States across the country have set a testable limit on the level of alcohol in someone’s bloodstream as a threshold in lieu of demonstrating impairment. While .08 has been established as a baseline legal limit for alcohol, there is no test available to readily measure the amount of THC, the active ingredient in marijuana, that is present in someone’s system. CNN.com took a long look at the issue, and at ongoing efforts to create a test similar to the breathalyzers that measure alcohol for purposes of prosecuting DUIs.
It is against the law for anyone to drive under the influence of alcohol or drugs, even if the drugs have been prescribed. Florida defines under the influence as “under the influence to the extent that normal faculties are impaired,” and other states use similar definitions. That’s why law enforcement use field sobriety exercises: they are meant to give the officers a chance to observe a driver to see if their faculties appear to be impaired. The shortcoming of those tests is their unreliability and that they are subjective: an officer will see impairment if they are looking for it. The breath and blood alcohol tests at least provide some consistency, though they are not impairment based. Time will tell if science can come up with something comparable for THC and other controlled substances.
Reliance on impairment based tests is challenging for law enforcement when a case goes to trial. Unless the impairment is clear, a jury may be reluctant to find it beyond a reasonable doubt. The subjectivity may matter more in a DUI than any other, and the outcome of a DUI is more dependent on the skill of the attorneys trying the case. The ambiguity cuts both ways, because the subjectivity of the field sobriety testing may lead to an arrest, and there is no dispositive scientific test to disprove the allegation. If a cop thinks you are impaired, you can be arrested, and the case may have to go to a jury trial. It will be interesting as this field becomes more important with the expansion of recreational and medicinal marijuana.