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Based on the officer’s observation, they must decide whether there is sufficient cause to command the drive to stop, either to conduct further investigations to determine if they suspect may be impaired, or for another traffic violation.  The officer is not committed to arresting the suspect for DWI based on the initial observation, but is concentrating on gathering all relevant evidence that may suggest impairment.  The officer’s second task during phase one is to observe the manner in which the suspect responds to the officer’s signal to stop, and to not any additional evidence of DWI violation.  If the initial observation discloses vehicle maneuvers or human behaviors that may be associated with impairment, the officer may develop an initial suspicion of DWI.  Based upon this initial observation of the vehicle in motion, the officer must decide whether there is reasonable suspicion to stop the vehicle.  At this point, the officer can either (1) stop the vehicle, (2) continue to observe the vehicle, or (3) disregard the vehicle.

Drivers who are impaired frequently exhibit certain effects or symptoms of impairment. These include

  • Slowed reactions
  • Impaired judgment as evidenced by a willingness to take risks
  • Impaired vision
  • Poor coordination

The National Highway Traffic Safety Administration sponsored research to identify the most common and reliable indicators of DWI.  This research identified 24 cues, each with an associated high probability that the driver exhibiting the cue is impaired.

Problems Maintaining Proper Lane Positions

  • Weaving
  • Weaving Across Lane Lines
  • Straddling A Lane Line
  • Swerving
  • Turning With Wide Radius
  • Drifting
  • Almost Striking Object or Vehicle

Speed and Braking Problems

  • Stopping Problems
  • Accelerating or Decelerating Rapidly
  • Varying Speed
  • Slow Speed

Vigilance Problems

  • Driving in Opposing Lanes or Wrong Way On One Way Street
  • Slow Response to Traffic Signals
  • Slow or Failure to Respond to Officer’s Signals
  • Stopping in Lane for no Apparent Reason
  • Driving without headlights
  • Failure to signal or signal inconsistent with action

Judgment Problems

  • Following too Closely
  • Improper or Unsafe Lane Change
  • Illegal or Improper Turn (Too fast, jerky, sharp, etc.)
  • Driving on Other Than Designated Roadway
  • Stopping Inappropriately in Response to Officer
  • Inappropriate or Unusual Behavior (Throwing Objects, Arguing, etc)
  • Appearing to Be Impaired; Examples of specific indicators might include:
    • Eye Fixation
    • Tightly gripping the steering wheel
    • Slouching in the seat
    • Gesturing erratically or obscenely
    • Face close to the windshield
    • Driver’s head protruding from vehicle

Post Stop Clues

  • Difficulty with motor vehicle controls
  • Difficulty exiting the vehicle
  • Fumbling with Driver’s License or Registration
  • Repeating questions or comments
  • Swaying, unsteady, or balance problems
  • Leaning on the vehicle or other object.
  • Slurred Speech
  • Slow to respond to officer/officer must repeat
  • Provides incorrect information, changes answers
  • Odor of alcoholic beverage from the driver

The second task during Phase One of the detection process is to observe the manner in which the driver responds to the officer’s signal to stop, and to note any additional evidence of a DWI violation.

Cues reinforcing the suspicion of DWI may be found in the stopping sequence.  After the command to stop is given, the impaired driver may exhibit additional important evidence of DWI.  These cues may include:

  • An attempt to flee
  • No response
  • Slow response
  • An abrupt swerve
  • Sudden stop
  • Striking the curb or another object

Common Questions  in DWI Detections

  • What is the vehicle doing?
  • Do I have grounds to stop the vehicle?
  • How does the driver respond to my signal to stop?
  • How does the driver handle the vehicle during the stopping sequence?

Phase 2

The officer’s first task is to observe and interview the driver face to face.  Based on the officer’s observation, they must decide whether there is sufficient cause to instruct the driver to step from the vehicle for further investigation.  The officer’s second task is to observe the driver’s exist and walk from the vehicle.

  • When I approach the vehicle, what do I see?
  • When I talk with the driver, what do I hear, see, and smell?
  • How does the driver respond to my questions?
  • Should I instruct the driver to exist the vehicle?
  • How does the driver exist?
  • When the driver walks toward the side of the road, what do I see?

Based upon the officer’s face to face interview and  observation of the driver, and upon previous observations of the vehicle in motion and the stopping sequence, the officer must decide whether there is sufficient reason to instruct the driver to step from the vehicle.

The Driver Interview

  • The sense of sight (Bloodshot eyes, soiled clothing, alcohol containers, drugs or drug paraphernalia, unusual actions.)
  • The sense of hearing (Slurred speech, admission of drinking, inconsistent responses, abusive languages, unusual statements.)
  • The sense of smell (Alcoholic beverages, marijuana, cover up odors, unusual odors.)

Pre-Exit Interview Techniques 

There are a number of techniques that officers use while the driver is still behind the wheel.  Most of these concepts apply the concept of divided attention.  They require the driver to concentrate on two or more things at the same time.

  • Asking for two things simultaneously forgets to produce both documents, produces documents other than the ones requested, fails to see the license, registration, or both while searching through their wallet or purse, fumbles or drops wallet, purse, license, or registration.
  • Asking interrupting or distracting questions  The second technique, asking interrupting or distracting questions, forces the driver to divide attention between searching for the license or registration and answering a new question.  Be alert for the driver who
    • Ignores the question and concentrates only on the license or registration search
    • Forgets to resume the search after answering the questions;
    • Supplies a grossly incorrect answer to the question.
  • Asking unusual questions The third technique, asking unusual questions, is employed after the officer has obtained the driver’s license and registration.  Using this technique, the officer seeks verifying information through unusual questions.  Unusually questions require the driver to precess information; this can be especially difficult when the driver does not expect to have to process information.


This technique requires the subject to recite a part of the alphabet.  The officer instructs the subject to recite the alphabet beginning with a letter other than A and stopping at a letter other than Z.  This divides the driver’s attention because the driver must concentrate to begin at an unusual starting point and recall where to stop.


This technique requires the subject to count out loud 15 or more numbers in reverse sequence.  This also divides attention because the driver must continuously concentrate to count backwards while trying to recall where to stop.

Finger Count

In this technique, the subject is asked to touch the typo of the thumb in turn to the tip of each finger on the same hand while simultaneously counting up one, two, three, four; then to reverse direction on the fingers while simultaneously counting  down four, three, two, one.

Exit Sequence

The officers decision to instruct the driver to step from the vehicle usually is made after you have developed a suspicion that the driver is impaired.  Even though that suspicion may be very strong, usually the suspect is not yet under arrest when you give the instruction.

How the driver steps and walks from the vehicle and actions or behavior during the exit sequence, may provide important evidence of impairment for the officer.  Examples include

  • Shows angry or unusual reactions
  • Cannot follow instructions
  • Cannot open the door
  • Leaves the vehicle in gear
  • Climbs out of the vehicle
  • Leans against vehicle
  • Keeps hands on vehicle for balance

Phase 3

The officer’s first task is to administer structured, formal pyscho-physical tests.  Based on these tests, the officer must decide whether there is sufficient probable cause to arrest the driver for DWI.  The officer’s second task is then to arrange for or administer a Preliminary Breath Test.  In each phase of detection, the officer must determine whether there is sufficient evidence to establish reasonable suspicion necessary to proceed to the next step in the detection process.

  • Should I administer field sobriety tests to the driver?
  • How does the driver perform those tests?
  • What exactly did the driver do wrong when performing the tests?
  • Do I have probable cause to arrest for DWI?
  • Should I administer a preliminary breath test?
  • What are the results of the preliminary breath test?