Law enforcement officers must provide the Miranda warnings to suspects who are (1) in custody and (2) subject to interrogation. Motorists who have been stopped by police officers aren’t free to leave and are inevitably subjected to some kind of questioning, which begs the question: Shouldn't officers read them their rights?
The Supreme Court has acknowledged that a traffic stop involves significant restraint of a driver’s freedom. But it has held that that a typical roadside detention doesn’t place the driver “in custody” because it doesn’t involve the kind of restraint that’s typical of a formal arrest (for example, handcuffing). That isn’t to say, however, that a traffic stop can’t evolve into a full-blown arrest that requires Miranda warnings for further questioning.
Because the initial detention didn’t place McNulty “in custody” within the meaning of Miranda, he wasn’t entitled to an advisement of his rights at that point. The statement that he had consumed “a few” beers will therefore be admissible in court. But once Officer Johnson formally placed him under arrest, he was “in custody.” Since Officer Johnson didn’t give the Miranda advisement, but nevertheless interrogated him, McNulty’s statements at the police station are inadmissible. (Berkemer v. McCarty, 468 U.S. 420 (1984).) (For more, see Miranda Rights: What Happens If Police Don't "Read Your Rights.")