What is Reasonable Suspicion & Probable Cause for a DUI Arrest

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In the world of DUI arrests, two key legal concepts play an important role in determining whether a police officer’s actions were justified: reasonable suspicion and probable cause. A police officer must have probable cause to make a DUI arrest, which means they must have enough evidence to believe that a crime has been committed.

On the other hand, reasonable suspicion is the lower standard that allows a police officer to pull over a vehicle for further investigation if they have a reasonable belief that the driver may be violating the law.

This article will cover the following topics:

reasonable suspicion for a dui stop

What is Reasonable Suspicion for a Traffic Stop

Reasonable suspicion is a legal standard that allows law enforcement officers to stop a vehicle if they believe a crime or traffic violation has occurred. In Arizona, this standard is defined by A.R.S. § 28-1594.

Reasonable suspicion must be based on specific and articulable facts.

For instance, if an officer observes erratic driving, speeding, or a broken taillight, they may have reasonable suspicion to make a traffic stop.

It is a lower standard than probable cause but requires more than a mere hunch.

In the context of a DUI investigation, an officer might stop a vehicle if they notice signs of impairment, such as swerving or delayed reaction times.

Exceptions for DUI Checkpoints

DUI checkpoints, also known as sobriety checkpoints, are exceptions to the general rule requiring reasonable suspicion for traffic stops.

Under Arizona law these checkpoints are legally permissible if conducted according to specific guidelines.

At DUI checkpoints, all drivers are stopped without individual suspicion. However, these checkpoints must adhere to constitutional guidelines to be valid. They must be publicly announced, conducted at a reasonable time and place, and operated under a predetermined plan to avoid arbitrary stops.

probable cause in a dui stop

What is Probable Cause For a DUI Arrest 

Probable cause is a critical legal standard that law enforcement must meet before making an arrest, including for DUI offenses. Under A.R.S. § 13-3883, probable cause exists when an officer has a reasonable belief, based on facts and circumstances, that a person has committed a crime.

For a DUI arrest, this means that the officer must have sufficient evidence to reasonably believe that the driver is impaired by alcohol or drugs.

In the context of a DUI, probable cause can be established through a variety of observations and evidence gathered during the traffic stop or checkpoint. This includes erratic driving behavior, physical signs of impairment such as bloodshot eyes or slurred speech, poor performance on field sobriety tests, and preliminary breath test results indicating a blood alcohol concentration above the legal limit.

Together, these elements provide the officer with a reasonable basis to believe that the driver is under the influence, thereby justifying an arrest under Arizona law.

Examples of Probable Cause in a DUI Investigation 

Probable cause in a DUI investigation arises from various observations and evidence that collectively suggest a driver is impaired. Here are some common examples:

Erratic Driving Behavior

Erratic driving is often the first indicator of potential impairment. Examples include:

  • Weaving or Swerving: The vehicle moves between lanes without signaling or control.
  • Abrupt Stops and Starts: The driver makes sudden, unexplained stops or accelerations.
  • Traffic Violations: The driver fails to obey traffic signals, stop signs, or speed limits.

Physical Signs of Impairment

During a traffic stop, an officer may notice physical signs that suggest intoxication, such as:

  • Bloodshot or Watery Eyes: Indicative of alcohol or drug use.
  • Slurred Speech: Difficulty articulating words clearly.
  • Smell of Alcohol: Detectable odor of alcohol on the breath or coming from the vehicle.
  • Flushed Face: Redness or unusual coloration of the face.


Field Sobriety Tests (FSTs)

Officers use standardized field sobriety tests to assess a driver’s physical and mental impairment. Examples include:

  • Walk-and-Turn Test: The driver struggles to maintain balance or follow instructions.
  • One-Leg Stand Test: The driver exhibits difficulty standing on one leg, hopping, or using arms to balance.
  • Horizontal Gaze Nystagmus Test: The driver shows involuntary eye movements when following an object side to side.

 Preliminary Breath Test (PBT)

Although not admissible in court as definitive evidence, a preliminary breath test can establish probable cause. If the test shows a blood alcohol concentration (BAC) above the legal limit of 0.08%, it supports the officer’s suspicion of DUI.


Admissions of Alcohol Consumption

Statements made by the driver can also contribute to probable cause. For example:

  • Admitting to Drinking: The driver acknowledges consuming alcohol before driving.
  • Describing the Amount: The driver specifies the number of drinks or types of alcohol consumed.


Observations of Behavior and Appearance

Other observations that can contribute to probable cause include:

  • Fumbling with Documents: Difficulty retrieving and handling license and registration.
  • Confused or Disoriented Behavior: Inability to follow simple instructions or answer questions coherently.
  • Presence of Alcohol Containers: Open or empty alcohol containers in the vehicle.

These examples illustrate how officers gather and assess evidence to meet the probable cause standard required for a DUI arrest in Arizona. Each piece of evidence adds to the overall picture, providing a reasonable basis to believe the driver is impaired.

What if the Officer Lacked Reasonable Suspicion or Probable Cause 

 When an officer conducts a traffic stop or makes an arrest without reasonable suspicion or probable cause, it violates the driver’s constitutional rights. In such cases, the evidence obtained during the stop or arrest may be challenged and potentially excluded from the case.

Consequences of Lacking Reasonable Suspicion

Reasonable suspicion is the threshold required for an officer to initiate a traffic stop. If an officer stops a vehicle without reasonable suspicion of a traffic violation or criminal activity, any evidence obtained as a result of that stop may be deemed inadmissible in court. This concept is based on the exclusionary rule, which prevents unlawfully obtained evidence from being used against a defendant.


Impact of Insufficient Probable Cause

Probable cause is necessary for an arrest, including a DUI arrest. If an officer arrests a driver without probable cause, the arrest itself may be invalid. This can lead to the suppression of evidence obtained during and after the arrest, such as results from breathalyzer tests, field sobriety tests, or blood tests. Without this evidence, the prosecution’s case may weaken significantly.

Filing a Motion to Suppress Evidence

A skilled DUI defense attorney can file a motion to suppress evidence if it can be demonstrated that the officer lacked reasonable suspicion for the stop or probable cause for the arrest. During a suppression hearing, the attorney will argue that the evidence was obtained in violation of the driver’s Fourth Amendment rights. If the court grants the motion, the suppressed evidence cannot be used in the prosecution’s case.

Potential Outcomes of Suppression

When critical evidence is suppressed, it often leads to favorable outcomes for the defendant, such as:

  • Dismissal of Charges: Without sufficient evidence, the prosecution may have no choice but to drop the charges.
  • Reduction of Charges: The prosecution may offer a plea deal with reduced charges or penalties.
  • Acquittal: If the case goes to trial, the lack of key evidence may result in a not guilty verdict.

Contact Criminal Defense Attorney Arja Shah

Arja Shah | Shah Law Firm Facing a DUI charge in Arizona requires expert legal representation, especially when challenging the conviction based on a lack of reasonable suspicion or probable cause.

At Arja Shah Law, we specialize in defending clients against such charges. Arja Shah is an experienced DUI defense attorney who meticulously examines every detail of your case.

If the traffic stop or arrest lacked legal justification, we will file motions to suppress unlawfully obtained evidence. This approach can lead to reduced charges or dismissal. 

With a proven track record of success, Arja Shah Law is dedicated to achieving the best possible outcome for you. Contact us today at (602) 560-7408  to schedule a consultation and take the first step towards protecting your future against unjust DUI charges.


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