Countless studies have shown that there is a direct correlation between increasing levels of ethanol in the blood and physical impairment. This relationship is the basis of our drunk driving laws.
However, what's not so well known is the relationship between illegal or prescription drugs and physical impairment.
Whenever a urine test or a blood test is taken as the result of an arrest for driving while intoxicated, substances other than alcohol can be detected in the specimen. Should any of these substances be detected, a driver may be looking at a DWI prosecution where the intoxication is alleged to have resulted from illegal drugs or prescription medications alone or combined with alcohol.
The problem with charging a motorist with driving while intoxicated by drug, is proving the relationship between the level of that drug detected and impairment. While the pharmacokinetics of a drug, that is, what the body does to the drug, are well-documented, the pharmacodynamics, or what the drug does to the body, are much murkier.
In general it is assumed that the higher the concentration of the drug, the greater the effect of the drug. This assumption does not, however, address the core question - whether the drug affected the driver's mental and/or physical faculties to the degree that the driver is intoxicated.
We are also left to wonder about the relationship between the concentration of the drug in the blood or urine and the concentration of the drug in the brain. For without knowing the concentration of the drug in the brain, it is impossible to know whether the detected amount could cause a driver to be intoxicated.
In fact, according to a report issued by the Virginia Institute of Forensic Sciences and Medicine, there is no statistical correlation between marijuana metabolite concentration and performance on NHTSA's standardized field sobriety test battery. Researchers were also unable to determine any per se concentration akin to a .08 alcohol concentration.
There are studies indicating the effects of marijuana on one's mental and physical faculties may last up to three hours; but traces of THC may still be in one's blood or urine more than four hours after smoking.
NHTSA conducted tests on the effects of marijuana and actual driving performance and came to the conclusion that "it is not possible to conclude anything about a driver's impairment on the basis of his/her plasma concentrations of THC...determined in a single sample." The study went on to conclude that the "[p]lasma of drivers showing substantial impairment in these studies contained by high and low THC concentrations; and, drivers with high plasma concentrations showed substantial, but also no impairment, or even some improvement."
The bottom line is that there is no level of illegal drug or prescription medication that can prove a driver was intoxicated at the time of driving.