For the purposes of DUI law, generally you are "impaired" if your ability to safely operate a motor vehicle is appreciably affected by having consumed alcohol, illicit drugs, or prescription medications. Since everyone is affected differently by different substances, "per se" laws set measurable limits. For instance, motorists who are shown to have a blood-alcohol concentration (BAC) of at least percent are assumed to be impaired. Similarly, some states assume that any amount of certain drugs in the bloodstream constitutes impairment. How Did DUI Laws First Come Into Existence?