In later decisions, the Supreme Court defined "custody" as "formal arrest or its functional equivalent." (Berkemer v. McCarty; California v. Beheler) Examples of the functional equivalent of formal arrest could be handcuffing and caging the suspect, pointing guns, or preventing the suspect from leaving the police station. (Orozco v. Texas) Miranda "custody" and Fourth Amendment "arrest" are not synonymous, because "custody" also includes detentions accomplished with arrest-like restraints; therefore, even though an officer may not have formally arrested a suspect, if the suspect is being subjected to the functional equivalent of arrest at the time of questioning, he is in "custody" for Miranda purposes.
But in the absence of arrest-like restraints or stationhouse settings, the mere fact that a suspect is being detained ("not free to leave") does not mean that he is in "custody." For example, during a pedestrian detention or a vehicle stop, the person being questioned is "not free to leave," but as long as he is not restricted by handcuffs, police-car cage or pointed firearms, he is not in Miranda "custody." The Supreme Court made this point in Berkemer, where a motorist stopped for suspected driving under the influence was questioned without warnings before being taken into custody:
"The comparatively non-threatening character of [pedestrian] detentions explains the absence of any suggestion in our opinions that Terry stops are subject to the dictates of Miranda. The similarly non-coercive aspect of ordinary traffic stops prompts us to hold that persons temporarily detained pursuant to such stops are not in custody for the purposes of Miranda." (Berkemer v. McCarty)
Although this passage from Berkemer made clear that Miranda does not apply merely because a person is detained ("not free to leave"), it was necessary for the Supreme Court to reverse the Pennsylvania Supreme Court on this very point four years later. In Pennsylvania v. Bruder, the commonwealth court had mistakenly ruled that Miranda applied at a car stop, because the driver was not free to leave. In a brief, four-paragraph decision, the U.S. Supreme Court reversed this ruling and repeated that temporary detentions lacking arrest-like restraints do not constitute "custody" for Miranda purposes.
If police officers, attorneys, and judges erroneously believed that Miranda required the exclusion of statements obtained without warnings and waiver at a time when the suspect was concededly not free to leave but not yet under formal arrest or its functional equivalent, crucial evidence of guilt could be needlessly lost. Miranda "custody" and "not free to leave" are immiscible. Don't try to mix them.