A familiar example helps illustrate the distinctions. An officer on night patrol sees a car ahead without lights on, weaving slightly within the lane. This is not PC to arrest, but it is reasonable suspicion to detain for investigation, so a stop is made. The driver might turn out to be sober, driving a rented vehicle and trying to find the light switch, which would explain both of the suspicious circumstances that justified the stop. Or, he might exhibit symptoms of being under the influence and might fail field sobriety tests. These additional facts, added to the erratic driving, would constitute PC to arrest for impaired driving. If his blood alcohol level were subsequently found to be above legal limits and he gave an admissible confession, all of the evidence combined would generally be enough to prove his guilt at trial, beyond a reasonable doubt.
As this example shows, probable cause is a fairly low level of suspicion when compared with courtroom standards of proof. In fact, the Supreme Court has pointed out that an officer can have PC even though the prosecutor might not be able to file charges or convict the person-and even though it might turn out that the person arrested was actually innocent, or that no crime had in fact been committed.
"There is a difference between what is required to prove guilt in a criminal case and what is required to show probable cause for arrest and search." (Brinegar v. U.S.)
"Innocence of the charge is largely irrelevant to [the question of the existence of probable cause]. The Constitution does not guarantee that only the guilty will be arrested." (Baker v. McCollum)
When is PC required?
As discussed above, the Fourth Amendment mandates that warrants be supported by probable cause. The Supreme Court has ruled that in limited circumstances, officers may make warrantless searches and seizures based on PC. These include vehicle searches and public arrests.
Vehicle searches: The court has given two reasons for allowing warrantless searches of a vehicle when police have PC to believe something seizable is inside, and when the vehicle is lawfully accessible (street, driveway, carport, parking lot). First, people have a reduced level of expectation of privacy in vehicles, because vehicles are constantly exposed to public view, are licensed and regulated, and are subject to safety inspections. Second, vehicles are inherently mobile, allowing them to be quickly removed from the jurisdiction while a warrant is being sought. The Supreme Court has therefore dispensed with the warrant requirement for vehicle searches. Such searches may lawfully be made based on lawful access and PC-even if there might have been time to get a search warrant. (Florida v. Meyers)
Public arrests: When officers have PC to believe a particular individual has committed or is committing any public offense, the person may constitutionally be arrested in a public place, without an arrest warrant. (U.S. v. Santana) To make a non-emergency, non-consensual entry into a residence in order to arrest, a warrant is usually required. (Payton v. New York)
When is PC not required?
Although PC is necessary to get a search or arrest warrant, to make a warrantless arrest where permissible, or to search a vehicle for suspected contraband or the fruits, instrumentalities, or evidence of crime, it is not necessary in some situations where many criminal justice professionals wrongfully insist on talking about it. When the law actually sets a lower standard than PC for certain investigative conduct, it can be harmful to police and prosecutors to talk in terms of "probable cause" for acts that do not need that much justification.
For example, there is no constitutional concept called "PC for the stop." A stop is a detention, either of a pedestrian or a vehicle. Detentions are lower levels of intrusion on a person's liberty than arrests. They may be made on the basis of a lower quantity and quality of suspicion. That lower level is called "reasonable suspicion." The Supreme Court has made the distinctions clear:
"The level of suspicion required for a detention [stop] is obviously less demanding than that for probable cause. The police can stop and briefly detain a person for investigative purposes if the officer has a reasonable suspicion that criminal activity may be afoot, even if the officer lacks probable cause." (U.S. v. Sokolow)
"Reasonable suspicion is a less demanding standard than probable cause not only in the sense that it can be established with [less information], but also in the sense that reasonable suspicion can arise from information that is less reliable than that required to show probable cause." (Alabama v. White)
Some other investigative steps may be justified at the lower "reasonable suspicion" level, and should not be misconnected in reports or testimony to the higher PC standard. Examples include weapons pat-downs or "frisks," which require only a reasonable suspicion that the person is armed and dangerous (Terry v. Ohio), safety sweeps of lawfully entered premises based on a reasonable suspicion that a potential assailant may be present (Maryland v. Buie), and making a no-knock entry to serve a warrant based on reasonable suspicion that knocking and announcing would imperil officers, facilitate escape, or permit the destruction of evidence. (Richards v. Wisconsin)
Officers do themselves and prosecutors a disservice when they carelessly speak of their PC for a stop, or a weapons frisk, or a safety sweep. The proper standard is "reasonable suspicion." Sometimes, PC is not required.
Devallis Rutledge is a former police officer and veteran prosecutor who now serves as Special Counsel to the Los Angeles County District Attorney. He is the author of the book "Investigative Constitutional Law" and is POLICE Magazine's resident legal writer.