As a matter of law (as upheld in an appellate decision) the prosecution must prove beyond a reasonable doubt that the drugs have affected the nervous system, brain, or muscles as to impair to appreciable degree the ability to operate a vehicle.
In other words, they must show that the driving was screwed up, that that the drug use actually impaired the driving ability. Otherwise there is insufficient evidence as a matter of law.
Drug DUI vs. Alcohol DUI
But unlike alcohol, where there is a defined legal limit (usually .08% BAC), there is no defined limit for drugs, no “threshold” — and thus no proof.
The challenge for the prosecution is to find enough admissible evidence to show that the person’s driving was impaired. They must show that 1) there were drugs in the system, and 2) the drugs were impairing the person’s driving at the time.
They can say, “We have 2000 nanograms of marijuana metabolite,” but maybe 2000 nanograms affects one person and makes him wacky, whereas the guy who smokes grass every day and builds up a tolerance—he barely gets a buzz. And then there’s the question: what’s the power and potency of the marijuana?
In addition, the prosecution must clearly prove how long ago the drug or drugs were consumed or introduced, since there are so many variables that affect how each person’s body metabolizes the drugs, and therefore no clear measurement guideline exists.
Thus they tend to focus on subjective, circumstantial evidence — how the person stands and walks, or a failed sobriety test. But that is not enough.
I have found that if I bring the case to a prosecutor and file a brief, I have very good luck getting DUI charges reduced to Exhibitions of Speed in these kinds of cases.