When The San Diego Union-Tribune Editorial Board endorsed Proposition 64, the November ballot measure legalizing recreational marijuana use for people over 21, our view was that marijuana use was already widespread throughout California because of a lax medical marijuana law that could be gamed by recreational users — and that it made sense to regulate and tax pot rather than continue with a laissez-faire status quo. But we worried about the lack of a well-regarded driver impairment field test for THC, marijuana’s primary active ingredient.

Now the San Diego Police Department is using a testing method to establish possible drug intoxication that has gotten mixed marks elsewhere. The department’s two Dräger DrugTest 5000 machines test oral mouth swipes for the presence of marijuana, cocaine, opiates, methamphetamine, amphetamine, methadone and benzodiazepines. Dräger says its machines only test for the presence of the active THC compound that creates the high, not residual THC. San Diego police say if a test shows the compound, drivers will be taken in for blood testing to establish their level of impairment.

But while this testing method is being used by more and more law enforcement authorities, it has its skeptics. Last year, Susan Price, the Orange County district attorney’s assistant head of courts, told the Orange County Register that in her agency’s experience, the test was more effective in detecting amphetamines and cocaine than marijuana and prescription drugs, citing several false positive and negative results. And the AAA Foundation for Traffic Safety reported last year that its research found that the claim that blood tests could show THC impairment “cannot be scientifically supported.” Defense attorneys are going to have potent tools to push back at marijuana DUI charges.


Thankfully, Proposition 64 provides funding for attempts to devise more reliable law enforcement tools. The UC San Diego Center for Medicinal Cannabis Research sees promise in field tests that measure reaction time, attention, coordination and perception instead of THC. For now, though, accurately testing drivers for marijuana impairment remains problematic — and legal headaches await authorities who insist otherwise.

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