Traffic Stops

Traffic stops have become a valuable tool not only for enforcing traffic laws but also for investigating drugs and other illegal activities; random stops can, however, be deemed unreasonable under the Fourth Amendment. More than 40 percent of citizen-police encounters occur in the context of a traffic stop. Individual drivers can be pulled over by police anytime they are observed violating the traffic code, whether for speeding, weaving in traffic, driving without a license plate light, or various other violations. The Supreme Court ruled in Whren v. United States (1996) that as long as the officer has probable cause for a traffic violation, he or she may stop a driver. The actual motivation for the traffic stop is immaterial, as long as the officer has probable cause of a traffic violation. Officers can use the pretext of a traffic violation to conduct a criminal investigation.

When a traffic stop is executed, an officer can perform other actions to gather further information: The officer can seek identification, engage the driver and occupants in conversation, and conduct a visual inspection of the vehicle's interior. This is extremely useful for drug interdiction because the officer has the driver (and passengers) in a position where they are not free to leave and where officers can use all of their senses, as well as communication skills, to illicit information regarding a crime.

A traffic stop is technically different from a “Terry stop”—named after the practice upheld by the Court in Terry v. Ohio (1968)—in that the latter is based on an officer's reasonable articulable suspicion that the driver or passengers are committing a crime. Whereas an officer can make a Terry stop of a vehicle based on suspicion of a crime—for example, in a case where the vehicle matches the description of a car associated with a robbery—most traffic stops are based on probable cause of a violation of the motor vehicle code and not reasonable suspicion.

During a traffic stop, plain view can be used to establish probable cause of non-traffic-related crimes. For example, an officer's plain view of a marijuana pipe in the center console of a vehicle constitutes probable cause for a drug paraphernalia arrest. Likewise, if the officer smells alcohol or marijuana in the vehicle, “plain smell” can prompt further action, such as asking the driver to exit the vehicle and submit to a roadside sobriety check. If the officer believes the driver or passengers pose a danger, the officer can remove them and frisk them for weapons.

If in the course of a traffic stop the officer determines that there is probable cause for an arrest—even on minor offenses, such as driving without proof of insurance—the officer can place the individual under arrest and then conduct a search of the individual incident to arrest. This includes a search of the arrestee's person, and if it is reasonable to believe that further evidence of the crime of arrest will be found (e.g., the driver was arrested for possession of drug paraphernalia), a full search of the passenger compartment can be conducted (see Arizona v. Gant [2009]).

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