Police officers must have a certain level of justification before they can pull a driver over or arrest them. That's because the U.S. Constitution prohibits "unreasonable" search and seizures, including traffic stops and arrests. When it comes to driving under the influence (DUI) or driving while intoxicated (DWI) cases, the police must have reasonable suspicion to pull over the driver, reasonable suspicion to begin a DUI investigation, and probable cause to arrest the driver.
Although these two standards – reasonable suspicion and probably cause – are often used as synonyms, they are not. Each constitutes a separate standard. It pays for every driver to understand these concepts before having an encounter with law enforcement.
Reasonable Suspicion for Traffic Stop
The police do not have the right to pull over a vehicle unless they have "reasonable suspicion" that the driver has committed or is committing a crime. Although some refer to this as "probable cause," that is a different standard. Under the reasonable suspicion standard, the law enforcement agent must be able to point to specific facts or circumstances that caused them to believe a crime was taking place. That means that they need an objectively reasonable basis for suspecting criminal activity.
A police officer cannot pull over a vehicle just for no reason or a reason unrelated to criminal behavior. For example, they cannot pull over a driver because they think the driver is attractive or because they don't like a bumper sticker on the vehicle. Under the Fourth amendment to the Constitution, the prohibition against unreasonable search and seizure, a traffic stop is legal only if the law enforcement officer has specific information or has witnessed certain actions that would lead another officer to believe that a crime was being committed.
Reasonable Suspicion of Crime
It is important to understand that the officer need not have a reasonable suspicion that the driver is operating the vehicle under the influence of alcohol or drugs. They must have reasonable suspicion that the driver has broken the law. Any law.
That is, the traffic stop that results in a DUI charge need not be premised on a suspicion of drunk driving. The police can pull over a driver for a traffic violation like having a tail light out, speeding or running a red light. As they talk to the driver, they may notice signs of being under the influence, like slurred speech or the smell of alcohol. They can also pull over a car for erratic driving associated with drunk driving, such as a driver who is swerving or creeping along at a dangerously slow pace. Or they may spot a driver taking a swig of beer, itself illegal. All of these traffic stops would be reasonable and valid.
Exception to Reasonable Suspicion Rule
Then there are sobriety checkpoints. At this type of checkpoint, the police stop and test every driver without having any reason to think individual drivers have broken any law. This type of stop was challenged under the Fourth amendment, and the U.S. Supreme Court ruled that these sobriety checkpoints are permissible exceptions to the reasonable suspicion rule. The Court said that the public interest in keeping drunk drivers off the road outweighs the inconvenience to motorists.
Reasonable Suspicion for DUI Investigation
The traffic stop is the first step in a DUI prosecution. The next step is the DUI investigation phase. This begins when the police officer begins to suspect that the individual is driving under the influence. During the investigation, the arresting officer can ask questions about whether the driver has been drinking and, if so, how much. They can also ask the driver to take a field sobriety test and a breath test with a handheld breathalyzer, but the driver can refuse until the time of arrest.
So what does an officer need to demonstrate reasonable suspicion for a DUI investigation? For an officer to legally detain and investigate someone for driving under the influence, they must be able to identify specific facts that would lead a reasonable officer to suspect that the driver is breaking DUI laws. This might be the smell of alcohol or marijuana in the car, physical symptoms of intoxication like watery eyes and a red face, or an open container in the car.
Probable Cause for DUI Arrest
At some point during the DUI investigation, the officer may arrest the driver for driving under the influence. In order to do that, the officer must have probable cause, a type of justification that is similar to reasonable suspicion but requires additional or stronger evidence. Reasonable suspicion exists if a reasonable officer suspects criminal activity, while probable cause exists if a reasonable individual who is not a police officer would suspect a crime based on the actions of the suspect.
To show probable cause, law enforcement must be able to point to specific facts that suggest that the driver was under the influence. Here, a mere suspicion is not enough. If the driver has a high blood alcohol content determined by a breathalyzer, that is sufficient probable cause. Likewise, if the driver cannot perform the field sobriety tests or shows physical symptoms of impairment or intoxication like slurred speech, that can constitute probable cause as well.
There is no legal requirement for the police to tell the driver all of the factors that went into a determination of probable cause. But all of them should be included in the arrest report.
Illegal DUI Stop or Arrest
If a driver is picked up for a DUI without reasonable suspicion for the stop or investigation or without probable cause for the arrest, the driver can raise this defense with the court. If the court agrees, any evidence against the driver that the police obtained through the illegal stop or arrest will not be allowed in court.
A driver in this situation would do well to consider bringing in an experienced DUI attorney. In order to ask the court to exclude illegally obtained evidence, it is necessary to file a motion to suppress evidence. A motion to suppress is based on a principle of law known as the “fruit of the poisonous tree.” That means that if any part of the traffic stop or arrest (the tree) was improper, any "fruit" from the tree that the officer finds cannot be used in court. The motion is to suppress the "fruit" and, if a judge suppresses evidence, that evidence cannot be used against the driver.
Teo Spengler earned a JD from U.C. Berkeley Law School. As an Assistant Attorney General in Juneau, she practiced before the Alaska Supreme Court and the U.S. Supreme Court before opening a plaintiff's personal injury practice in San Francisco. She holds both an MA and an MFA in English/writing and enjoys writing legal blogs and articles. Her work has appeared in numerous online publications including USA Today, Legal Zoom, eHow Business, Livestrong, SF Gate, Go Banking Rates, Arizona Central, Houston Chronicle, Navy Federal Credit Union, Pearson, Quicken.com, TurboTax.com, and numerous attorney websites. Spengler splits her time between the French Basque Country and Northern California.