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The driving under the influence (DUI) detection process begins when a police officer suspects that a DUI violation is occurring and ends when the police officer decides that there is probable cause to arrest the driver for a DUI violation. Typically, the DUI detection process is comprised of three separate and distinct phases1. Each phase requires the police officer to make observations and decisions. The phases of the DUI detection process are: phase one, the police officer’s observations of the vehicle in motion; phase two, the police officer’s initial contact with the driver; and phase three, the police officer’s pre-arrest screening of the driver. In each phase of the DUI detection process the police officer must determine whether there is the necessary reasonable suspicion to proceed to the next phase of the detection process2.

Phase One: Vehicle in Motion

In phase one of the DUI detection process, the police officer is observing the driver’s operation of the vehicle and looking for indicators3 of impairment. These indicators of impairment are divided into four sub-groups. The four sub-groups are: problems maintaining lane position; speed and braking problems; vigilance problems; and judgment problems.

The indicators of impairment in sub-group one, problems maintaining lane position, the police officer is looking for are:

  1. Weaving;
  2. Weaving across lane lines;
  3. Straddling a lane line;
  4. Swerving;
  5. Turning with wide radius;
  6. Drifting; and
  7. Almost striking a vehicle.

The indicators of impairment in sub-group two, speed and braking problems, the police officer is looking for are:

  1. Stopping problems (too far, too short, too jerky);
  2. Accelerating or decelerating rapidly;
  3. Varying speed; and
  4. Slow speed (10 m.p.h. under the speed limit).

The indicators of impairment in sub-group three, vigilance problems, the police officer is looking for are:

  1. Driving in opposing lanes/wrong way on a one-way street;
  2. Slow response to traffic signals;
  3. Slow or failure to respond to officer’s signals;
  4. Stopping in lane for no apparent reason;
  5. Driving without headlights at night; and
  6. Failure to signal/signal inconsistent with action.

The indicators of impairment is sub-group four, judgment problems, the police officer is looking for are:

  1. Following to closely;
  2. Improper/unsafe lane change;
  3. Illegal/improper turn (too fast, jerky, sharp);
  4. Driving on other than designated roadway;
  5. Stopping inappropriately in response to officer;
  6. Inappropriate/unusual behavior (throwing objects, arguing) and
  7. Appearing to be impaired.

Phase one of the DUI detection process, requires the police officer to conduct two tasks: task one, observe the vehicle in motion; and task two, observe the stopping sequence of the vehicle.

Task one, observing the vehicle in motion, requires the police officer to observe the driver’s ability to operate the vehicle and note any of the aforementioned indicators of impairment. If, after observing an indicator or indicators of impairment the police officer must determine if there is reasonable suspicion to stop the vehicle.

Task two, observing the stopping sequence, requires the police officer to observe the driver’s ability to operate the vehicle, after the police officer’s signal to stop is given, and note any of the aforementioned indicators of impairment. Note, the signal to stop creates a new situation with which a driver must cope. Flashing emergency lights and/or a siren demand and divert a driver’s attention, requiring the driver to divide their attention between driving and responding to the police officer’s signal to stop. An impaired driver may not be able to handle this more complex task and the aforementioned indicators of impairment are more likely to be observed.

Phase Two: Personal Contact

In phase two of the DUI detection process, the police officer observes and interviews the driver while they are still in the vehicle. During this face-to-face contact with the driver the police officer, like phase one, is looking for indicators of impairment. The police officer is trained to use their sense of sight, hearing and smell to identify indicators of impairment.

In phase two there are a number of indicators of impairment that the police officer is looking for. Some of the more common indicators of impairment that the police officer may see are:

  1. Bloodshot eyes;
  2. Droopy eyelids;
  3. Soiled clothing (urine/vomit);
  4. Fumbling fingers;
  5. Alcohol containers; and
  6. Drugs and/or drug paraphernalia.

In phase two there are a number of indicators of impairment that the police officer is listening for. Some of the more common indicators of impairment that the police officer may hear are:

  1. Slurred speech;
  2. Admission to drinking or drug use;
  3. Inconsistent responses;
  4. Abusive language; and
  5. Unusual statements.

