Banner for The Criminal Justice Systems: A Guide for Law Enforcement Officers and Expert Witnesses in Impaired Driving Cases

Understanding Criminal Cases

The Role of Law Enforcement Officers and Expert Witnesses in Impaired-Driving Cases

Prosecutors handling impaired-driving cases may decide to use one or more witnesses to prove their case. Each witness’s testimony serves to prove some fact or element of the case against the defendant. Witnesses may include Standardized Field Sobriety Test officers, drug recognition evaluators/experts, toxicologists, crash reconstructionists, optometrists, and other medical personnel.

Never underestimate the value of your role in establishing a defendant’s guilt based upon the totality of the evidence. Just as a puzzle with many pieces missing is difficult or impossible to put together, a jury (or judge) must find a defendant “not guilty” if important pieces of evidence are absent.

Generally speaking, only witnesses who have firsthand knowledge of the facts of a case are permitted to testify at trial. Their testimony is limited to their personal knowledge—what they heard or saw or personally know about the defendant or victim. Thus, the citizen who witnesses a crash and the first law enforcement officers on the scene have personal knowledge of the events leading to the criminal case and will likely be called to testify about what they saw and heard. Other law enforcement officers may have additional contact with the defendant during or after his arrest. Each of these witnesses has direct knowledge of some fact important to the criminal case.

The expert witness, on the other hand, is not required to have firsthand knowledge of the facts of the particular case, and in fact, often does not. Rather, the expert witness testifies to the meaning of the facts (gives an opinion). For example, assume there is a fire that destroys a house. The prosecution is arguing that the fire was deliberately set and was not the result of an accident. Burn patterns resulting from arson typically vary from those caused by an accident. Laboratory testing of evidence gathered at the scene may reveal that an accelerant was used to help start and spread the fire. The average person would not have the experience, training, or knowledge to understand the significance of burn patterns or the meaning of laboratory results. An arson expert would testify about these facts, and then state his or her opinion that the cause of the fire was “deliberate” and not accidental. The expert was not present when the fire started, yet his specialized training allows him to testify about the cause.

Who is an expert?

 Federal Rule of Evidence (FRE) 702 states: “If scientific, technical, or other specialized knowledge will assist the trier-of-fact to understand the evidence or to determine a fact in issue, a witness qualified as an expert by knowledge, skill, experience, training, or education, may testify thereto in the form of an opinion or otherwise.”

Even States that have not adopted the FRE have usually adopted, either by statute or case law, rules similar to the FRE governing the qualification of expert witnesses. 

Before qualifying any witness as an expert, the proponent must establish that the evidence is beyond the knowledge of the average juror (or judge) and that expert testimony will assist the trier-of-fact in determining a fact at issue.

Then, if the court agrees with the test as stated above, the proponent must establish that this witness has the requisite knowledge, skill, experience, training or education to offer an opinion on a fact of importance in this case.

It is possible for the court to agree that expert witness testimony would be helpful and therefore admissible, but then decide that the particular person a prosecutor or defense attorney has called as an expert witness is not, in fact, qualified as an expert. Perhaps the expert’s credentials were exaggerated (the masters degree and doctorate on his curriculum vitae were from a diploma mill and not an accredited university) or the expert claims such a broad range of quasi-expertise that he is in fact a “jack of all trades and master of none.”