Traffic Stops Based on a License Plate Reader Alert

The legal standard: reasonable suspicion

A traffic stop is a seizure under the Fourth Amendment. To make a brief investigative stop, police need reasonable suspicion: under Terry v. Ohio, “specific and articulable facts” suggesting criminal activity may be afoot. It’s a lower bar than probable cause, but it requires more than a hunch. Courts judge it on the totality of the circumstances known to the officer at the time.

This page is about the stop. Whether collecting or searching ALPR data was itself a search is a separate question. See is ALPR a Fourth Amendment search?

Is an unconfirmed alert enough?

An ALPR alert tells an officer that the system’s plate text matched a hotlist entry. As explained in ALPR hotlists and alerts, that match doesn’t confirm the plate was read correctly, that the entry was accurate and current, or that the vehicle is the one the entry describes.

The point appears in court decisions, database rules and vendor guidance:

• Ninth Circuit: in Green v. City and County of San Francisco (2014), the court stated that “an unconfirmed hit on the ALPR does not, alone, form the reasonable suspicion necessary” for the stop. The defendants didn’t contest that point.
• FBI’s NCIC manual: “An NCIC 2000 hit alone is not probable cause to arrest,” and a hit must be confirmed with the agency that entered the record before an arrest or seizure based on it.
• Flock’s own policy: users “should confirm the computer translation prior to taking any action based on an LPR alert.”
• Agency policy (example): Knoxville’s policy says an ALPR hit “does not provide legal authority for enforcement action” and requires visual and database verification first.

Green was a civil rights case in one federal circuit, not a criminal suppression ruling, and other courts may analyze alert-based stops differently. But together these sources make verification a central issue in any alert-based stop.

Assuming the owner is driving: Kansas v. Glover

Many hotlist entries are tied to a vehicle’s registered owner, such as a warrant or a revoked license. In Kansas v. Glover (2020), an officer ran a plate, learned the owner’s license was revoked, and stopped the truck without seeing any traffic violation. The Supreme Court, 8–1, upheld the stop: “when the officer lacks information negating an inference that the owner is the driver of the vehicle, the stop is reasonable.”

The Court stressed the limits of that rule:

• “The presence of additional facts might dispel reasonable suspicion.” Its example: an officer who knows the owner is middle-aged but sees a much younger driver.
• Justice Kagan, concurring, said the inference depends on context. It was strong in Glover because Kansas revokes licenses mainly for serious or repeated driving offenses. She noted that Kansas, like many states, also suspends licenses for reasons “having nothing to do with road safety,” such as “failing to pay parking tickets, court fees, or child support,” and suggested the analysis could differ there. She also wrote that a defendant may show that “additional information dictates the opposite result,” including through statistical evidence.

Glover involved an officer running a plate, not an ALPR alert, but its reasoning is often relevant to alert-based stops. It also connects to a theme of this site: vehicle identification is not driver identification.

What tends to strengthen or weaken an alert-based stop

This is a summary of how the cited sources reason, not a checklist courts are required to follow. Each court weighs the totality of the circumstances.

When the alert was wrong

Wrong alerts happen in two main ways, a misread plate or a wrong match, and both have led to serious stops. In Green, a misread plate led to a high-risk stop at gunpoint. In Aurora, Colorado, a correct plate number matched a stolen motorcycle from another state. See misreads and accuracy for those cases and the data on how often alerts are wrong.

If a stop was unlawful, evidence found as a result may be challenged in a motion to suppress as fruit of the poisonous tree. Courts apply exceptions, including for good-faith reliance on database records. See exceptions to the exclusionary rule. How that plays out depends on the facts and jurisdiction.

Questions a defense attorney may investigate

• What exactly did the alert say, and what did the officer know before the stop?
• Did the officer visually confirm the plate and state? Is that recorded, for example on dash or body camera?
• Was the hotlist entry confirmed as active, and with whom?
• Did the vehicle match the description in the entry?
• Was there anything suggesting the driver wasn’t the person the entry concerned?
• What does the agency’s ALPR policy require, and was it followed?