In order to stop a vehicle, a police officer must have reasonable suspicion that the driver is committing a violation of Illinois law. The Fourth Amendment of the United States Constitution requires that law enforcement have specific and articulable facts that a crime has been committed, or is about to be committed, to justify the stop of a vehicle. An exception to this requirement is known as the community caretaking function, which allows a police officer to investigate if the driver appears to be in need of assistance.
Of course, there are a wide variety of offenses that can justify a vehicle stop. In DUI cases, the officer does not need to have a basis to believe that the driver is under the influence at the time of the stop. Minor moving violations or even equipment violations (i.e. a burnt out taillight or cracked windshield) are valid grounds to stop a vehicle. Most Illinois DUI investigations will begin with an allegation of improper lane usage, speeding or other common moving violations under the Illinois Vehicle Code.
When conducting an Illinois DUI arrest, the police officer must have probable cause to believe that a driver is under the influence. Probable cause is a higher standard than the reasonable suspicion necessary to stop a vehicle. Once a police officer has stopped a vehicle, that officer must be able to articulate specific facts supporting a belief that the driver is under the influence of alcohol, illegal drugs, medical cannabis, or another intoxicating compound in order to arrest the person for DUI. This can be based on factors relating to the driver’s speech, appearance, and odor. Specifically, officers will often point to sign of impairment including bloodshot and/or glassy eyes, slurred speech, soiled clothing, unusual actions, inconsistent responses and the odor of alcohol and/or marijuana.