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?
(For information on a related topic, see Searches After Traffic Stops: Not Always Allowed.)
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.
Example: Officer Johnson is driving on the freeway when he notices a car swerving in and out of its lane. He turns on his patrol car’s flashing lights. The driver pulls off the freeway and comes to a stop. Officer Johnson pulls up behind it.
Officer Johnson asks the driver, named McNulty, to get out of the car. McNulty can’t even stand up straight. He blatantly fails the initial field sobriety test that Officer Johnson administers. Officer Johnson asks McNulty whether he has been using any drugs or alcohol. McNulty responds that he’s had “a few” beers. Officer Johnson then formally places him under arrest, handcuffing him and putting him in the patrol car.
Officer Johnson drives McNulty to the police station, where he asks him a series of questions about his level of intoxication. He never gives McNulty the Miranda warnings. McNulty talks at length about his “bad habit” of driving drunk and the fact that he had imbibed approximately seven beers before driving.
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.")