University Camden School of Law.

Instead, McNichol said, the DRE process is a product of "police science"—techniques created by police officers to use in their work. Few scientific studies have tried to determine how often DREs get it right. But the existing evidence suggests they're "not much better than a coin toss," McNichol said.

Despite that, DRE programs and training are federally funded, and more than 8,000 DREs work in police departments nationwide, according to the International Association of Chiefs of Police. In addition, McNichol points out, a "spinoff" of the DRE has recently made its way into job sites: workplace impairment recognition experts, or WIREs, who are certified to detect and prevent on-the-job drug impairment.

Not long ago, when marijuana was uniformly illegal in the U.S., people would land in hot water for simple possession or use of the drug. Now that it's legal in many states, McNichol said, there is an urgent need for scientifically valid, reliable methods for detecting marijuana impairment. That, he added, will require scientists in the substance abuse field to get involved.

A published in the same issue of JSAD echoes that last sentiment. Collaborations between law enforcement and scientists who are not invested in either supporting or refuting the status quo is the best path forward, write Thomas D. Marcotte, Ph.D., and Robert L. Fitzgerald, Ph.D., from the Center for Medicinal Cannabis Research at the University of California San Diego.

"Developing more robust tools to identify cannabis-impaired drivers in an unbiased fashion is essential to keeping our roadways safe," they write. These authors also provide recommendations for improving the detection of drug-impaired driving.

As for how to fund that type of research, McNichol said a source already exists: taxes from legal marijuana sales.

"The money is there," he said, "if only it can be allocated properly."