Probable Cause, Reasonable Suspicion, and California DUI

Under the United States Constitution, law enforcement officers must have (1) "reasonable suspicion" to stop and investigate you for California DUI,1 and (2) "probable cause" to arrest you for a DUI.2

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

These standards apply regardless of whether you are suspected of Vehicle Code 23152(a) VC driving under the influence or Vehicle Code 23152(b) VC driving with a BAC of 0.08% or greater.

It is common for California DUI defense lawyers to challenge whether an officer had reasonable suspicion/probable cause as part of a California DUI defense strategy. This is usually done by filing a Penal Code 1538.5 PC motion to suppress evidence.3

Below, our California DUI defense attorneys answer the following frequently asked questions about probable cause, reasonable suspicion, and California DUI traffic stops, investigations and arrests:

1. Does a police officer need probable cause to pull me over for suspected California DUI?

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 stop your car for no reason. Instead, s/he must be able to point to specific facts that led him/her to suspect that a crime, including DUI, 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 DUI. Any potential traffic infraction or violation--such as speeding or a broken taillight--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. The car is slowing down and speeding up erratically as it drives down the street, and then the driver runs a stop sign.

The fact that the car came from an area where lots of alcohol is consumed, the erratic driving, and the running of the stop sign are all specific facts that could support reasonable suspicion that the driver is DUI.

Thus, Jeannette is justified in pulling the driver over for a DUI traffic stop.

There is an exception to this "reasonable suspicion" rule for California DUI sobriety checkpoints. There is no reasonable suspicion/probable cause requirement to stop a driver at a DUI checkpoint that follows all the relevant legal requirements.7

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

Law enforcement officers also need reasonable suspicion to begin a DUI investigation after a traffic stop.

A DUI investigation is the next step after a traffic stop. The officer will detain you for an investigation if, after stopping you, s/he believes you may be guilty of driving under the influence. An investigation usually involves:

With each phase of the DUI--that is, the stop, then the detention/investigation, and then the arrest--comes a higher requirement for probable cause/reasonable suspicion.  So while it may be fairly easy for an officer to satisfy the reasonable suspicion requirement to stop your car, the level of proof necessary to detain and investigate you is higher.

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

For an officer to legally detain and investigate you for DUI, there must be some specific articulable facts that would lead a reasonable officer to suspect that you are, in fact, breaking California DUI laws.

For example, officers routinely ask drivers whom they stop at night if they've been drinking. Even if you've only consumed a marginal amount of alcohol, a yes answer may give rise to reasonable suspicion/probable cause and a subsequent California DUI investigation.

Other facts that might create reasonable suspicion for a California DUI detention and investigation are:

  • The officer observes an open container of alcohol in the car;
  • The officer smells the odor of an alcoholic beverage on your breath; or
  • The officer observes red/watery eyes or a flushed face.

However, if none of these factors was readily observable, the results of the investigation (including FST or PAS breath test results) may be excluded from evidence.

Example: Let's take Officer Jeannette from our example above. She pulls over the driver of the car that was starting and stopping suddenly and that ran a stop sign.

The driver of the car is Carlos. Jeannette asks him if he has had anything to drink, and he says no.

Carlos explains that he was distracted by his cell phone because he had just gotten a text message telling him that  a family member was very ill, and that that probably caused him to drive erratically. Carlos is not slurring his speech and appears alert. His face is not flushed, and his eyes are not bloodshot.

Jeannette probably does not have probable cause to initiate a DUI investigation. If she does so anyway, any evidence obtained from the investigation may not be admissible against Carlos later on.

3. Does an officer need probable cause to arrest me for California DUI?

Finally, an officer must have probable cause to make a DUI arrest.8 The standard required for an arrest is higher than that for a traffic stop or an investigation.

This means that the officer must be able to articulate exactly why he/she believes you were DUI. A hunch or mere suspicion won't suffice.

Because an arrest is such an absolute invasion of your personal privacy, it requires the highest level of personal protection available. This is why an arresting officer must be able to articulate the exact facts that led him/her to believe you were DUI--for example, a poor performance on an FST, or distinct signs of intoxication.

man being placed into handcuffs by police officer
Probable cause is required for a DUI arrest in California.

Although the officer won't necessarily share these facts with you at the time of your arrest, they should be contained in the police report. More importantly, the officer must be able to convince the court that his/her actions were lawful and reasonable under the "probable cause" standard.

Example: Let's return one more time to Officer Jeannette and Carlos in the example above. 

Let's say Carlos is feeling testy because he is worried about this sick relative and Jeannette is keeping him from getting home. He doesn't show any signs of being intoxicated, and his story checks out, but he is rude to Jeannette and makes a nasty comment about her physical appearance.

Jeannette decides to arrest Carlos for DUI because she is angry about his behavior. Once he has been arrested, Carlos is required to take a DUI chemical (blood or breath test) to avoid DUI chemical test refusal penalties. His DUI breath test shows that his BAC is 0.08%--right at the legal limit.

If Carlos is charged with VC 23152(b) driving with a BAC of 0.08% or higher, he may be able to argue that his DUI arrest was illegal because there was no probable cause. That could lead to his post-arrest DUI breath test result being suppressed.

4. What can I do if I was stopped or arrested for DUI without probable cause or reasonable suspicion?

If your California DUI defense attorney suspects that the officer lacked reasonable suspicion or probable cause during any of the above stages, he/she will likely move to suppress any illegally obtained evidence through a PC 1538.5 motion to suppress.9 

A motion to suppress is based on a legal principle known as the "fruit of the poisonous tree". It means that any evidence obtained as a result of an illegal procedure--a traffic stop, a DUI investigation or a DUI arrest--is subject to suppression. When a judge "suppresses" evidence, it means that evidence may not be used against you.

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

A PC 1538.5 motion to suppress will lead to a hearing that can be referred to as either a "suppression hearing," a "probable cause hearing" or a "PC 1538.5 hearing."  If your California DUI defense attorney can convince the judge at this hearing that probable cause/reasonable suspicion didn't exist for either the stop, the investigation/detention or the arrest, your DUI charges could be reduced or even dismissed.  

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."

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 at Shouse Law Group.

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.

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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