Under the Fifth Amendment to the United States Constitution, the state cannot force you to give evidence against yourself (you have “the right to remain silent”; you can “take the Fifth”). Under the Sixth Amendment to the United States Constitution, you have the right to a lawyer when you are charged with a crime.
A famous case, Miranda v. Arizona 384 U.S. 436 (1966), held that the police must inform you of the right to remain silent, that anything you say may be used against you, that you have the right to consult with attorney before any police questioning and that a lawyer will be appointed if you are unable to afford one. This is often referred to as “reading me my rights”.
The United States Supreme Court reasoned that if someone were not aware of his rights, then having those rights would not be of any use to them. Therefore, Miranda determined that in the proper circumstances, the police are required to inform you of your rights.
In a case known as Mapp v. Ohio, 367 U.S. 643 (1961), the court had, before the Miranda case, held that if the police violate your constitutional rights, the appropriate remedy is to exclude the evidence that the police gathered illegally. This is knows as the “exclusionary rule”.
To realize the importance of Miranda rights for someone who is arrested for DUI, you have to understand how and when the exclusionary rule applies. Miranda rights apply only to “custodial interrogations”.
If the police stop you for a traffic violation, they are only required to demonstrate that the stop was based upon a reasonable suspicion that you committed, or were about to commit, a crime. This is known as a “Terry stop”, named for another Supreme Court case, Terry v. Ohio, 392 US 1 (1968).
During the period of a Terry stop, you are not under arrest and the interrogation is not custodial. The police are merely investigating, finding out what is going on, not trying to gather evidence for a criminal case.
However, at some point, based upon the length of the detention and its nature, a Terry stop becomes custodial. Only at the point do your Miranda rights apply.
Second, even if you are in custody and even if the police do not read you your rights, that only means that any statements you made cannot be used against you. If the state can still prove its case without the statements, you will face criminal charges. Not having your rights read to you is not an automatic “get out of jail free” card.
In many DUI arrests, the point at which the stop becomes a custodial situation is clear. But in other cases, the car is stopped for one reason and then the encounter subtly expands. At that point, it may become custodial, triggering the right to Miranda warnings.