Police officers can be called as expert witnesses to testify either strictly on matters of law enforcement or on other police-related matters where the court recognizes their expertise. They are not permitted to testify about legal matters or the minutiae of the law (that is the purview of lawyers). Although law enforcement officers can form conclusions based on evidence presented in a case in their “expert opinion,” they cannot make statements as to a defendant’s guilt or innocence, given a defendant’s right to a fair trial. Police officers acting as expert witnesses will often testify on police procedure, rules of arrest, or correct steps for gathering evidence.
Additionally, even police officers who are not recognized as expert witnesses by a court can testify to certain things because of their training and experience. Traffic police, for example, are often extensively trained to accurately recognize the speed of a moving vehicle, usually within a few miles per hour, without the use of equipment such as a radar gun, and courts will generally recognize that estimation as evidence, depending on the circumstances.