525 U.S. 113 (1998)
A police officer in Newton, Iowa, stopped petitioner Patrick Knowles for speeding after clocking him driving 43 miles per hour in a 25 miles per hour zone.1 The officer issued a citation to Knowles for the speeding violation, although Iowa law allowed for his arrest.2 Following the issuance of the citation, the officer conducted a full search of the car and located a bag of marijuana and a pot pipe under the driver's seat.3 Knowles was arrested at that point and charged with violations of state laws regarding controlled substances.4
Knowles moved before trial to suppress the evidence found during the search.5 During the hearing on the motion, the officer stated that he had neither obtained Knowles' consent nor possessed probable cause to search the vehicle.6 The officer instead relied upon provisions of Iowa law that authorized searches in connection with the issuance of citations.7 The trial court denied the suppression motion and found Knowles guilty of the charges.8
The Supreme Court of Iowa, sitting en banc, affirmed the conviction by a divided vote.9 The United States Supreme Court granted certiorari to consider the validity of the search.10
Whether the issuance of a traffic citation rather than effecting a custodial arrest authorizes an officer to conduct a full search of the automobile consistent with the Fourth Amendment?11
In United States v. Robinson, the Court identified two historical rationales for the search incident to arrest exception. The first is the need to disarm the suspect to take him into custody. The second is the need to preserve evidence for later use at trial.12 These rationales justify a bright-line rule only when a custodial arrest actually occurs. Neither supports extending full search authority to the far less intrusive encounter that follows issuance of a citation.13
No. The rule requires an actual custodial arrest before the twin rationales of officer safety and evidence preservation can justify a full search of the vehicle.14 Here the officer issued only a citation and performed the search before any arrest took place, so the safety concerns that accompany taking a suspect into custody and transporting him were never triggered.15 The encounter remained brief and more analogous to a Terry stop than to a formal arrest, reducing the danger to the officer below the level that supports a full field search.16 All evidence needed to prosecute the speeding offense had already been obtained once the citation was written, eliminating any justification based on preservation of evidence.17
Iowa's statutory authorization for a search incident to citation cannot enlarge Fourth Amendment authority.18 Officers retain other tools to address safety, including ordering occupants from the vehicle and conducting limited patdowns on reasonable suspicion.19 The remote chance that a search might uncover evidence of some other undetected crime supplies no basis for the greater intrusion of a complete search of the passenger compartment.20
Because neither rationale supporting the search-incident-to-arrest exception was present, the search violated the Fourth Amendment.21