In addition to probable cause or reasonable suspicion for the initial stop, the officers must have probable cause for the arrest.  Usually this means that the officers reasonably believe a felony has occurred or a misdemeanor occurred in their presence.  A seizure occurs when a reasonable person would not feel free to leave or could not leave.  In traffic cases passengers are seized when the driver is seized.  Police may search a vehicle incident to an arrest only if the suspect is within reaching distance of the passenger compartment or there is reason to believe evidence of the offense is in the vehicle. The following U.S. Supreme Court cases and other resources provide the basis for this line of reasoning:

Probable Cause
Brinegar v. United States , 338 U.S. 160, 69 S.Ct. 1302 (1949)
Probable cause exists where "the facts and circumstances within their [the officers'] knowledge and of which they had reasonably trustworthy information [are] sufficient in themselves to warrant a man of reasonable caution in the belief that" an offense has been or is being committed. Carroll v. United States,267 U. S. 132, 162.

De facto arrest requires probable cause
Dunaway v. New York, 442 U.S. 200 (1979)
We first consider whether the Rochester police violated the Fourth and Fourteenth Amendments when, without probable cause to arrest, they took petitioner into custody, transported him to the police station, and detained him there for interrogation. . . .
No intervening events broke the connection between petitioner's illegal detention and his confession. To admit petitioner's confession in such a case would allow "law enforcement officers to violate the Fourth Amendment with impunity, safe in the knowledge that they could wash their hands in the `procedural safeguards' of the Fifth."

Kessler, David K.  Free to Leave? An Empirical Look at the Fourth Amendment’s Seizure Standard. 99 The Journal of Criminal Law and Criminology 1 (2009): 51-88. – This Article presents the first empirical study of whether people would actually feel free to leave in two situations in which the Court has held that people would: on public sidewalks and on buses.

Passenger seized
Brendlin v. California 551 U.S. 249 (2007),
When police make traffic stop, a passenger in the car, like the driver, is seized for Fourth Amendment purposes and so may challenge the stop’s constitutionality.

Search incident to an arrest
Arizona v. Gant , 556 U.S. 332 (2009), 129 S.Ct. 1710 (2009)
Police may search a vehicle incident to a recent occupant's arrest only if the arrestee is within reaching distance of the passenger compartment at the time of the search or it is reasonable to believe the vehicle contains evidence of the offense of arrest.

Singh, Angad. “Stepping out of the Vehicle: The Potential of Arizona v. Gant to End Automatic Searches Incident to Arrest beyond the Vehicular Context.” 59 American University Law Review 6 (August 2010): 1759-1795. – This Comment argues that Gant not only enhances Fourth Amendment protections overall by limiting authority to search an automobile upon arrest, but that its first holding also undermines other cases permitting automatic searches incident to arrest in nonvehicular situations. Part I outlines the judicial origin of search incident to arrest law and its schizophrenic history. Part II argues that the standard governing Gant’s second holding is vague, and is rather than a quantum of proof analysis prevalent in standards such as probable cause and reasonable suspicion. Part III analyzes the effect of applying Gant’s first holding to an automatic search of containers on the person incident to arrest, while Part IV applies Gant to certain automatic home searches incident to arrest. Part IV also address some counterarguments and potential pitfalls.

NCSC Library Resources:
Available from the NCSC Library by contacting
The Drunk Driver and Jail. Washington, D.C.: U.S. Department of Transportation, National Highway
Traffic Safety Administration, American Correctional Association, 1987. (Available in the NCSC library: KF2231 D78 1987).