Phases of Detection

Sometimes, a police stop while you are driving can come out of the blue. On this page, we outline the phases of detection that a police officer will follow before they make a traffic stope. This includes observing a vehicle and determining if there is probable cause to pull a driver over, contact with the driver, and pre arrest screening. Read on for more information.


Police officers are trained to break DUI stops into three separate phases. Phase one is vehicle in motion. Phase two is personal contact. Phase three is pre-arrest screening. The first phase is based on observation of the vehicle’s movement and the officer’s attempt to observer the driver. The second phase gives the police officer time to observe and speak with the driver face-to-face. The third phase is the administration of field sobriety tests. DUI detection doesn’t always include all three phases. Sometimes there are cases in which one phase is absent because of issues like a car accident or when a driver refuses to submit to field sobriety tests.

Phase One of Detection: Vehicle in Motion

In phase one, the police officer has to make a decision based on visual observation. The officer must decide whether or not there is good reason to stop a driver. Police are looking for unusual driving. This includes weaving or very slow speed, or something like a burnt out tail light. During this phase, the officer has to determine if there is enough evidence to legally grant reasonable suspicion to carry on to the next step in the DUI detection process. The officer must determine:

  • What is the vehicle doing?
  • Do I have grounds to stop the vehicle?
  • If I give a signal to stop, how does the driver respond?
  • How does the driver react when pulled over?

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During the first phase of police DUI detection of observing a driver, the police officer has to make one of three choices:

1)    Stop the car.

2)    Continue to observe.

3)    Disregard the car.


Drivers who operate under the influence tend to exhibit symptoms of slowed reactions and impaired judgment. They also might exhibit impaired vision and poor coordination. An impaired driver will exhibit visual clues because of the following complexities of operating a motor vehicle:

  • Steering the car.
  • Pressing the accelerator.
  • Using turn signals.
  • Pressing the brake.
  • Observing traffic.
  • Watching signal lights, stop signs, etc.
  • Making decisions about where to turn.

Police look for additional visual clues of impairments such as:

  • Weaving. Car movement toward one side of the roadway and then quickly toward the other side, like a zigzag movement.
  • Weaving across lane lines. Extreme cases of weaving in which the vehicle’s wheels move across lane lines before the driver corrects their path.
  • Straddling lane line. The vehicle is moving straight ahead with the center lane in between both the left and right wheels of the car.
  • Swerving. A jarring turn away from a straight course.
  • Turning with a wide radius. A much wider turn than normal in terms of the center of the turn being much greater than normal.
  • Drifting. A straight-line movement of a car at a slight angle in relation to the road.
  • Almost striking an object or vehicle.

Speeding and braking problems also can be taken into consideration. Some of these include:

  • Stopping issues. Stopping with a jerking motion, stopping abruptly, stopping too far from a curb, and stopping either too short or beyond a stop bar at an intersection.
  • Accelerating or decelerating rapidly. Any driving that is either too fast or too slow in relation to driving conditions.
  • Varying speed. Alternating between speeding up and slowing down.
  • Very slow speed. A vehicle that is being driven less than 10 mph than posted limit.

Vigilance Issues

Vigilance issues include:

  • Driving in an opposite lane or the wrong way on a one-way street. A vehicle that is going into an opposing lane, not yielding the right of way, driving the wrong way, etc.
  • Slow response to traffic signals. The observed vehicle exhibits a response to a traffic signal that is delayed; for example, a driver that is not responding to a green light.
  • Slow or failing to respond to a police officer’s signals. Driver’s reaction time is very slow to react to an officer’s hand signals, lights, or sirens.
  • Stopping in a lane for no reason. No justifiable reason for the vehicle to stop in the traffic lane.

Phase 2 of Detection: Personal Contact

Police officers approach and observe a potential drunk driver still in the vehicle. They note any indications of impairment. During the face-to-face interaction, the police officer tries to determine if the driver is impaired. Based on the up close observation of the driver, combined with previous observations of the car, the officer determines if there is legitimate reason to tell the driver to exit the vehicle.

