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 new editorial 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.