Probable Cause, Reasonable Suspicion, and California DUI

Under the United States Constitution, law enforcement must have a certain level of justification before they can initiate a traffic stop or arrest anyone. In the context of a California DUI, such level is usually expressed as:

  1. "reasonable suspicion" for a DUI traffic stop,1 or
  2. "probable cause" for a DUI arrest.2

These standards apply in all types of California DUI cases, including:

In theory, "reasonable suspicion" and "probable cause" are distinct standards. But in practice, the two terms are often used interchangeably in DUI cases.

Whatever this level of justification is called, however, the police may not always have it.

As a result, a California DUI defense strategy often includes a challenge to reasonable suspicion/probable cause. This is usually done by filing a Penal Code 1538.5 PC motion to suppress evidence.3

To help you better understand the probable cause or reasonable suspicion needed for a DUI traffic stop, investigation and arrest, our California DUI defense lawyers discuss, below:

1. Does a police officer need probable for a DUI traffic stop?

In order to pull over a motorist for DUI, a police or California Highway Patrol (CHP) officer must have "reasonable suspicion" that a crime is or was taking place. This is similar to, but not exactly the same thing as, "probable cause."4

The Fourth Amendment to the United States Constitution prohibits unreasonable searches and seizures.5 This means that an officer can't simply pull a driver over for no reason. Instead, s/he must be able to point to specific facts that led him/her to suspect that a crime was taking place.6

rolled up constitution on top of american flag
The 4th Amendment to the US Constitution is the origin of the "probable cause" requirement in California DUI law.

This doesn't mean that the officer must specifically suspect that the driver is under the influence of alcohol and/or drugs. Any potential traffic infraction or violation is enough. As long as the officer is able to articulate some reason for initiating a traffic stop, probable cause/reasonable suspicion is usually satisfied at this level.

Example: Jeannette, a police officer, observes a car driving away from a street where a number of bars are located. She then sees the driver run a stop sign.

Specific facts that could support a reasonable suspicion that the driver is under the influence include:

Exception: DUI sobriety checkpoints

There is one exception to this "reasonable suspicion" rule. It applies to California DUI sobriety checkpoints

The police do NOT require reasonable suspicion of a crime in order to stop drivers at a lawful DUI checkpoint.7

2. Do police need probable cause to start a California DUI investigation?

Law enforcement officers need reasonable suspicion to begin a "DUI investigation." The DUI investigation is the next step after a traffic stop or sobriety checkpoint.

An investigation begins when an officer believes a driver may be guilty of driving under the influence. This investigation usually involves:

In some counties, an officer may also swab a driver's cheek if the officer suspects DUI of marijuana.

Note that a driver is not required to take any FSTs or a PAS test or to submit to a cheek swab if these are requested BEFORE a driver has been arrested.

Note, too, that as each phase of a DUI stop progresses, there is a slightly higher requirement for the level of justification needed. It may be fairly easy for an officer to satisfy the reasonable suspicion required for a traffic stop. But the level of cause needed to detain and investigate a driver is higher.

man blowing into breathalyzer
A field sobriety test is typically part of a DUI investigation.

More than a mere suspicion of drinking and/or drug use is required

For an officer to legally detain and investigate someone for driving under the influence, he or she must be able to identify certain specific facts. These can be any facts that would lead a reasonable officer to suspect that the driver is breaking California DUI laws.

For example, officers routinely ask drivers they stop at night if they've been drinking. Even if someone has only consumed a marginal amount of alcohol, a yes answer may be enough to create the cause needed for an investigation. 

Other facts that might create enough cause for an officer to detain and investigate a driver include:

  • The officer observes an open container of alcohol in the car;
  • The officer smells the odor of an alcoholic beverage on the driver's breath; and/or
  • The officer observes red/watery eyes, a flushed face and/or other physical symptoms consistent with DUI.

3. Does an officer need probable cause to arrest someone for driving under the influence?

An officer must have probable cause to make an arrest for driving under the influence.8 An arrest is an invasion of privacy. Therefore, the justification needed to support an arrest is higher than for a traffic stop or an investigation.

The officer must be able to point to specific facts that show why he/she believed the driver was under the influence. A mere hunch or vague suspicion won't suffice.

Facts that might constitute probable cause include:

  • A high blood alcohol content ("BAC") result on a preliminary breath test,
  • Poor performance on field sobriety tests, or
  • Distinct physical signs of intoxication (such as slurred speech or dilated pupils).
man being placed into handcuffs by police officer
Probable cause is required for a DUI arrest in California.

The officer won't necessarily share these facts with the driver at the time of the arrest. But they should be contained in the police report.

More importantly, the officer must be able to convince the court that the "probable cause" standard was satisfied.

Example: While at a bar with his friends, Carlos gets a text that his wife is ill. Worried, he races into his car and speeds home. Officer Jeanette sees him leaving the bar and driving over the posted speed limit. She pulls him over and asks if he has been drinking. Carlos says "no," he is just worried about his sick wife. He shows Jeanette the text message and asks if he can leave.

