You have probably read or heard somewhere that police need probable cause to arrest someone. This is true. It comes from the Fourth Amendment to the U.S. Constitution, which protects the public from unreasonable searches or seizures. Among other things, the Fourth Amendment generally requires the police to secure a warrant before they can search a suspect’s home.

When you read about these protections and requirements, you may wonder how they apply to people who are suspected of drunk driving. Do police need probable cause to arrest someone on suspicion of DUI?

The answer is yes, but as with so many other questions in the law, it is complicated.

Put simply, “probable cause” means the police have adequate reason to believe a crime has been committed. Courts generally interpret this to mean that the officer had a reasonable belief that it was more likely than not that a crime has been committed. For instance, if a police officer sees a man grab a necklace from a jewelry store and run out the door, the officer has probable cause to believe the man committed a crime, and can arrest the man.

However, most cases are not so simple. Police often have to detain a person to learn more about what happened. A detainment is a temporary stop that falls short of an arrest. For this, police need only a reasonable suspicion. If, during the detention, the officer gathers more evidence that supports probable cause, the officer may lawfully arrest the suspect. For example, in the jewelry store example above, imagine that the officer sees a man running while holding a diamond necklace. The officer stops the man and says, “Where are you going?” While talking to the man, a jewelry store clerk comes running, shouting, “Stop that man! He stole a necklace from my store!” The officer now has probable cause to arrest the man.

In the context of DUI arrest, when a police officer sees a car weaving between lanes or driving erratically, the officer can formulate a reasonable suspicion that the driver is intoxicated and can detain the driver. During this traffic stop, if the officer smells alcohol on the driver’s breath, or if the driver is slurring words, this provides more evidence of intoxicated driving.

Here is where enforcement of DUI laws is different from enforcement of other criminal laws. California law holds that drivers have implied their consent to further testing when they are pulled over on suspicion of DUI. This includes submitting to a field sobriety test and a chemical breath test. With these so-called Breathalyzer tests, police can submit strong evidence to show that a driver had a blood alcohol concentration above the legal limit, and prosecutors will use that evidence against the driver in court.

What’s more, California law provides that police can stop drivers almost at random at a designated DUI checkpoint, even if they don’t see the driver acting erratically. During this short detention, if the officer notices signs of intoxication, this can provide them with a reasonable suspicion to gather more evidence, and this can turn into probable cause for an arrest.

All of this is to note that the police have great power to stop and arrest you on suspicion of drunk driving. However, if you have been accused of DUI, you have the right to a defense. An attorney can advise you on your options for defending your rights.