Can Testing Measure Marijuana Impairment?

Most Americans are familiar with the number 0.8: this is the percentage of Blood Alcohol Concentration (BAC) at and above which a person is considered too intoxicated to drive. In other words, be over that limit while driving and you could be arrested. This is a straightforward, black-and-white policy. The state of Oregon, as it nears its public election on the issue of Marijuana legalization, is facing the question of whether Marijuana use could similarly be tested to draw a hard line for identifying impaired driving.

In Washington and Colorado, where Marijuana has already been legalized, similar practices of measurement have already been put into place. There, more than 5 nanograms of THC in the bloodstream is the threshold for an impaired driver. Oregon's ballot measure does not include this (or any) legal limit for THC; instead, it would allow police officers to decide whether a driver should be considered impaired.

While establishing a hard limit may seem the only way to consistently uphold a law on driving while influenced, some argue that such a measurement for THC would cause inaccuracies and possibly lead to wrongful convictions. An article released in the Oregonian cites a British study demonstrating that while THC levels were highest 10 minutes after a person used Marijuana, sensations of impairment were highest after 30 minutes. This would mean that those who were actually more of a risk would be less likely to be found guilty of driving while impaired.

In Washington and Colorado, where Marijuana has already been legalized, similar practices of measurement have already been put into place. There, more than 5 nanograms of THC in the bloodstream is the threshold for an impaired driver. Oregon's ballot measure does not include this (or any) legal limit for THC; instead, it would allow police officers to decide whether a driver should be considered impaired.

While establishing a hard limit may seem the only way to consistently uphold a law on driving while influenced, some argue that such a measurement for THC would cause inaccuracies and possibly lead to wrongful convictions. An article released in the Oregonian cites a British study demonstrating that while THC levels were highest 10 minutes after a person used Marijuana, sensations of impairment were highest after 30 minutes. This would mean that those who were actually more of a risk would be less likely to be found guilty of driving while impaired.

It is hard not to feel troubled, though, about leaving behind a scientific measurement (albeit flawed) for subjective judgment. Substances can affect some individuals more dramatically than others in their appearance and behavior. Certainly, the job of observing an individual and identifying whether he is in fact over the threshold of what should be considered an impaired state will be a tricky one. Oregon police officers—if the proposed law is voted into effect—will have a heavy burden in this judgment. One can only hope their eyes are better than imprecise testing.

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