Detection Clues

The face-to-face observations and interview of the driver by a police officer is based on sight, hearing, and smell. In terms of sight, there are a variety of things that a police officer is looking for to determine alcohol and/or drug use including:

  • Bloodshot eyes.
  • Dirty clothes.
  • Fumbling of hands/fingers.
  • Containers of alcohol.
  • Drugs or drug paraphernalia.
  • Unusual actions.

In terms of hearing, the police officer listens for:

  • Slurring of speech.
  • Any admission of drinking.
  • Abusive language.
  • Unusual statements.
  • Responses that are inconsistent.

In terms of smell, the police officer notices:

  • Smell of alcohol.
  • Smell of marijuana.
  • Air freshener scents.
  • Breath sprays/mouthwash.
  • Unusual odors.

Divided Attention

A technique that police officers use in terms of establishing evidence of alcohol intoxication is the concept of “divided attention.” These tasks require the driver to focus on two or more things at the same time. Some of the techniques used are asking for two things at the same time, asking interrupting questions, and asking unusual questions. One of these techniques is to ask for both the driver’s license and the vehicle registration at the same time. Also, police officers are trained to look for drivers who:

  • Forget to produce both documents.
  • Produce documents other than those requested.
  • Fail to see license, registration or both while looking through wallet or purse.
  • Drops or fumble through wallet, purse, etc.
  • Cannot retrieve documents using fingertips.

When police officers ask interrupting or distracting questions, it forces the driver to divide attention between searching for a driver’s license and answering a new question. Police officers are trained to look for drivers that:

  • Ignore the question and concentrate on the license and registration search.
  • Forget to keep searching after answering the question asked by police.
  • Supply an incorrect answer to the question.

Police use the technique of asking unusual questions after getting your driver’s license and registration. An example would be a police officer asking you your middle name. These types of questions require a driver to process information. This can be difficult when a driver doesn’t expect to have to answer them. Another technique used is having a driver recite part of the alphabet. Police officers will often instruct a driver to begin with a letter other than the letter A and stop at a letter other than the letter Z. An example would be asking a driver to begin with the letter G and ending with the letter W.

Another technique is to require the subject to count multiple numbers in reverse. For example, counting backwards from the number 88 to the number 53. This task divides the attention of a driver. The driver has to continue to concentrate throughout the whole process.

Detection Exiting the Vehicle

If a police officer does instruct you to exit the vehicle, it means that they think you are impaired. When a driver steps out of and walks away from a vehicle, a police officer is looking for the following behavior:

  • Shows anger towards an officer.
  • Can’t follow instructions.
  • Can’t open the door.
  • Leaves the car in gear.
  • Climbs out slowly.
  • Leans against the car.
  • Keeps hands on the car for balance.

Remember, the police are trying to find sensory evidence of alcohol/drug influence to make a DUI arrest.

Police officers that are successful in DUI stops have the ability to recognize the sense-based evidence of alcohol and/or other drugs. They have the ability to describe that evidence clearly in a police report. For accurate DUI enforcement, police officers train to use detection to collect evidence and describe their observations in a police report. Through a police report, a police officer communicates with many different parties in a DUI case. Also, some of these parties include superior officers such as a lieutenant or sergeant, the prosecutor, judge, jury, and defense attorneys.

Phase 3 of Detection: Pre Arrest Screening

After an officer establishes probable cause, they will oftentimes try to recover more evidence that the driver is under the influence of alcohol. At this time, the officer might ask the driver to perform field sobriety tests. These tests are not required in every state although consent to a blood, breath, or urine test varies depending upon the state. Also, these tests, which are usually conducted on the side of the road, do nothing to prove a person’s sobriety, and often hurt the driver’s case. The police cannot force a driver to perform these tests, so the driver should politely decline to take them.

Do you have police detection related questions? If so, please contact our office.

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