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 California, a person found in a parked vehicle who is suspected of being under the influence can be subject to a DUI investigation. California law considers a person to be driving under the influence if they have a blood alcohol concentration (BAC) of 0.08% or higher, or if they are impaired by drugs or alcohol regardless of BAC. The definition of 'driving' can be interpreted broadly to include being in 'actual physical control' of the vehicle, even if the vehicle is not moving. Factors such as the location of the person in the vehicle, whether the engine is running, the person's level of consciousness, and the location of the keys can all influence whether an individual is considered to be in control of the vehicle. If the police find someone in a parked car who appears to have been drinking, they may conduct a welfare check, which can lead to a DUI investigation if there is reasonable suspicion of impairment. This can include field sobriety tests and a request for a breath or blood test. Refusal to submit to these tests can result in automatic license suspension and other penalties under California's implied consent laws. An arrest for DUI can occur if the officer has probable cause to believe the person was in actual physical control of the vehicle while impaired.