The United States Supreme Court, in a case known as McNeely v. Missouri, issued an important decision related to Driving Under the Influence, or DUI, and search warrants. The Court decided that in most cases, before the police can force a DUI suspect to give blood, they must obtain a search warrant.
The Fourth Amendment to the United States Constitution prohibits unreasonable searches and seizures. It further states that no search warrant shall issue without probable cause.
In essence, the government cannot search you or your home without a warrant issued by a judge. The judge should not issue the warrant unless the police provide evidence to the judge that there is some good reason to approve the warrant and authorize a search.
There are many exceptions to the warrant requirement. A number of the exceptions apply to motor vehicle stops, including DUI arrests.
Exceptions to the warrant requirement include searches made during an arrest (police can pat you down for weapons in the process of an arrest) and searches in which the police are already in legitimate contact with you and observe something "in plain view" (stopped for a traffic ticket; during the stop, the police see an open container on the floorboard). In addition, searches done with your consent (do you mind if I search your trunk? No go right ahead) and "stop and frisk" searches (officer observes what appears to be a street corner drug transaction and approaches the suspect to question him and then pats him down for weapons) do not require a warrant.
Another exception applies to emergency situations, where the evidence can be easily moved, destroyed or otherwise made to disappear before a warrant can be issued (drugs flushed down the toilet). This exception was pertinent to the McNeely decision.