The 4th Amendment governs roadblocks by police
Many roadblocks set up by the police are illegal. These are controlled by the 4th Amendment of the United States Constitution or a similar state constitutional provision or state statute that limits "searches and seizures."
Roadblocks must be in accordance with established guidelines that assure no discrimination or profiling has occurred
The police must have a legally sufficient reason to stop your vehicle as a general rule. This reason can be based upon an observation of a traffic offense, defective equipment, an expired tag or inspection sticker, or some observable evidence that a crime is being committed or is about to be committed. In most states, however, roadblocks have been approved for use in stopping "impaired" drivers, so long as the roadblock is LEGALLY established and operated under strict, established guidelines that assure no discrimination or profiling has occurred.
Roadblocks for only finding drunk drivers can be valid
Roadblocks are valid under the ambit of a State's police powers to insure that the roads are safe from drunk drivers. The United States Supreme Court upheld this authority under the federal constitution in the case of Michigan v. Sitz. The standards set in this case cannot be ignored by any state, since federal law is superior to all state laws. A state may add additional, more protective standards for citizens' rights for roadblocks in that state, or even ban roadblocks as being violative of the state constitution.
Parameters of a valid roadblock
The exact legal parameters of a valid roadblock vary from state to state. However, most states that allow roadblocks have developed rules that include the following: (1) the decision to implement the roadblock must be made by supervisory personnel rather than by law enforcement officers in the field; (2) the roadblock operation must be carried out pursuant to specific, prearranged procedures requiring all passing vehicles to be stopped; and (3) the delay experienced by passing motorists must be minimal.
Some states have added further requirements for roadblocks
Decisional law (appellate law) in some states has added other requirements. For example, Massachusetts requires adequate lighting at the roadblock site and the visible presence of signs that warn motorists of a roadblock ahead. Several states have held these DUI-DWI roadblocks to be inherently unconstitutional under their state constitutions, including Michigan, Oklahoma, Arizona and South Dakota.
An invalid roadblock can be a defense to criminal prosecution
Thus, if the police did not follow precisely the legally required guidelines set forth in your state, or if these DUI-DWI roadblocks are not allowed in your state, your criminal defense attorney can use this as a defense to the charges against you. To win a case based upon an illegal roadblock, you will likely need to have one of the top DUI-DWI specialists in your state when you make your legal challenge. See this informative article: http://www.duiblog.com/2005/05/03#a159.