As a Kansas criminal defense attorney, I regularly deal with warrantless BAC chemical tests in DUI cases. The Supreme Court of the United States released a drunk driving opinion yesterday morning about this, in Missouri v. McNeely. They held that that in drunk driving investigations, the natural dissipation of alcohol in the bloodstream does not constitute an exigency in every case sufficient to justify a blood test without a warrant. In essence, they ruled that law enforcement should seek warrants before testing the blood of drivers suspected of being intoxicated. However, several of the Supreme Court Justices had concerns that the decision would leave police confused about how to handle their drunk driving investigations.
SCOTUS was obviously torn between the 4th amendment values verse keeping citizens safe from drunk drivers on our roads. Their middle ground was to demand warrants, in certain cases, based on the totality of the circumstances. Their reasoning was that advances in technology and communications have made compliance with the Fourth Amendment more practical. All nine justices agreed that since alcohol dissipates from the bloodstream at a steady pace, law enforcement should be able to order blood tests at least in circumstances when the breathalyzer tests have been refused and the warrant is too difficult to obtain.
However, there are no solid bright line ways to make the determination as of now. Rather it is a totality of the circumstances test. Justice Sonia Sotomayor said, "Whether a warrantless blood test of a drunk-driving suspect is reasonable must be determined case by case, based on the totality of the circumstances." The point which is clear is that police must judge each case individually. In the McNeely circumstance, a warrant should have been obtained.
The case was originally from Missouri, where a warrantless blood test was administered in the middle of the night. The blood test showed a BAC level which was almost twice the legal limit of .08.
The question addressed and somewhat answered in this case was, If a warrantless blood draw is permissible under the Fourth Amendment. Under their decision, it just depends.
As a Kansas DUI defense lawyer, I have handled hundreds of cases. However, it is rare to actually see a warrant obtained to draw blood. It does happen though. I suspect that police will begin to procure warrants more often now. Currently, the law in Kansas requires someone to take the Intoxilyzer 8000 breathalyzer test. Otherwise, you are charged with a serious crime. In light of this case, I think it's clear that such a law is Unconstitutional and will likely find its way before an appellate court soon.