Most states now have implied consent laws and since a drivers license is a privilege and not a right they do not need probable cause to ask you to submit to a DUI test. In fact, your 5th Amendment right does not apply because once again driving is a privilege granted by the state not a right. Therefore, if you refuse to blow it can mean automatic administrative suspension of your license with no "hardship privileges" regardless of your guilt.
Where this came from was people refusing to blow and prosecutors having a tough time with the conviction based purely on officer's testimony. This is a Constitutional end around brought to you by MADD. Refuse to blow and you are still off the streets.
DUI you are considered guilty until proven innocent by a little machine you have to blow into. DUI checkpoints violate your 4th and 5th Amendment rights, even though 6 of the US Supreme Court Justices disagreed. 
The Constitution of the United States pretty clearly says that police can’t just stop someone and conduct an investigation unless there are “articulable facts” indicating possible criminal activity. So how can they do exactly that with DUI roadblocks?Good question. And it was raised in the case of Michigan v. Sitz (496 U.S. 444), in which the U.S. Supreme Court reviewed a decision of the Michigan Supreme Court striking down drunk driving roadblocks as unconstitutional. In a 6-3 decision, the Court reversed the Michigan court, holding that roadblocks were consitutionally permissible. Chief Justice Rehnquist began his majority opinion by admitting that DUI roadblocks (aka “sobriety checkpoints”) do, in fact, constitute a “seizure” within the language of the 4th Amendment. In other words, yes, it’s a blatant violation of the Constitution. However….
However, it’s only a little one, and there’s all this “carnage” on the highways MADD tells us we’ve got to do something about. The “minimal intrusion on individual liberties”, he wrote, must be “weighed” against the need for and effectiveness of roadblocks. In other words, the ends justify the (illegal) means….aka, “the DUI exception to the Constitution”.
The dissenting justices pointed out that the Constitution doesn’t make exceptions: The sole question is whether the police had probable cause to stop the individual driver. As Justice Brennan wrote, “That stopping every car might make it easier to prevent drunken driving…is an insufficient justification for abandoning the requirement of individualized suspicion.” Brennan concluded by noting that “The most disturbing aspect of the Court’s decision today is that it appears to give no weight to the citizen’s interest in freedom from suspicionless investigatory seizures”.
Rehnquist’s justification for ignoring the Constitution rested on the assumption that DUI roadblocks were “necessary” and “effective”. Are they? As Justice Stevens wrote in his own dissenting opinion, the Michigan court had already reviewed the statistics on DUI sobriety checkpoints/roadblocks: “The findings of the trial court, based on an extensive record and affirmed by the Michigan Court of Appeals, indicate that the net effect of sobriety checkpoints on traffic safety is infinitesimal and possibly negative”.
p.s. The case was sent back to the Michigan Supreme Court to change its decision accordingly. But the Michigan Supreme Court sidestepped Rehnquist by holding that DUI checkpoints, if permissible under the U.S. Constitution, were not permissible under the Michigan State Constitution, and ruled again in favor of the defendant — in effect saying to Rehnquist, “If you won’t protect our citizens, we will”. The State of Washington has since followed Michigan.
BTW: I'm not an attorney and have never had a DUI, but I helped someone else get through the process after she refused to blow. Fortunately, the cop read her the wrong consent law (it was on the video - he read her the CDL consent) and they had to drop the administrative as well as the criminal charges. Neither the prosecutor, nor her attorney caught that. I had to do all the leg work for her attorney and just hand it to him on a silver platter. There are two types of attorneys - those that know the law and those that know the judge. We hired the latter and I did all the work.
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