ALPR Expert Witnesses

What an ALPR expert can address

Depending on the case and their qualifications, an expert might explain:

• how the camera captures images and how software converts them to plate text, and the conditions known to cause misreads, such as angle, lighting, motion blur and plate design. See misreads and accuracy;
• how hotlist matching works and why a correct read can still produce a wrong alert. See hotlists and alerts;
• what time stamps and camera locations represent, and how they can be wrong;
• what search and audit logs show, and what they don’t. See audit logs;
• what can and can’t be concluded from a set of reads, including about who was driving.

An expert can also be a consultant who never testifies, helping counsel understand the records, frame discovery requests and prepare cross-examination.

The legal standard: Rule 702

Federal Rule of Evidence 702 lets a qualified expert give an opinion only if “the proponent demonstrates to the court that it is more likely than not that”:

1. the expert’s specialized knowledge will help the jury understand the evidence or determine a fact in issue;
2. the testimony is based on sufficient facts or data;
3. it is the product of reliable principles and methods; and
4. the opinion reflects a reliable application of those methods to the facts of the case.

The “more likely than not” language was added in a 2023 amendment. The Advisory Committee’s note says it was meant to “clarify and emphasize” that the side offering the expert must establish these requirements by a preponderance of the evidence. States have their own versions of this rule, and some use different standards.

The judge as gatekeeper

In Daubert v. Merrell Dow Pharmaceuticals (1993), the Supreme Court described the trial judge’s gatekeeping role over scientific testimony. It listed factors a court can consider, including, for a particular technique, “the known or potential rate of error” and “the existence and maintenance of standards controlling the technique’s operation.” In Kumho Tire Co. v. Carmichael (1999), the Court held that this gatekeeping obligation applies not only to scientific testimony but also to “technical” and “other specialized” knowledge. That covers testimony about how ALPR systems work.

Error rates matter here because, as our accuracy guide explains, independent, published per-read error rates for ALPR systems are scarce.

When the prosecution offers technical testimony

The same standard applies to witnesses the prosecution calls to explain ALPR results, such as an analyst or a vendor representative. If a witness offers opinions based on specialized knowledge, for example that the system is accurate or that reads show a pattern of travel, the defense can ask whether those opinions meet Rule 702. The defense can also challenge them on cross-examination or with its own expert.

If you can’t afford an expert

• Federal court. Under the Criminal Justice Act, counsel for a person “financially unable to obtain investigative, expert, or other services necessary for adequate representation” may request them in an ex parte application. The court authorizes them if it finds they are necessary and the person can’t pay (18 U.S.C. § 3006A(e)).
• Constitutional floor. In Ake v. Oklahoma (1985), the Supreme Court held that when an indigent defendant’s sanity is likely to be a significant factor at trial, the state must provide access to a psychiatrist’s assistance. How far that principle reaches for other kinds of experts is decided by lower courts and varies.
• State procedures. States have their own funding processes, often through the public defender’s office or a court application.

Questions about a proposed expert

• What is their experience with ALPR systems specifically, and with this vendor’s system?
• Have they testified before about ALPR or similar technology, and were they qualified as an expert?
• What records will they need to see, such as images, read data, logs and system documentation?
• Will they be a consultant, a testifying witness, or both?

We don’t recommend or refer to specific experts. Your attorney will decide whether an expert is needed.