Encounters between citizens and law enforcement fall into 3 categories: voluntary encounters, temporary detentions, and arrests. When a police officer turns on his lights and sirens, you must pull over. Traffic stops are not voluntary, but you are not under arrest. This is a temporary detention, but the protections of the 4th Amendment of the Constitution related to search and seizure still apply. Every traffic stop must either rise to "reasonable suspicion" or "probable cause." From DWI/DUI cases to narcotics cases, traffic stops are the key to tons of arrests. Unlike a house or apartment, police do not need a search warrant to search you or your motor vehicle. Challenging the legality of the officer's initial temporary detention is often a key issue in criminal cases.If the officer's temporary detention was unlawful or unreasonably long, a court can suppress all evidence seized after the detention. This could include field sobriety results, in toxilizer results, large quantities of drugs, or even a murder weapon!"Reasonable suspicion" is sufficient reason to temporarily detain a citizen to investigate a potential crime
"Probable cause" is sufficient evidence to arrest a citizen for a crime. In a DWI case, an officer may have probable cause to stop someone if they were committing a traffic violation like speeding. On the other hand, non-violation behavior like weaving in the lane may provide reasonable suspicion that a DWI offense is occurring. Many times, the officer may have both probable cause of a traffic violation and reasonable suspicion of a DWI offense. Once the officer initiates a traffic stop, the driver and all passengers have been "seized" under the 4th amendment.
An officer is allowed to temporarily stop a suspect during a temporary detention and gather more information to confirm or disprove criminal activity. If the officer can explain that he believes the suspect might be armed with a weapon, he can pat down the outer clothing to search for weapons.
Reasonable suspicion is based on facts and reasonable inferences from those facts articulated during the officer's testimony. The facts must have been known to the officer at the time the stop was made (not learned later), but the officer's training and experience are considered. The judge decides if reasonable suspicion existed not whether the suspects initial conduct was actually unlawful.
Reasonable suspicion of a traffic violation is sufficient to allow a temporary detention in the form of a traffic stop. An officer's observations do not have to be sufficient by themselves to establish the traffic offense, but an officer's honest but mistaken understanding of a traffic law will render a temporary detention unlawful. The detention is supposed to last long enough to resolve the traffic stop issue that was the reason for the detention. While investigating the traffic stop, the officer can ask the driver to get out of the car, driver's license check, check for warrants, ask for information about vehicle ownership and insurance, look at visible areas in the vehicle, and ask questions about the driver's condition, and alcohol consumption.Any of these areas may develop additional reasonable suspicion through observations and questioning. An officer can also always ask for consent to search a vehicle during the investigation of a traffic stop. Additional reasonable suspicion and consent to search allow the officer to prolong the detention to continue their investigation. This could involve steps like having the driver do field sobriety tests.
If an officer can explain why his safety or the safety of others may be at risk, he may handcuff an occupant of a vehicle. He may also pat down the outer clothing of the driver and any passenger, if the officer can explain his belief that the person is armed. These pat downs can lead to the lawful discovery of drugs, weapons, or contraband based on the "plain feel" rule, if the officer can immediately identify the substance to be contraband during the pat down.Sometimes officers can justify a traffic stop without reasonable suspicion or probable cause if one of the occupants of a vehicle is in distress
Courts will look at the nature of the distress, the location of the individual, who else is in the car,and the level of danger the situation poses to the individual or others. This is called "community care taking." Sometimes, community care taking is sometimes successfully used to justify a traffic stop for non-traffic violation weaving where no one else is in the car and the vehicle poses a danger to others on the road. There is technically no violation for "weaving." It is called "failing to drive within a single lane."
Sometimes police use trained drug dogs to sniff around a vehicle to detect narcotics. If the dog "alerts" on a vehicle for narcotics, the officer has probable cause to search the vehicle and places within the vehicle that narcotics could be hidden. The Supreme Court has said that this is not a search because a citizen does not have an expectation of privacy related to the smell of their vehicle. Where these searches are challenged usually relates to how long it took to get the canine unit to the scene. Prolonging a traffic stop"unreasonably" (judged on a case by case basis) will render the search unlawful.
An officer's intent in initiating a traffic stop is not a relevant question for a legal challenge. As long as their was an objective reason for the stop, the stop is valid. Texas does have a prohibition against racial profiling in Article 2.131 of the Code of Criminal Procedure. As a practical matter, proving an officer's motives or intent is nearly impossible.