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

Law Enforcement Testimony

Specific DWI Trial Recommendations

  1. Never give the numerical alco-sensor/PBT reading of the defendant when asked by the prosecutor. The answer to this question is whether the result was positive or not for the presence of alcohol. However, if the defense attorney asks you for the NUMERICAL reading, give it to him unless otherwise directed by the judge. It might be a good idea to ask the judge if it is okay to answer this before offering the number so as not to risk a mistrial.
  2. Always demonstrate how you conducted field sobriety evaluations. If the prosecutor forgets to ask you to come off the witness stand to demonstrate, suggest that it will aid your testimony. Be certain, however, that you can do in court all the evaluations you asked the defendant to perform at the time of the arrest. Obviously, if you cannot do them, the jury will not expect the defendant to have done them properly.
  3. Know the reasons for giving field sobriety evaluations:
    • They are divided attention tests, designed to detect when a person is impaired by alcohol and/or drugs.
    • They provide evidence of intoxication in case the defendant refuses to take a State-administered test under implied consent.
    • They prevent an arbitrary decision to arrest and allow an officer to articulate the reasons for concluding a driver was DWI to someone not present at the scene.
  4. You are not required to know anything about the Intoxilyzer 5000 or your jurisdiction’s breath test instrument, its internal workings, or anything other than how to operate it and take a breath sample from a defendant. You are merely an operator of an instrument, and while you have been taught something about how the instrument works when you became certified as an operator, never testify to its internal workings, or the defense attorney will discredit you as someone who “thinks he knows it all.” You know your cell phone works when you make and receive calls and could testify to its proper operation without knowing how it does what it does.

NOTE: DO NOT bring the breathalyzer operator’s manual to court, or the log, unless instructed to by the prosecutor. Discuss any subpoena to produce with the prosecutor before complying with the subpoena.

  1. YOU MUST BE ABLE TO EXPLAIN AT TRIAL “INVALID SAMPLE” AND THE NEED FOR A SUBSEQUENT TEST. If you get an “Invalid Sample” the instrument has either detected residual mouth alcohol or the defendant has failed to give an adequate deep-lung air sample. You must restart the 15- or 20-minute waiting period and repeat the test, or, if allowed in your jurisdiction, take the subject for a blood test. Remember to write the blood drawer’s name and title (doctor, nurse, etc.) on the police report. It is also a “best practice” to witness the blood draw yourself – this may allow the prosecutor to avoid having to call other hospital personnel as a witness. FOLLOW ALL PROCEDURES FOR COLLECTION AND CHAIN OF CUSTODY OF THE BLOOD KIT.
  2. When testifying about field sobriety evaluations, remember to discuss the level of impairment of the defendant, not just the “numbers” or the conclusion that he “passed” or “failed.” Generally, officers can testify to numerical scores on field sobriety tests, including Horizontal Gaze Nystagmus (HGN), and can testify to the level of impairment. Describe each of the tests you asked the defendant to perform and how he did, step by step. For example, on the Walk and Turn, you might testify, after describing the test, that the defendant put out his arms for balance, did not count properly by counting “one, two, four, four, five,” and when putting one foot in front of the other, he failed seven out of nine times to touch heel-to-toe and then gave up doing the test. In many jurisdictions, based upon your specialized training and experience, you could say, after describing in detail the steps involved in the HGN test: “the defendant scored six out of a possible six clues on the HGN, and four clues is considered impaired.” Thus, you would offer your opinion that he failed the tests and was intoxicated. However, see number 8 below.
  3. If you are IACP/NHTSA-trained and testify as to the accuracy of the field sobriety tests, it is best practice to stay away from discussing the numbers and their significance! The numbers referenced in the various studies commissioned by NHTSA are wide open for misinterpretation. Follow the dictates below.

Law enforcement officers need to be aware that NHTSA has done validation studies, and the SFST is considered very useful in aiding the determination of whether or not a defendant is driving while intoxicated. However, the officer doesn’t have to know the numbers, or care, because in this case, this defendant was impaired.

If the officer was unable to conduct any of the three standardized tests—for instance, because the defendant truly has a bad knee and thus can’t do the One Leg Stand and Walk and Turn—the officer should be free to use other tests—reciting the alphabet from, say, D to M, counting in multiples of 10 from 20 to 100, or similar observations. While these are not the preferred tests based upon NHTSA’s scientific studies, they can, nevertheless, provide evidence of intoxication, along with all of the other observations, to support a guilty verdict when the SFSTs are not practical or possible, and may be useful in addition to the SFSTs. Just be sure to note in your report why you used these tests and what the results were.

  1. If the prosecutor files a proper Motion in Limine (a pre-trial request for a decision from the judge on a particular legal point), you might be allowed to testify only as to the observations you made on the field sobriety evaluations; you would not testify about the numbers of clues or whether the defendant “passed” or “failed” any tests. It is very important that you discuss this option with the prosecutor in advance of trial. You would only testify as to what you observed regarding the defendant’s manifestations of intoxication and performance of the field sobriety evaluations. Thus, you would be testifying to the totality of the circumstances to explain why you made the arrest and why the defendant was in fact DWI.