The Volokh Conspiracy

Mostly law professors | Sometimes contrarian | Often libertarian | Always independent

Volokh Conspiracy

Impressions from the oral argument in Kansas v. Glover

On reasonable suspicion and how to calculate it.


The Supreme Court held argument Monday in Kansas v. Glover, the Fourth Amendment case I blogged about last week.  The transcript is up here, and I thought I would blog some reactions.

1. Does the officer need to testify?  Several Justices were particularly interested in whether the officer needs to take the stand and testify that, in his training and experience, a positive hit on an owner-license-suspension query correlates sufficiently with the premise that the owner with a suspended license is driving.  A few Justices suggested that this could be important to provide actual evidence in support of the inference that this was likely.

I gather this question was a focus in part because the parties in Glover stipulated the facts. It was agreed that the officer assumed that the positive hit meant the owner was driving, and as a result he did not  testify about his "training and experience" with that.

There was also a recognition that, as Justice Gorsuch noted, requiring an officer to take the stand might be pretty formalistic. After all, the officer will take the stand and say that he was trained to draw that distinction, or that his impression is that the inference is justified.  If that's all that is required, then officers will be trained accordingly and cross examination about that training and experience may not add much.

This is a really interesting issue.  The general question, I take, is this: In assessing cause, what is the zone of judicial assessment and what is the zone of officer experience?  When are conclusions properly a call for judges to make, and when are conclusions something for an individual witness?  Off the top of my head, I don't think there's a well-developed theory for what the line is between those two.

For what it's worth, my own intuitions tend to make the general question something for judges but the experience with special cases something up to witnesses.  If that's right, I would think that the officer's testimony shouldn't be required in a case like Glover, but that there will be Glover cases where it would be relevant.

Here's my thinking.  If we want to get a ballpark sense of whether an intuition is justified, the ballpark sense is probably something best generalized by courts.  I would think it would be a relatively uncommon case in which the officer's training and experience can shed substantial light on the generalized answer.   Relying on the officer's training seems particularly unhelpful because it's presumably circular.  In the future, the officer's training should depend on how the Court rules in Glover.  So that doesn't seem all that useful.

I would think that an officer's testimony about his experience will be pretty limited in most cases.  As the Chief Justice noted, some officers will be new.  Others will have no strong recollections of the correlation between a positive database hit and the owner driving.  And none will be well-equipped to offer a statistical analysis of the inference anyway, such as whether the factors that went into their past experience were relevant to this case.  Whether the inference is justified strikes me as something often or typically beyond the officer's own personal experience.  Given that, I'm skeptical that such testimony should be required.

I see this as a lot like Harris, the dog sniff case.  The officer didn't need to testify that, in his experience, the alert of a well-trained dog was ordinarily sufficient to create probable cause.  That was the Court's assessment, not the officer's.  Here was the key language from Harris:

If a bona fide organization has certified a dog after testing his reliability in a controlled setting, a court can presume (subject to any conflicting evidence offered) that the dog's alert provides probable cause to search. The same is true, even in the absence of formal certification, if the dog has recently and successfully completed a training program that evaluated his proficiency in locating drugs.

The ballpark intuition came from the Court.

With that said, I would also think that the officer's experience is certainly relevant in some Glover cases.  Officer experience can be important when an officer has a lot of experience with a recurring fact pattern.  That experience can impart significant knowledge that may not be obvious to outsiders.  For example, an officer who does undercover buys often may know how drug deals work, and that may give the officer knowledge of what lessons to draw from something he sees during a drug buy that someone without that experience would lack. The idea is that the officer can tell us about a world we don't know, serving as a sort of expert to guide us.

If I'm right about that, I would think there will be circumstances in which testimony about an officer's experience is relevant.  Imagine an officer has a lot of localized experience with a particular kind of stop in a particular place and time.  If the officer has a lot of experience with past stops similar to that one, the officer's experience could give useful localized information about whether the stop in those same circumstances justifies the usual inference or instead a departure from it.

Again, it seems a lot like Harris.  The Court gives a ballpark assessment, and then you can get testimony about whether a particular situation fits that ballpark assessment.

2.  Quantifying probable cause and reasonable suspicion.  Chief Justice Roberts pushed a line of questioning in which he discussed ballpark numbers for probable cause and reasonable suspicion.  He quantified probable cause, which he said had been held to be less than fifty percent:

CHIEF JUSTICE ROBERTS: Okay. Well, we know that probable cause is not 50 percent.


