Some States (Arizona, Delaware, Georgia, Indiana, Illinois, Iowa, Michigan, Minnesota, Nevada, North Carolina, Ohio, Pennsylvania, Rhode Island, South Dakota, Utah, Virginia, and Wisconsin) have passed “per se” laws, in which it is illegal to operate a motor vehicle if there is any detectable level of a prohibited drug, or its metabolites, in the driver’s blood. Other State laws define “drugged driving” as driving when a drug “renders the driver incapable of driving safely” or “causes the driver to be impaired.”

http://www.drugabuse.gov/publications/drugfacts/drugged-driv...