The Fourth Amendment to the U.S. Constitution prohibits "unreasonable" search and seizures. (State constitutions typically have similar provisions applying to searches and seizures that might provide greater protections.) Traffic stops and arrests are considered seizures. So, the legality of a DUI prosecution depends—among other factors—on whether the stop and subsequent arrest were reasonable.
Generally, searches and seizures require a warrant. For example, police typically can't enter someone's home unless they first get a search warrant from a judge. However, seizures of vehicles are treated differently. All police need to stop your vehicle is a reasonable suspicion to believe you've broken the law.
In terms of a DUI arrest, the original stop might or might not be based on an officer's suspicion of drunk driving. For instance, officers normally pull drivers over when they observe indications of impairment such as swerving or ridiculously slow driving. But in other cases, an officer might stop a motorist for a minor traffic violation (like a stop sign or speeding violation) and notice signs of intoxication (like the odor of alcohol) during the traffic stop. In both situations, the stop is valid, and the officer is allowed to conduct a reasonable DUI investigation.
So, what about DUI checkpoints? With DUI checkpoints police stop every driver without having any reason to believe the individual drivers have broken the law. The Supreme Court has said that DUI checkpoints (and some other types of roadblocks) are a permissible exception to the reasonable-suspicion rule. Basically, the reasoning was that the public's interest in keeping intoxicated drivers off the road outweighs the inconvenience to motorists.
The legality of the traffic stop is just the first part of the analysis. For a DUI prosecution to be lawful, the driver's arrest must be supported by probable cause. In other words, police need a reasonable basis to believe the motorist was driving under the influence.
In most states, a DUI is defined as being in actual physical control of a vehicle while:
In some cases, the motorist's driving pattern will provide some of an officer's probable cause. And during the stop, the officer might note other symptoms of drug or alcohol use such as:
It's also common for officers to ask drivers to complete field sobriety tests (FSTs) or a roadside breath test. FSTs and prearrest breath tests are typically optional. But if a driver consents, the results are fairly considered in the probable cause determination.
If, on balance, the facts could lead a reasonable person to believe the driver is under the influence, the officer is legally justified in making a DUI arrest.
So, what happens if there was no reasonable suspicion for the stop or probable cause for the arrest? Generally, any evidence obtained subsequent to the illegal police action will be inadmissible in court.
To exclude illegally obtained evidence from consideration at trial, a defendant would normally need to file a motion to suppress evidence in court. A successful motion to suppress can ultimately lead to dismissal of a DUI charge. (Of course, a motion to suppress is just one of the many ways to fight a DUI charge.)
After being arrested for a DUI, you should seek the advice of an attorney. Search-and-seizure and DUI laws are complicated. And the facts of each case are unique. A qualified DUI lawyer can apply the law to the facts of your case and let you know your options.