Thirty-two drugs were detected. A single agent was detected in only 34 percent (47) of the cases. The most frequently found drugs, expressed as percent of positive cases, were phencyclidine, 47 percent; marijuana, 47 percent; benzodiazepines, 22 percent; barbiturates, 15 percent; opiates, 11 percent; and cocaine, 9 percent. Most multiple drug cases involved popular illicit drug mixtures such as cocaine and morphine ('speedballs') or phencyclidine on marijuana ('whack'). All the drivers in this survey had displayed inappropriate or impaired operation of a motor vehicle to the extent that a police officer had stopped and charged them for DUID. In at least 81 percent of the drug-positive cases, persons impaired in the operation of a motor vehicle from a drug or drugs other than alcohol were impaired from deliberate self-intoxication with illicit or controlled substances. Data indicate DUID reflects illicit drug abuse and may be an epidemilogic indicator of local drug abuse. 1 table, 38 references. (Author summary modified)