CHIEF JUSTICE ROBERTS: It's somewhat less than 50 percent.


He then asked about a quantification of reasonable suspicion:

CHIEF JUSTICE ROBERTS: Well, what — what reasonable suspicion cutoff do you think? Do you think it's one out of five?

MS. HARRINGTON: I can't say because this Court has said repeatedly that —


MS. HARRINGTON: — none of us can say, right?

As President of the  Why Probable Cause Shouldn't Be Quantified Club, I thought I would push back on this.  First, contrary to the Chief's statement, the Court has not held that probable cause is less than 50%.  Granted, there are cases saying that the government doesn't need to show more than a 50% likelihood to establish probable cause.  But that's because probable cause is not quantifiable, not because it is quantifiable and the target number is below 50%.

The Court has put he point this way: "The test for probable cause is not reducible to precise definition or quantification. Finely tuned standards such as proof beyond a reasonable doubt or by a preponderance of the evidence  have no place in the probable cause decision." Florida v. Harris (2013), (cleaned up)   I think the same goes for reasonable suspicion. Although some Justices have hinted at wanting to quantify it, such as in Justice Scalia's dissent in Navarette, I don't think it is a quantifiable idea.

3.  Can cause be based on purely statistical information?  At the bottom of page 30, Justice Kagan asked whether reasonable suspicion could be based on no more than a statistical study:

Suppose that a municipality has a law that says everybody has to carry their driver's license with them at all times. And suppose that a particular police department actually did a kind of survey or, you know, a study of their practices and found that actually 50 percent of teenagers do not carry their driver's license with them at all times. All right? So now it's like common sense that if you see a teenager, she won't be carrying her driver's license with her. Does that give the police officer the ability to stop every teenager that he sees?

The scholarship on cause thresholds has pondered similar hypotheticals.  For example, in my quantifying probable cause article, I asked whether the government could get a warrant to search a Harvard dorm for marijuana based solely on a reliable study showing that 60% of Harvard dorms have marijuana.  It's the same basic question, I gather: When can the government rely on statistics alone?

In the context of a warrant, I argued, the problem with relying on statistical evidence and no more is that the actual probability of finding evidence was likely no where near like the percent found in the study.  The reason is the missing information.  Someone had selected that particular dorm room to search, and investigators apparently had found no other evidence justifying the search beyond the study to include in the warrant affidavit.  To know the actual likelihood drugs would be found, I argued, we really need to know the backstory of why that room was selected and why no other evidence had been offered.  When you filled in that missing information, the actual probability quickly became wildly different from 60%.

A question raised by Justice Kagan's hypothetical is whether the same is true for a reasonable suspicion determination in the field in a Glover-like case.   If there are statistics showing how often a license-suspension hit leads to a driver behind the wheel, can a court rely on them in a particular case?  I'm not sure.  Off the top of my head, I think the basic dynamic is at least partly valid.  There could be lots of reasons to think that the particular case where a stop is made is not representative of the whole.

At the same time, I can also think of reasons why the dynamic might be less pronounced in that setting.  I gather that the decision to run a license check is normally not as involved as that for obtaining a warrant, and therefore involves less question of why a particular search or seizure was made and what investigation was tried.  Not sure where I come out, but it's an interesting question.

4. A note on the criminal/civil divide.  One additional small thought I had is that I gather all parties are assuming that driving without a license is a crime, not just a civil offense.  Under Terry and its progeny, evidence of an ongoing crime permits the stop.  So the court is treating Glover as a Terry case, and asking if a Terry stop is justified. That makes sense, as driving on a revoked or suspended license is a misdemeanor offense in Kansas.

I flag that because driving on a suspended license is a civil offense instead of a criminal offense in at least some jurisdictions, and the standard for a stop in the case of a civil infraction is less settled.  Whren held that probable cause to believe a civil traffic violation was committed justifies a stop. My recollection is that some lower courts have held that mere reasonable suspicion of a civil traffic violation also permits a stop, and that lower courts are divided on the question.

Given the apparent disagreement, it seems important to note that Glover is only about when the facts of a case satisfy the reasonable suspicion standard of Terry.  It is not about the distinct question of whether a stop can always be made when reasonable suspicion exists that a person is driving on a suspended license.