Law enforcement officers in Iowa cannot just randomly pull a vehicle over to check whether the driver may be driving while intoxicated. The U.S. Constitution, as interpreted by the Supreme Court, places limits on the authority of police to stop and question individuals. The Supreme Court has ruled that an officer must have a reasonable suspicion a crime has been committed before stopping an individual to question them. This rule applies whether the officer stops a pedestrian on a sidewalk or pulls a vehicle over on the highway.
Reasonable suspicion for an OWI stop is usually based on the officer's observation of the vehicle. Certain driving behaviors such as weaving, excessive braking and driving too slowly may give the officer grounds to pull the vehicle over. Reasonable suspicion can also be based on information from the public, as when a witness calls police, reports an intoxicated person leaving a bar and driving away, and provides a vehicle description.
Reasonable suspicion alone is not sufficient to make an arrest for OWI. To make an arrest the officer must meet the higher standard of probable cause. While reasonable suspicion means the officer has enough information to believe a crime may have been committed, probable cause means the officer has information indicating a crime has probably been committed. To establish probable cause for an OWI arrest, police generally need to administer field sobriety tests or blood alcohol content tests.
When a police officer pulls a vehicle over without sufficient basis for a reasonable suspicion of OWI, the illegal stop can be challenged in court. If successful, the challenge can result in dismissal of the charges.
Source: Findlaw.com, "What is Reasonable Suspicion for a DUI Stop?" accessed May 18, 2015