How Police Investigate DUI Cases: The 3 Phases of a Drunk Driving Investigation

When a police officer suspects a motorist of driving under the influence, the investigation does not begin with a breath test or field sobriety test. The investigation often begins before the officer ever approaches the driver.

Police are trained to make observations throughout a DUI investigation and to document those observations as potential evidence. Based on insight I gained while vigorously cross-examining police witnesses during a DUI death resulting trial, DUI investigations can generally be understood as occurring in three phases: Vehicle in Motion, Face-to-Face Interaction, and Testing.

Understanding these phases can help explain how police build a DUI case—and how a criminal defense attorney may later analyze and challenge the evidence collected during each phase.

Phase 1: Vehicle in Motion

The first phase begins while the suspected motorist’s vehicle is still moving.

Before conducting a traffic stop, police will observe the vehicle and look for driving behavior or traffic violations that may provide a basis for the stop. Officers may look for conduct such as:

• Speeding or driving too slowly for conditions
• Reckless driving
• Crossing a solid line
• Striking a curb or another object
• Failing to yield
• Failing to obey a traffic sign or signal

These observations can become important because the legality of the initial traffic stop may become one of the first issues examined by a DUI defense attorney.

Why the Traffic Stop Matters in a DUI Case

A DUI defense should include a careful review of why the officer stopped the vehicle in the first place.

The police report should identify the observations or alleged traffic violations that caused the officer to initiate the stop. A defense attorney can compare those claims against other available evidence and determine whether there is a legal basis to challenge the stop.

When there is a valid legal issue concerning the stop, the defense may consider filing a motion to suppress evidence obtained as a result of it.

That makes the Vehicle in Motion phase important even though it occurs before most people would consider the DUI investigation to have formally begun.

Phase 2: Face-to-Face Interaction With the Driver

Once the officer approaches the vehicle, the investigation enters the second phase: the face-to-face interaction between the police officer and the suspected motorist.

At this point, the officer is trained to make observations that may later be attributed to intoxication.

These may include observations concerning:

• Bloodshot or watery eyes
• Slurred speech
• Unusual statements or responses
• The condition of the driver’s clothing
• Difficulty locating a driver’s license, registration, or insurance information
• Producing the wrong document
• Fidgeting
• Difficulty standing without assistance
• Using a vehicle or another object to maintain balance

These observations can eventually appear in the police report and may be relied upon by the prosecution as evidence of impairment.

What You Say and Do Can Become Evidence

The longer an interaction continues, the more opportunities an officer has to observe the driver.

Statements made by the motorist may also become part of the government’s case. This is why what happens during the roadside interaction can become a significant part of a subsequent DUI prosecution.

When defending a DUI charge, an attorney should carefully review the officer’s report and other available evidence to determine exactly what the officer claims to have observed and whether those observations actually support the government’s allegations.

Phase 3: DUI Testing

The third phase involves testing. According to the video series, there are two principal categories discussed here: field sobriety testing and the chemical breath test.

Because these tests are very different, it is helpful to examine them separately.

Phase 3, Part 1: Field Sobriety Tests

During the roadside investigation, an officer may ask a motorist to perform various exercises.

The video explains that these may begin with preliminary exercises involving activities such as reciting letters of the alphabet or counting. Mistakes made during these exercises may be documented in the police report and characterized by the officer as evidence supporting intoxication.

A motorist may then be asked to perform standardized field sobriety tests.

What Are the Three Standardized Field Sobriety Tests?

The three tests discussed in the video are:

Horizontal Gaze Nystagmus (HGN). The officer has the motorist follow a stimulus, such as a pen or flashlight, with his or her eyes while looking for particular observations.

Nine-Step Walk-and-Turn. The motorist is asked to walk heel-to-toe according to instructions, turn, and continue walking.

One-Leg Stand. The motorist is instructed to stand on one leg while following the officer’s directions.

The video emphasizes that these exercises provide officers with additional opportunities to observe balance, coordination, the ability to follow instructions, and other behavior that may later be documented as evidence.

The video also explains that participation in field sobriety testing is voluntary and recommends respectfully declining these roadside tests.

Phase 3, Part 2: The Chemical Breath Test

The chemical breath test occurs at a different stage of the investigation.

Unlike roadside field sobriety exercises, the breath test discussed in the video is offered at the police station after the motorist has already been arrested on suspicion of DUI. The driver is asked to provide a breath sample to a machine designed to estimate blood alcohol concentration.

The video explains that a BAC of 0.08 is the commonly applicable legal limit for drunk-driving cases and that the breath test is designed to determine whether the driver’s BAC is above or below that level.

What Happens if You Refuse a Chemical Breath Test?

This issue is more complicated than field sobriety testing.

The video explains that while a driver may decline a chemical breath test, doing so can have consequences under the laws of states including Rhode Island and Massachusetts. Those consequences may include driver’s license suspension, fines, community service, and specialized DUI classes.

At the same time, providing a breath sample may give law enforcement additional evidence for use in a DUI prosecution.

This creates an important distinction between field sobriety testing and chemical breath testing: refusing a chemical test can itself carry legal consequences.

Anyone facing a refusal allegation should therefore have an attorney examine both the DUI case and the consequences associated with the alleged refusal.

Can Breath Test Evidence Be Challenged?

A numerical breath test result should not necessarily end the defense investigation.

The video raises several issues concerning the breath-testing machine itself, including its maintenance history, previous problems, software updates, manufacturer recalls, age, and other potential issues.

Depending on the circumstances of a particular case, the defense may investigate how the test was administered and the reliability and admissibility of the resulting evidence.

The important question is not simply “What number did the machine produce?” but also whether the government can establish that the evidence was lawfully and reliably obtained.

How a DUI Defense Attorney Reviews the Entire Investigation

A DUI defense should not focus exclusively on a breath test result or field sobriety test.

Instead, the investigation should be reconstructed from the beginning:

Vehicle in Motion → Traffic Stop → Face-to-Face Observations → Field Sobriety Testing → Arrest → Chemical Testing

Each stage may produce evidence, and each stage may present issues for the defense.

For example, a defense attorney may examine the stated reason for the traffic stop, the officer’s observations, statements attributed to the motorist, roadside testing, breath-testing evidence, police reports, and other available evidence.

The objective is to determine not only what evidence the government claims to possess, but how that evidence was obtained and whether it can withstand legal scrutiny.

Charged With DUI in Rhode Island?

If you have been charged with DUI, the police report should be carefully reviewed to determine what occurred during each stage of the investigation.

Attorney John L. Calcagni, III can review the circumstances surrounding the traffic stop, the officer’s alleged observations, field sobriety testing, chemical testing, and other evidence to identify potential defenses.

The Law Office of John L. Calcagni, III represents individuals facing DUI and other criminal charges in Rhode Island.

Call (401) 351-5100 for a free consultation.