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DUI Law: DUI Tests & Equipment Used by Law Enforcement

Standardized Field Sobriety Testing

The Standardized Field Sobriety Test (SFST) is a battery of three tests administered and evaluated in a standardized manner to obtain validated indicators of impairment and establish probable cause for arrest. These tests were developed by the National Highway Traffic Safety Administration (NHTSA). Training was developed to help police officers become more skillful at detecting DWI suspects, describing the behavior of these suspects, and presenting effective testimony in court. The three tests of the SFST are:
  • The horizontal gaze nystagmus (HGN)
  • The walk-and-turn
  • The one-leg stand.

HGN Testing

Horizontal gaze nystagmus is an involuntary jerking of the eyeball which occurs naturally as the eyes gaze to the side. Under normal circumstances, nystagmus occurs when the eyes are rotated at high peripheral angles. However, when a person is impaired by alcohol, nystagmus is exaggerated and may occur at lesser angles. An alcohol-impaired person will also often have difficulty smoothly tracking a moving object. In the HGN test, the officer observes the eyes of a suspect as the suspect follows a slowly moving object such as a pen or small flashlight, horizontally with his eyes. The examiner looks for three indicators of impairment in each eye: if the eye cannot follow smoothly, if jerking is distinct when the eye is at maximum deviation, and if the angle of onset of jerking is within 45 degrees of center. If, between the two eyes, four or more clues appear, the suspect likely has a BAC of 0.10 or greater. NHTSA research indicates that this test allows proper classification approximately 77% of suspects.

Divided Attention Testing

The walk-and-turn test and one-leg stand test are "divided attention" tests that are easily performed by most sober people. They require a suspect to listen to and follow instructions while performing simple physical movements. Impaired persons have difficulty with tasks requiring their attention to be divided between simple mental and physical exercises.

Walk-and-Turn Test

In the walk-and-turn test, the subject is directed to take nine steps, heel-to-toe, along a straight line. After taking the steps, the suspect must turn on one foot and return in the same manner in the opposite direction. The examiner looks for seven indicators of impairment: if the suspect cannot keep balance while listening to the instructions, begins before the instructions are finished, stops while walking to regain balance, does not touch heel-to-toe, uses arms to balance, loses balance while turning, or takes an incorrect number of steps. NHTSA research indicates that 68% of individuals who exhibit two or more indicators in the performance of the test will have a BAC of 0.10 or greater.

One-Leg Stand Test

In the one-leg stand test, the suspect is instructed to stand with one foot approximately six inches off the ground and count aloud by thousands (One thousand-one, one thousand-two, etc.) until told to put the foot down. The officer times the subject for a 30 seconds. The officer looks for four indicators of impairment, including swaying while balancing, using arms to balance, hopping to maintain balance, and putting the foot down. NHTSA research indicates that 65% of individuals who exhibit two or more such indicators in the performance of the test will have a BAC of 0.10 of greater. The effectiveness of SFST in court testimony and evidence depends upon the cumulative total of impairment indicators provided by the three-test battery. The greater the number of indicators, the more convincing the testimony. Because SFST is administered according to national standards and is supported by significant research, it has greater credibility than mere subjective testimony.

Alternative Testing Methods

Sometimes, an officer will encounter a disabled driver who cannot perform the SFST. In such cases, some other battery of tests such as counting aloud, reciting the alphabet, or finger dexterity tests may be administered. Several appellate court decisions have indicated that, if you administer a test that requires the subject to respond orally in other than a routine information-giving fashion, a Miranda warning may be required.

DWI Sobriety Checkpoints

DWI sobriety checkpoints are a special form of roadside safety checks. While some states have ruled such checkpoints illegal under their state constitutions, the majority and the U.S. Supreme Court have found checkpoints to be legal when conducted in a manner minimally intrusive on the rights of the traveling public.

Roadside sobriety checkpoint locations should be determined by law enforcement commanders or first-line super-visors, rather than being selected on an ad hoc basis by the line officers who conduct them.

Roadside checks prohibits the constitutionally impermissible random stopping of vehicles and complies with the provisions of the U.S. Supreme Court decision in Delaware v. Prouse. This case can be complied with by either stopping every vehicle, so that each driver has an equal chance of being stopped, or by stopping of every tenth or every twentieth vehicle so that the officer does not exercise individual discretion in deciding which vehicle is to be stopped, and all cars have an equal chance of being selected.

Drug Recognition Experts

Often the behavior of suspects is abnormal for alcohol impairment alone, or field or breath tests indicate that the suspect's BAC is lower than the level of impairment suggests. Either of these observations is common when encountering a DWI investigation by alcohol, drugs, or a combination.

Drug recognition experts (DREs) are officers who have been specifically trained to recognize the effects of drug impairment. The DRE's examination also provides evidence of observable drug effects to help confirm the lab analysis. Recognizing Drug "Signatures".

DREs are trained to recognize distinguishable "signatures" of certain categories of drugs, identified through five observations by the DRE: vital signs (pulse, temperature, and blood pressure); psychophysical responses (coordination of mind and body); signs of administration of drugs (such as injection sites); eye responses (horizontal and vertical gaze nystagmus, eye convergence, and pupil size under varying light intensities); and physical and behavioral characteristics (such as muscle rigidity or flaccidity, hyperactivity).

A DRE's observations cannot substitute for the chemical test or lab analysis. Only such analysis by qualified forensic chemists can accurately identify or quantify a particular drug.

