Police in New York have the option to inspect the exterior of a stopped vehicle with a drug-sniffing dog. A police canine that signals that drugs are present enables officers to search a vehicle without a warrant. Although dogs have a sense of smell far superior to human olfactory capabilities, police canines have a rocky record when it comes to accuracy. An audit of 139 traffic stops showed that 45 percent of the vehicles searched after dogs alerted to the presence of narcotics did not contain any drugs.

Some of these false positives produced by police canines may happen because trace amounts of drugs or drug residues were, in fact, in the vehicles. Canine research indicates that dogs can detect odors in concentrations as little as one part per trillion. In some cases, dogs appeared to have alerted to drug odor because the drivers had admitted to recent marijuana use, and the scent lingered on their clothing.

Police canines also perform in the context of a working relationship with handlers. Conscious or subconscious cues from a handler may influence a dog to alert when drugs are not present. A handler who persists in leading a dog around a vehicle over and over may motivate a dog to make a positive reaction for drugs. Some research has shown that dogs alert to drugs more often on vehicles of black or Latino people. The racial prejudices of handlers may be influencing their dogs.

The circumstances behind a police search that resulted in accusations of drug crimes may play an important part in a person’s defense strategy. An attorney representing a drug suspect might question the validity of law enforcement’s actions. Legal challenges to a prosecutor’s case may inspire a reduction of charges or a case dismissal.