Although Carlos shows no signs of intoxication, Jeanette smells alcohol on his breath. She asks him to blow into a handheld Breathalyzer. Carlos declines. But he is angry that Jeanette is keeping from home. He makes a nasty comment about Jeanette's physical appearance.

Jeannette then gets angry and arrests Carlos for driving under the influence. Carlos then takes a DUI post-arrest breath test. It shows a BAC of 0.08%--right at the legal limit.

Carlos' California DUI lawyer may be able to argue that Carlos' arrest was illegal. Carlos can take the position that there was no probable cause to lead Jeanette to reasonably believe he was under the influence. If the judge agrees, the breath test result will be excluded from evidence.

4. What can a driver do if he/she is arrested for DUI without probable cause?

The most common remedy for an unlawful DUI stop, investigation, or arrest is a PC 1538.5 motion to suppress evidence.9 

A motion to suppress is based on a legal principle known as the "fruit of the poisonous tree". It means that if any stage was improper (the "tree"), anything that the officer discovers as a result (the "fruit") is tainted. This "fruit" is subject to "suppression." When a judge "suppresses" evidence, it means that such evidence may not be used against you.

Example: A San Diego Police officer is patrolling a residential neighborhood at night. An expensive car drives by with four young men wearing hoodies and baseball caps. They are singing loudly and waving their arms around. Even though the driver has not broken any traffic laws, the officer is suspicious.

He pulls the car over and asks the driver, George, whether the men have been drinking or using drugs. George answers truthfully that his passengers have smoked a little weed, but that he is the designated driver and is sober. He exhibits no signs of intoxication.

The officer nevertheless asks George for a DUI marijuana swab (which is being tested in some California counties). When it comes up positive, he arrests George and requires a DUI blood test to test for drugs. The blood test comes up positive for marijuana.

George's DUI defense lawyer will probably file a motion to suppress the blood test results. The lawyer will argue that because George obeyed all the traffic laws and showed no signs of intoxication, there was no probable cause for the officer to arrest him.

Judge's Gavel
A motion to suppress under PC 1538.5 can challenge a DUI arrest or investigation that occurred without probable cause.

Getting evidence excluded in a California DUI case is rare

It is important to note that PC 1538.5 motions to suppress for lack of probable cause usually are not granted in DUI cases. However, as Oakland DUI defense attorney John Murray10 explains:

 "Many experienced California DUI defense lawyers will still run a motion to suppress for lack of probable cause for strategic reasons. This is an important pre-trial opportunity to cross-examine the officer and to discover any weaknesses in the prosecution's case--weaknesses that may result in reduced or even dismissed DUI charges down the line."

Need help challenging a DUI arrest or traffic stop in California? Call us for help . . . 

female operator smiling
Call us for help

If you or your loved one is charged with a DUI and think that an officer might not have met probable cause/reasonable suspicion requirements, we invite you to contact us for a free consultation.

Call us at 855-LAWFIRM to discuss your case with an experienced California DUI lawyer.

We have local offices in Los Angeles, the San Fernando Valley, Pasadena, Long Beach, Orange County, Ventura, San Bernardino, Rancho Cucamonga, Riverside, San Diego, Sacramento, Oakland, San Francisco, San Jose and throughout California.

We also have Las Vegas and Reno offices that can represent people who were arrested for a Nevada DUI without probable cause.


Legal references:

  1. Terry v. Ohio, (1968) 392 U.S. 1, 20.
  2. United States Constitution, amendment IV.
  3. Penal Code 1538.5 PC -- Motion to return property or suppress evidence.  ("(a)(1) A defendant may move for the return of property or to suppress as evidence any tangible or intangible thing obtained as a result of a search or seizure on either of the following grounds: (A) The search or seizure without a warrant was unreasonable [as may be the case with California DUI arrests made without probable cause].")
  4. Terry v. Ohio, endnote 1 above.
  5. United States Constitution, amendment IV.
  6. Terry v. Ohio, endnote 1 above, at 21 ("And in justifying the particular intrusion the police officer must be able to point to specific and articulable facts which, taken together with rational inferences from those facts, reasonably warrant that intrusion [with respect to a California DUI investigation].")
  7. Ingersoll v. Palmer (1987) 43 Cal.3d 1321, 1327.  ("[T]he primary purpose of the [DUI checkpoint] stop here was not to discover evidence of crime or to make arrests of drunk drivers but to promote public safety by deterring intoxicated persons from driving on the public streets and highways. We therefore conclude the propriety of the sobriety checkpoint stops involved here is to be determined not by the standard pertinent to traditional criminal investigative stops [reasonable suspicion/probable cause], but rather by the standard applicable to investigative detentions and inspections conducted as part of a regulatory scheme in furtherance of an administrative purpose.")
  8. United States Constitution, amendment IV.
  9. Penal Code 1538.5 PC -- Motion to return property or suppress evidence, endnote 3 above.
  10. Oakland DUI defense attorney John Murray is a leading expert in California DUI defenses, including lack of probable cause or reasonable suspicion for DUI traffic stops, investigations and arrests. He has extensive experience both in the court systems of Los Angeles County and Ventura County and in California DMV hearings.

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