NHTSA Prerequisites

NHTSA has individual, departmental, and jurisdictional prerequisites for training of DREs. The trainees should already be proficient in using standardized field sobriety testing techniques and should demonstrate a commitment to DWI and drug enforcement.

NHTSA has also established specific prerequisites as part of its DRE training curriculum. The student must be employed or under the direct control of a public criminal justice agency or an institution involved in providing training services to officers of law enforcement agencies. He must achieve the learning objectives of a two-day pre-school, demonstrate proficiency in the use of the SFST, possess good communication skills or a demonstrated ability to testify in court, and be willing to serve as a DRE upon completion of the training.

DRE Training and Certification Process

Once the prerequisites have been met, DRE training is a three-step process. Phase I is a two-day orientation to the techniques and procedures for evaluating drug-impaired suspects. Phase II is seven days of instruction in drug evaluation, physiology, effects of drugs, and legal considerations. At its conclusion, students are required to pass a written exam. Phase III consists of supervised field training and working with actual drug-impaired suspects. After a student has competently performed a minimum of 12 suspect evaluations identifying three of the seven different drug categories, he must complete a comprehensive written examination before obtaining IACP certification. Certified DREs must renew their certification every two years. Recertification requires each DRE to perform a minimum of four acceptable evaluations since the date of the last certification, successfully complete eight hours of IACP-approved recertification training, and submit updated documentation of DRE activity. A DRE will be decertified if he fails to maintain standards and certification requirements, or demonstrates substantial unethical or unprofessional behavior.

DWI Breath Testing Instruments

NHTSA annually publishes a list of breath testing instruments rigorously examined for accuracy and approved by NHTSA for their ability to accurately determine breath alcohol concentration, and thus blood alcohol concentration.

Captured Samples

Exhaled air can be categorized into essentially three types of samples: tidal breath air, reserve breath air, and alveolar breath air. Tidal breath air is air exhaled in the course of normal breath-ing. It is the most shallow of the three types. Reserve breath air is exhaled when the body is exerted. It is produced through deeper breathing than tidal breath air, but great volumes of air are both inhaled and exhaled with little residence in the lung. Alveolar breath air is deep lung air. Since breath testing instruments are intended to measure indirectly the concentration of alcohol in the blood, it is essential for accuracy that the breath sample captured by the instrument for analysis be representative of the air in the alveoli of the lung, because it is in the alveoli that the 2100:1 equilibrium ratio between alcohol in the breath and alcohol in the blood occurs.

Infrared Instruments

Infrared breath measuring instruments operate on the principle that each chemical compound has unique infrared energy absorption characteristics. Ethyl alcohol absorbs energy in the 3.42 micron region of the infrared spectrum. The amount of alcohol contained in a sample can be calculated by observing energy loss when a known energy is applied to the sample. In the infrared devices, infrared energy is projected through a breath sample. A photo-detector identifies a decrease in wave amplitude caused by the absorption of energy by the alcohol. The amount of energy absorbed is equal to the breath alcohol concentration. The greater the alcohol concentration, the lower the wave amplitude. A computer on the instrument determines the breath alcohol content based upon the amount of energy loss, and then applies the 2100:1 conversion ratio to provide a digital readout of the suspect's blood alcohol content.

Because infrared instruments are based upon infrared absorption spectra, which are chemically unique, they cannot be influenced by compound such as acetone, which may have some chemical characteristics in common with ethyl alcohol. In fact, some infrared instruments also provide data on the concentrations of other compounds contained in the breath sample as well as that of alcohol.

Preliminary Breath Testing Instruments

PBT instruments are portable instruments for the purpose of BAC screening as part of the pre-arrest field testing. The suspect driver blows for several seconds through a plastic or glass tube, and the PBT provides an instantaneous determination of blood alcohol content.

In most jurisdictions, the legal basis for the use of these instruments is contained in the implied consent laws. While results of a PBT generally are not admissible as evidence of DWI, they do provide officers with additional objective information to establish probable cause for arrest and further chemical testing. They also help to detect persons who may be suffering from an illness or injury such as diabetes or head injury and are in need of chemical treatment, but would otherwise be mistaken for an intoxicated person. There are essentially three types of PBTs: electro-chemical, semi-conductor, and disposable chemical.

In electro-chemical PBTs, alcohol in the breath is absorbed into a fuel cell where it is oxidized, producing electrical current. The higher the alcohol content of the breath, the greater the current output of the fuel cell. By measuring the current produced, the instrument determines the breath alcohol content, and the BAC conversion is displayed with the aid of a computer chip. In semi-conductor PBTs, alcohol increases the electrical output of the semi-conductor. By measuring the voltage output, the breath alcohol content can be determined and the BAC conversion is displayed.

Disposable chemical PBTs are glass or plastic tubes containing a measured amount of the chemical, which is reactive with alcohol. As the suspect exhales through the tube, alcohol contained in the breath reacts with the chemical contained within. The greater the breath alcohol content, the greater the chemical reaction observed.

Passive Alcohol Sensors

Passive alcohol sensors (PAS) are instruments that detect the presence of alcohol in normally expelled breath. They require no cooperation from the driver. During the roadside interview of the driver and examination of documents, the officer places the PAS within six inches of the driver's mouth. It contains a small fan which samples the ambient air for examination. An electro-chemical mechanism analyzes the air for the presence of alcohol. Some instruments are concealed within a flashlight and can be used as a passive or active detector.

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