Even when police have stopped and arrested you for DUI, it is not the entire story. In order for you to be convicted, police must have made a valid arrest in the first place. If not, the charges against you can be dismissed.
In order to make a traffic stop, the officer needs to have a reasonable suspicion that someone is under the influence of alcohol. This suspicion could be based on a report to law enforcement or the officer’s observation of the driver. If an officer sees someone driving erratically or recklessly, they may have this reasonable suspicion.
In order for the officer to make a legal arrest, they must have probable cause to believe that someone was driving under the influence. Probable cause is defined as a reasonable basis to believe that a crime has been committed. The officer needs a valid reason to make the arrest. This could be because they have noticed signs of intoxication or administered a field sobriety test.
A police officer simply cannot stop and arrest anyone that they want. While reasonable suspicion is a lower standard than probable cause, the officer must be able to articulate a legally valid reason why they stopped someone on the road. The officer may pull someone over for a minor traffic infraction and notice signs of intoxication. That would be a valid traffic stop.
Arresting someone is considered a seizure. If there was not a reasonable basis to support it, any evidence obtained as a result of an illegal search and seizure will not be admissible in court.