Probable Cause for a DUI Arrest

The evidence required for an officer to make a traffic stop and arrest someone on suspicion of drunk driving.

Drunk driving is and probably always will be one of the most serious dangers drivers and vehicle passengers face when traveling on roads and highways. To address this problem, law enforcement is tasked with patrolling the streets to identify and remove impaired drivers from the public roadways. But constitutional protections prevent police from arresting drivers without a certain amount of evidence—called "probable cause"—that a crime or violation has been committed. This article will explain when an officer can lawfully make a DUI stop and arrest and the consequences of officers failing to follow constitutional requirements.

What Makes a Traffic Stop Legal

DUI arrests usually start with some sort of traffic stop. But for a traffic stop and a subsequent DUI arrest to be legal, the officer must have a valid reason for stopping the vehicle in the first place. Quite often, DUI investigations start with the officer making contact with the suspect during a routine traffic stop for some sort of violation (like speeding or running a red light), DUI checkpoint stop, a vehicle collision investigation. But in many other DUI cases, the initial stop is based on the officer noticing something about the suspect's driving (such as swerving) that could indicate driver intoxication. An officer might also discover evidence of drunk driving in conducting welfare checks of drivers in parked vehicles.

Traffic violations. An officer who has articulable suspicion of a traffic violation (more than just a hunch) can lawfully stop a vehicle and investigate. Generally, any traffic violation justifies a traffic stop. Some of the more common reasons police stop vehicles include rolling through a stop sign, exceeding the speed limit, and driving without headlights or some other equipment violation. But when an officer notices indicators of impairment (like the odor of alcohol, bloodshot eyes, or slurred speech) during a traffic stop, he or she will normally start to investigate the possibility that the motorist is driving under the influence.

DUI checkpoints. DUI checkpoints are basically roadblocks that law enforcement set up to enforce DUI laws. At these checkpoints, officers stop every vehicle and normally ask drivers if they've had anything to drink and quickly look at the driver for clues of impairment before allowing them to pass. When officers notice indications of impairment, they'll typically ask the driver to pull over to the side of the roadways so they can conduct further investigation. This investigation might involve the officers requesting that the driver participates in field sobriety tests or some sort of alcohol testing (like blowing into a breathalyzer). Drivers have challenged the legality of DUI checkpoints, but the courts have generally come down on the side of law enforcement. However, some states have laws prohibiting or limiting the use of DUI checkpoints.

Vehicle collision investigations. As impaired driving is a leading cause of vehicle fatalities, officers investigating a collision will generally check for impairment. If the officers investigating the collision notice anything indicating alcohol or drugs might have been involved, they'll typically look into the possibility that one of the drivers could have been in violation of state DUI laws.

Regular DUI stops. Driving under the influence is, of course, unlawful. So officers can legally stop a vehicle if they have a reasonable basis for thinking the driver is drunk or otherwise impaired. A reasonable basis for making this type of stop might include indicators like a motorist swerving, driving the wrong way on a roadway, or driving unusually slow.

Welfare checks. Although somewhat uncommon, some DUI investigations begin with a driver in a parked car asleep behind the wheel. In this situation, the vehicle is already stopped, so an officer might approach to check on the wellbeing of the driver. If in doing so, the officer suspects the driver to be under the influence, he or she might decide to conduct further investigation to determine whether the driver is in violation of state DUI laws. In most states, actual driving isn't required to be convicted of a DUI—being in actual physical control of a vehicle is enough.

When a DUI Arrest is Legal

Assuming the initial traffic stop was justified, the officer still needs probable cause to make a DUI arrest. In other words, the officer needs a reasonable basis to believe the motorist was driving (or in actual physical control of a vehicle) while under the influence of drugs or alcohol. Absent some sort of indication of driver impairment, officers generally can't lawfully make an arrest or detain someone longer than necessary to complete the traffic stop.

General observations. Officers often start DUI investigations with some standard questions regarding where the driver came from and if he or she had anything to drink. How the driver answers the questions (for example, if the driver admits to drinking prior to the stop) can supply police with evidence of a DUI violation. But even if the driver denies drinking, the officer's observation during the question can provide probable cause. For instance, the driver might reek of alcohol, have slurred speech, or have bloodshot eyes—all indications of possible intoxication.

Field sobriety testing. Generally, an officer who suspects a driver of being impaired will ask the driver to step out of the car and complete field sobriety tests. These are tests were designed to determine the impairment level of drivers. Common tests include the horizontal-nystagmus eye test, the walk-and-turn, and the one-leg-stand. Drivers generally don't have to agree to participate in field sobriety tests. But when drivers do participate, the officers can consider field sobriety test performance if deciding whether to make an arrest.

Preliminary breath test. It's also common for officers to ask the drivers to take a roadside breath test. Prior to being arrested, drivers generally don't have to take a breathalyzer test. However, if a driver consents to testing, the results are fair game in terms of probable cause.

Illegal Stops and DUI Suspect Arrests

If the prosecutor can't prove the officer had reasonable suspicion to stop the driver or probable cause to make the arrest, the evidence obtained during the stop generally can't be used against the driver in court. In many cases, where an illegal stop or arrest results in evidence being thrown out, the prosecution will drop the charges. For this reason, it is often crucial to have a skilled attorney review your case for any possible defenses.

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