When police don't read you your rights while asking you questions, there are times when evidence obtained during the questioning can't be used against you. You can petition to have evidence suppressed, which means the judge rules a prosecutor cannot present it.
In DWI cases, an experienced DWI defense lawyer can sometimes help defendants avoid conviction by successfully petitioning to suppress key evidence like a BAC test.
However, some DWI defendants are under the mistaken impression they can keep evidence out of court whenever they weren't given a Miranda warning. This is not necessarily true. You can keep evidence from being used if you were pulled over illegally or if your Fourth Amendment rights were violated, but there are lots of situations where police can collect evidence even without first mirandizing you.
A Miranda Warning Isn't Always Required for DWI Evidence to be Used Against You
A Miranda warning, or mirandizing, refers to police reading you your rights. You've heard the warning on TV cop and lawyer shows: you have the right to remain silent and right to a lawyer, and anything you say will be used again you.
A Miranda warning is required during a custodial interrogation. This is where the confusion comes in. If you are pulled over by police, you naturally assume questions they ask are part of a custodial interrogation and thus your answers can't be used in court if you weren't read your rights first. When police ask if you've been drinking, or ask you how many drinks you had, your answers indeed can incriminate you - so this could be considered an interrogation.
You may also believe you are in custody, since you aren't free to just drive away.
Courts disagree, however, that you are always being subject to a custodial interrogation when police pull your car over and ask you questions. In Berkemer v. McCarty, the court ruled there's no requirement police give you a Miranda warning before questioning you during a traffic stop. No Miranda warning is necessary because you are not in custody. The court's decision reads: "In short, the atmosphere surrounding an ordinary traffic stop is substantially less "police dominated" than that surrounding the kinds of interrogation at issue in Miranda itself."
You don't have to answer questions when you are pulled over, although you should be polite to avoid creating a potentially dangerous situation. If you do answer questions, anything you say can be presented by a prosecutor although you weren't first read your rights.
If police decide to arrest you on suspicion of drunk driving, you are actually being taken into custody. If you aren't read your Miranda rights at that point, anything you subsequently say in response to police questions can likely be suppressed in court. Your attorney can help you to determine at what point you were officially in custody.