A person who believes they have had too much to drink to safely or legally drive may sit, lie, or fall asleep in their vehicle in a parking lot or parking space outside of a bar or restaurant, or may pull off the road and park to “sleep it off.” When the police observe a person occupying or sleeping in a parked car they may investigate if the car is illegally parked (on the side of the road or elsewhere) or for the stated purpose of checking on the well-being of the occupant of the vehicle.

After knocking on the window and attempting to wake the occupant of the vehicle the police will typically ask if the occupant was recently operating the vehicle and whether the occupant has had anything to drink. Based in part on the occupant’s answers to these questions the police may request the occupant submit to a breath or blood test, perform field sobriety tests, and may ultimately arrest the occupant for DUI/DWI.

Laws defining DUI/DWI criminal offenses vary from state to state and often include a person who is in physical control of a vehicle as being within the definition of operating a vehicle. Based on such a broad statutory definition of operating a vehicle—and depending on other factors such as (1) whether the occupant was in the driver’s seat of the vehicle or the back seat or passenger seat; (2) whether the vehicle’s engine was running; (3) whether the occupant was awake or asleep; and (4) whether the key, if any, was in the ignition—the police may arrest the occupant for DUI/DWI.

In Illinois, a person found in a parked vehicle who is suspected of being under the influence can be subject to a DUI investigation. Illinois law considers a person to be in 'actual physical control' of a vehicle if they have the capability to operate the vehicle, regardless of whether they are driving at the time. Factors such as the person's location in the vehicle, whether the engine is running, the person's state (awake or asleep), and the position of the keys can all influence whether they are deemed to be in control of the vehicle. If the police find someone in a parked car and suspect DUI, they may conduct a well-being check and inquire about recent vehicle operation and alcohol consumption. Based on the occupant's responses and other observations, the police may administer field sobriety tests or request a breath or blood test. If the evidence suggests the person was in physical control of the vehicle while impaired, they may be arrested for DUI. It's important to note that simply being in a parked car does not guarantee immunity from DUI charges if other factors suggest control of the vehicle.