Although it may seem so at times, police generally do not initiate DUI traffic stops for no reason or for arbitrary reasons. The best advice to avoid being pulled over for any vehicle-related stop remains, “Don’t give them a reason.” If you do get pulled over and the police officer suspects you of drunk or drugged driving, he still needs enough certainty to establish probable cause before arresting you. He can establish some of that certainty, in some situations, with the use of test devices like breathalyzers and blood test equipment. It may even not be long before, in at least one state, police can use a “textalyzer” to see if you were using your handheld device at the time of an accident.

Tests to detect alcohol in your system have been around long enough and have been refined enough that they can – allegedly, at least – measure even trace alcohol amounts in your blood. Hence the .08 DUI standard that you are likely familiar with.

But not every drug lends itself so readily to equipment-based detection, and on top of that in the case of at least one illegal drug – marijuana – it can be difficult to decide how much you need to have consumed or how influenced by it you need to be to qualify as being too high to drive.

It isn’t that marijuana does not have a negative effect on driving. Studies have concluded that marijuana leads to problems like difficulty maintaining consistent speed, lane weaving, and reduced ability to react to sudden events. Taken together with alcohol, marijuana tends to heighten the bad effects of alcohol such as speeding, overconfidence and risk-taking. For example, in Washington state (one of a few in the country where recreational use of marijuana is legal under state law), about one of every five fatal crashes involves a driver with marijuana in his or her system.

But what is the marijuana equivalent to that “.08” standard? Does it exist? How is it measured? Is it the same for everyone? Recent studies suggest that establishing a threshold limit to establish being under the influence of marijuana is a questionable exercise; “meaningless” is the term that one of the studies’ authors used to describe marijuana threshold levels for impaired driving.

As an alternative to hard-to-define numerical measurement standards, one recommendation from the studies is that states should instead rely on police officers trained to become proficient at recognizing marijuana impairment through “Drug Recognition Expert” (DRE) programs.

The idea underlying DRE programs is that the specially-trained police officers can be called upon to perform special testing of people suspected of being under the influence of marijuana at the time of their vehicle stop (if the DRE officer concludes that the subject is intoxicated, a blood test will usually follow).

Oklahoma already has a DRE program in place to train police officers to identify drug-impaired drivers, and not just for marijuana use. Under current law in this state having any amount of marijuana in your system is illegal when operating a vehicle, but if the trend toward legalization of not only recreational but also medical use of marijuana continues the question of whether to reconsider its illegality may one day also force a re-examination of how to spot with certainty a drug-impaired driver, and how to prove that conclusion.