Driving Under the Influence

Driving Under the Influence

This recent article in the Washington Post reports that “a police department in Maryland is training officers to spot the signs of driving high by watching people toke up in a tent”. The story features Lt. John O’Brien, who is described as a “trained drug recognition expert”. It also quotes a defense attorney who says, “There are real questions about the scientific validity of what they’re doing.”

As it happens, the scientific validity of Drug Recognition Experts is one of the examples in my forthcoming book, Probably Overthinking It. The following is an excerpt from Chapter 9: “Fairness and Fallacy”.

In September 2017 the American Civil Liberties Union (ACLU) filed suit against Cobb County, Georgia on behalf of four drivers who were arrested for driving under the influence of cannabis. All four were evaluated by Officer Tracy Carroll, who had been trained as a “Drug Recognition Expert” (DRE) as part of a program developed by the Los Angeles Police Department in the 1970s.

At the time of their arrest, all four insisted that they had not smoked or ingested any cannabis products, and when their blood was tested, all four results were negative; that is, the blood tests found no evidence of recent cannabis use.

In each case, prosecutors dismissed the charges related to impaired driving. Nevertheless, the arrests were disruptive and costly, and the plaintiffs were left with a permanent and public arrest record.

At issue in the case is the assertion by the ACLU that, “Much of the DRE protocol has never been rigorously and independently validated.”

So I investigated that claim. What I found was a collection of studies that are, across the board, deeply flawed. Every one of them features at least one methodological error so blatant it would be embarrassing at a middle school science fair.

As an example, the lab study most often cited to show that the DRE protocol is valid was conducted at Johns Hopkins University School of Medicine in 1985. It concludes, “Overall, in 98.7% of instances of judged intoxication the subject had received some active drug”. In other words, in the cases where one of the Drug Recognition Experts believed that a subject was under the influence, they were right 98.7% of the time.

That sounds impressive, but there are several problems with this study. The biggest is that the subjects were all “normal, healthy” male volunteers between 18 and 35 years old, who were screened and “trained on the psychomotor tasks and subjective effect questionnaires used in the study”.

By design, the study excluded women, anyone older than 35, and anyone in poor health. Then the screening excluded anyone who had any difficulty passing a sobriety test while they were sober — for example, anyone with shaky hands, poor coordination, or poor balance.

But those are exactly the people most likely to be falsely accused. How can you estimate the number of false positives if you exclude from the study everyone likely to yield a false positive? You can’t.

Another frequently-cited study reports that “When DREs claimed drugs other than alcohol were present, they [the drugs] were almost always detected in the blood (94% of the time)”. Again, that sounds impressive until you look at the methodology.

Subjects in this study had already been arrested because they were suspected of driving while impaired, most often because they had failed a field sobriety test.

Then, while they were in custody, they were evaluated by a DRE, that is, a different officer trained in the drug evaluation procedure. If the DRE thought that the suspect was under the influence of a drug, the suspect was asked to consent to a blood test; otherwise they were released.

Of 219 suspects, 18 were released after a DRE performed a “cursory examination” and concluded that there was no evidence of drug impairment.

The remaining 201 suspects were asked for a blood sample. Of those, 22 refused and 6 provided a urine sample only.

Of the 173 blood samples, 162 were found to contain a drug other than alcohol. That’s about 94%, which is the statistic they reported.

But the base rate in this study is extraordinarily high, because it includes only cases that were suspected by the arresting officer and then confirmed by the DRE. With a few generous assumptions, I estimate that the base rate is 86%; in reality, it was probably higher.

To estimate the base rate, let’s assume:

  • All 18 of the suspects who were released were, in fact, not under the influence of a drug, and
  • The 28 suspects who refused a blood test were impaired at the same rate as the 173 who agreed, 94%.

Both of these assumptions are generous; that is, they probably overestimate the accuracy of the DREs. Even so, they imply that 188 out of 219 blood tests would have been positive, if they had been tested. That’s a base rate of 86%.

Because the suspects who were released were not tested, there is no way to estimate the sensitivity of the test, but let’s assume it’s 99%, so if a suspect is under the influence of a drug, there is a 99% chance a DRE would detect it. In reality, it is probably lower.

With these generous assumptions, we can use the following table to estimate the sensitivity of the DRE protocol.

SuspectsProb PositiveCasesPercent
Not impaired140.405.606.2

With 86% base rate, we expect 86 impaired suspects out of 100, and 14 unimpaired. With 99% sensitivity, we expect the DRE to detect about 85 true positives. And with 60% specificity, we expect the DRE to wrongly accuse 5.6 suspects. Out of 91 positive tests, 85 would be correct; that’s about 94%, as reported in the study.

But this accuracy is only possible because the base rate in the study is so high. Remember that most of the subjects had been arrested because they had failed a field sobriety test. Then they were tested by a DRE, who was effectively offering a second opinion.

But that’s not what happened when Officer Tracy Carroll arrested Katelyn Ebner, Princess Mbamara, Ayokunle Oriyomi, and Brittany Penwell. In each of those cases, the driver was stopped for driving erratically, which is evidence of possible impairment. But when Officer Carroll began his evaluation, that was the only evidence of impairment.

So the relevant base rate is not 86%, as in the study; it is the fraction of erratic drivers who are under the influence of drugs. And there are many other reasons for erratic driving, including distraction, sleepiness, and the influence of alcohol. It’s hard to say which explanation is most common. I’m sure it depends on time and location. But as an example, let’s suppose it is 50%; the following table shows the results with this base rate.

SuspectsProb PositiveCasesPercent
Not impaired500.4020.028.8

With 50% base rate, 99% sensitivity, and 60% specificity, the predictive value of the test is only 71%; under these assumptions, almost 30% of the accused would be innocent. In fact, the base rate, sensitivity, and specificity are probably lower, which means that the value of the test is even worse.

The suit filed by the ACLU was not successful. The court decided that the arrests were valid because the results of the field sobriety tests constituted “probable cause” for an arrest. As a result, the court did not consider the evidence for, or against, the validity of the DRE protocol. The ACLU has appealed the decision.


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