4) An officer who reasonably believes that criminal activity may be afoot in a public place is authorized to stop any person who is suspected of participating in that criminal activity and conduct a carefully limited search of the suspect's outer clothing for weapons that may be used against the officer (see Terry v. Ohio, 392 U.S. 1 [1968]). The officer may also ask for identification, but the suspect is under no obligation to produce it. However, A suspect's refusal to identify himself together with surrounding events may create probable cause to arrest (see People v. Loudermilk, 241 Cal. Rptr. 208 (Cal. App. 1987). This kind of warrantless search, called a Terry stop or a Terry frisk, is designed to protect officers from hidden weapons. Accordingly, items that do not feel like weapons, such as a baggie of soft, granular substance tucked inside a jacket pocket, cannot be seized during a Terry frisk, even if it turns out that the item is contraband.

5) Warrantless searches, seizures, and arrests may be justified by "exigent" circumstances. To determine whether exigent circumstances justified police conduct, a court must review the totality of the circumstances, including the gravity of the underlying offense and whether the suspect was fleeing or trying to escape. However, the surrounding circumstances must be tantamount to an emergency. Shots fired, screams heard, or fire emanating from inside a building have all been considered sufficiently exigent to dispense with the Fourth Amendment's warrant requirement.

6) The Supreme Court has upheld brief, warrantless seizures at fixed roadside checkpoints aimed at intercepting illegal aliens (see United States v. Martinez-Fuerte, 428 U.S. 543 [1976]) and drunk drivers (see Michigan v. Sitz, 496 U.S. 444 [1990]). Both checkpoint programs passed constitutional muster because they were tailored to remedying specific problems that law enforcement could not effectively address through more traditional means, namely problems relating to policing the nation's border and ensuring roadway safety. However, when the primary purpose of a checkpoint is simply to detect ordinary criminal activity, the Supreme Court has declared it violative of the Fourth Amendment (see Indianapolis v. Edmond, 531 U.S. 32 [2000]).

7) Searches, seizures, and arrests made pursuant to a defective warrant may be justified if the officer was proceeding in "good faith." The Supreme Court has said that a search made pursuant to a warrant that is later declared invalid (i.e., it fails to meet the requirements for a valid warrant enumerated above) will still be considered reasonable under the Fourth Amendment so long as the warrant was issued by a magistrate and the defect was not the result of willful police deception (see United States v. Leon, 468 U.S. 897 [1984]). This exception to the warrant requirement was created so as not to punish honest police officers who have done nothing wrong while acting in accordance with an ostensibly valid warrant.