As an initial matter, people need to understand that they can be arrested for driving under the influence of alcohol or drugs (i.e. DUI) even if the police never see them driving a vehicle. If a person is in actual physical control of the vehicle, that is sufficient for a DUI arrest if the person is impaired. The classic case of actual physical control is when the police officer sees a person in his/her vehicle with the keys in the ignition, whether the vehicle is running or not. In those cases, the state has a valid argument that the defendant was in actual physical control of the vehicle and guilty of DUI although no one was actually driving the vehicle at the time.
However, the police cannot just detain a person who is laying down in or sleeping in a vehicle whether the keys are in the ignition or not. A common DUI case occurs when the police get a call about, or see, someone apparently drunk or asleep in a vehicle parked somewhere. The police officer will approach the vehicle and see that the person is asleep or passed out in the driver’s seat. The police officer can assume the driver is passed out drunk, but without specific evidence that the person has been drinking or using drugs, the police officer cannot act on that assumption alone. The police officer can knock on the window and ask the driver some questions to investigate further. However, based on this evidence alone, the police officer cannot tell the driver to get out of the vehicle, tell the driver to take the keys out of the ignition or block the driver’s vehicle with his/her own police vehicle.
The bottom line is the police officer cannot force someone to do something or give the person any indication that he/she cannot leave based on an assumption that the driver is impaired from alcohol or drugs. If, after a consensual discussion with the driver, the police officer develops evidence that the person has been drinking or using drugs (such as odor of alcohol or incriminating statements), then the police officer can ask the driver to turn off and step out of the vehicle. However, without specific evidence of impairment, detaining the driver for further investigation or arresting him/her for DUI is improper and should result in the DUI case getting thrown out.