In phase two there are a number of indicators of impairment that the police officer may smell. Some of the more common indicators of impairment that the police officer may smell are:

  1. Alcoholic beverages;
  2. Marijuana;
  3. “Cover up” odors like mouthwash; and
  4. Any other unusual odors.

Phase two of the DUI detection process, requires the police officer to conduct two tasks: task one, approach, observe and interview the driver while they are still in the vehicle; task two, observe the manner in which the driver exits the vehicle.

Phase two, task one, of the DUI detection process, requires the police officer to use their sense of sight, hearing and smell to identify and note the aforementioned indicators of impairment. Based on the observed indicators of impairment in phase one, vehicle in motion, and phase two, personal contact, the police officer must determine if there is reasonable suspicion of impaired driving: and, whether they will have the driver exit the vehicle to conduct pre-arrest screening (i.e., field sobriety tests).

Phase two, task two, of the DUI detection process requires the police officer to observe the manner in which the driver exits the vehicle and note any additional indicators of impairment. The most common indicators of impairment in the exit sequence are:

  1. Cannot open the vehicle’s door;
  2. Leaves the vehicle in gear;
  3. Leans against the vehicle for balance; and
  4. Uses hands on vehicle for balance.

Phase Three: Pre-arrest Screening

In phase three of the DUI detection process, the police officer administers three scientifically validated psychophysical (field) sobriety tests. Based on the driver’s performance on the sobriety tests and the indicators of impairment in phase one and two, the police officer must determine if there is probable cause to arrest the driver for DUI. Note, depending on the police officer, the police officer may request that the driver conduct a preliminary breath test (PBT). Police officers use the PBT to determine the chemical basis of the driver’s impairment (i.e., drugs and/or alcohol).

The three scientifically validated psychophysical (field) sobriety tests that the police officer will conduct in phase three are: Horizontal Gaze Nystagmus Test; Walk-and-Turn Test; and One-Leg Stand Test. In each test the police officer is looking for clues of impairment.

The clue for impairment that the police officer is looking for when administering the horizontal gaze nystagmus test is nystagmus. Nystagmus refers to the involuntary jerking of the eyes. Involuntary jerking of the eyes becomes more noticeable when a person is impaired. Specifically, as a person’s blood alcohol concentration increases, the eyes will begin to jerk sooner as they move to the side.

The eight clues for impairment that the police officer is looking for when administering the walk-and-turn test are:

  1. Driver cannot balance during the instructions;
  2. Driver starts test to soon;
  3. Driver stops while walking;
  4. Driver doesn’t touch heel-to-toe;
  5. Driver steps off line;
  6. Driver uses arms to balance;
  7. Driver loses balance on turn or turns incorrectly; and
  8. Driver takes the wrong number of steps.

The four clues for impairment that the police officer is looking for when administering the one-leg stand test are:

  1. Driver sways while balancing;
  2. Driver uses arms to balance;
  3. Driver hops; and
  4. Driver puts foot down.

The police officer’s first task, in phase three, of the DUI detection process, is to administer structured, formal psychophysical tests (i.e., field sobriety tests). The police officer must observe and note any/all the clues of impairment that the police officer observes. Based on the driver’s performance on the field sobriety tests and the indicators of impairment observed in phase one and phase two the police officer must decide whether there is sufficient probable cause to arrest the driver for driving under the influence (DUI). The police officer’s second task, in phase three, is to arrange for or administer a preliminary breath test.

Understanding the three phases of the DUI detection process is necessary for an attorney defending someone charged with a DUI. Most importantly, the three phases of the DUI detection process can provide the necessary evidence to obtain an acquittal for someone charged with a DUI.

1 The driving under the influence (DUI) detection process does not always include all three phases. For instance, phase one, the vehicle in motion phase, may be absent when the police officer arrives at the scene of an accident, at a roadblock, or when you the police officer responds to a request for motorist assistance.
2 Effective police officers in DUI detection do not leap to the arrest/no arrest decision. Rather, they proceed carefully through a series of intermediate steps, each of which helps to identify the collective evidence.
3 Indicator/Cue- a reminder or prompting as a signal to do something. A suggestion or a hint. (See, NHTSA manual.)