To the untrained eye, Katelyn Ebner seems completely sober during her 28-minute roadside encounter with Cobb County, Georgia, police officer Tracy Carroll, who has pulled the 23-year-old waitress over for failing to maintain her lane as she made a left turn. But Carroll, who was designated a "drug recognition expert" (DRE) after undergoing 160 hours of special training, perceives "numerous indicators" that Ebner is under the influence of marijuana. Ebner repeatedly assures him she does not "smoke weed" or "do any of that stuff" and volunteers to prove it by taking a drug test. "You're going to jail, ma'am," he replies. "I don't have a magical drug test that I can give you right now."

Carroll does not need a magical drug test, because he is a magical drug test—or so the Cobb County Police Department would have you believe. But the experiences of innocent motorists like Ebner, who were arrested for driving under the influence of marijuana based on Carroll's hunch, only to be cleared by negative blood tests, suggest otherwise. This week three of them, including Ebner, filed a federal lawsuit that casts doubt on the drug-detecting abilities of DREs like Carroll.

The plaintiffs, who are represented by the ACLU of Georgia, were all stopped for briefly touching or crossing the line at the edge of their lanes—an offense that every driver on the road probably has committed at some point. They were all evaluated by Carroll, who deemed them stoned despite their protests to the contrary. They were all arrested for DUI and spent a night in jail. And in all three cases, as WXIA, the NBC station in Atlanta, revealed in an exposé last May, the DUI charges were eventually dropped after blood tests found no trace of marijuana—neither active THC nor inactive metabolites.

"As a result of their prosecutions," the ACLU complaint says, "Plaintiffs suffered the loss of liberty, extensive monetary losses, reputational damages, humiliation, and emotional distress," all "because a police officer had a hunch, based on deeply flawed drug-recognition training, that they might have been smoking marijuana." The ACLU notes that Carroll used a "watered-down version" of the 12-step DRE protocol, which it says "is itself riddled with flaws, based on discredited studies, and irresponsibly entrusts police officers with performing essentially medical or scientific tests."

The arbitrariness and subjectivity of the DRE tests can be seen in the dashcam video of Ebner's traffic stop. Ebner, who says this is the first time she has ever been stopped by police, is lucid, calm, polite, and cooperative throughout the video, despite her mounting anxiety about the length of the detention and her growing realization that she is going to jail even though she has committed no crime. But in Carroll's mind, the most innocent detail confirms his suspicion that she must be high.

Carroll notes that Ebner' eyes are watery, which she says is a reaction to a cleaning solution she uses at work. (Other supposed signs of intoxication include bloodshot eyes, glassy eyes, constricted pupils, and dilated pupils.) Carroll examines Enber's tongue, presumably looking for the "green coating" that police dubiously claim indicates recent marijuana use. He performs a horizontal gaze nystagmus test, which looks for an eye twitch that is a good indicator of drunkenness but has not been validated as a sign of cannabis consumption.

Carroll has Ebner blow into a breathalyzer, which confirms that she has not been drinking. He has her perform several roadside sobriety tests, including balancing on one foot, walking heel to toe on an imaginary line, extending her arms and touching her nose, and closing her eyes for 30 seconds. As the Supreme Judicial Court of Massachusetts noted in a recent ruling, the usefulness of such tests in detecting marijuana intoxication is still a matter of scientific dispute.

Ebner, in any case, seems to perform all of her assigned tasks just fine. Yet Carroll is determined not to be satisfied. As Ebner walks the imaginary line, betraying no obvious sign of intoxication, Carroll mutters, "improper number of steps." Presumably he also counted against her the extra six seconds she kept her eyes closed while mentally timing half a minute.

"You're showing me indicators that you have been smoking marijuana," Carroll says as he handcuffs Ebner. "I believe you're an impaired and less safe driver. That's why you were unable to maintain your lane."

Ebner is dismayed. "I can 1,000 percent guarantee you that I don't do that stuff," she says. "I can take a drug test. I can do any of that. You can call my mother. You can search my car. You can do everything."

The lawsuit argues that Ebner and her two co-plaintiffs, Princess Mbamara and Ayokunle Oriyomi, both college students, felt they had no choice but to submit to drug tests, especially after Carroll told them that state law required them to do so and that their driver's licenses would be suspended if they refused. Under the circumstances, the complaint says, the consent was not genuine, and since there was no warrant the blood tests violated the Fourth Amendment's ban on unreasonable searches and seizures. Ebner, Mbamara, and Oriyomi also argue that Carroll had no "justifiable basis" for subjecting them to sobriety tests and no probable cause to arrest them.

"Defendant Carroll's pattern and practice of enforcing DUI-drug infractions," the complaint says, "was to arrest an individual based on nothing more than a hunch, which would be invariably ratified by the results of an ad hoc smattering of tests he administered, which were divorced from any rigorous methodology and were without the foundational underpinning necessary to amount to legal justification to arrest….The way that Cobb County Police Officers such as Defendant Carroll are taught to and do administer their testing for the detection of impairment by drugs is designed to make innocent behavior appear incriminating and to make exculpatory behavior appear irrelevant." The ACLU argues that the Cobb County Police Department licensed, endorsed, and encouraged such pseudoscientific methods, "allowing officers artificially knighted with 'Drug Recognition Expert' status to falsely believe that they have a special and unique ability to detect marijuana use."

Even after blood tests confirmed that Ebner, Mbamara, and Oriyomi were telling the truth when they denied being under the influence of marijuana, the Cobb County Police Department defended Carroll's methods. Amazingly, the complaint notes, his superiors "continued to state that even if Defendant Carroll had known of the negative results of Plaintiff Ebner's blood test at the time she was arrested, nonetheless there would have been probable cause for her arrest."

Who are you going to believe? Some fancy lab test or Officer Carroll's gut?