"Drug per se laws are not quite analogous to the alcohol impaired-driving per se laws now in effect in every State make it illegal to operate a motor vehicle with a blood alcohol concentration (BAC) of .08 grams per deciliter or greater. Alcohol-impaired driving per se laws are based on evidence that all drivers are impaired at .08 BAC. Drug per se laws are more analogous to zero-tolerance laws that make it illegal to drive with certain drugs in the system."
Lacey, John, Brainard, Katharine, and Snitow, Samantha. (2010). Drug Per Se Laws: A Review of Their Use in States. (DOT HS 811 317). Washington, DC: National Highway Traffic Safety Administration, p. 1.