We should note at the outset that the three-phase sequence -- vehicle in motion, personal contact and pre-arrest screening -- do not need to be followed in order. It is possible for a police officer to skip the first phase and go directly to the second; a common way this can happen is at a DUI checkpoint, or when the officer is responding to the scene of a car accident.
The purpose of the personal contact phase is for the officer to observe the driver while still in the car and to assess whether signs exist that suggest that the driver may be impaired. Depending on how this face-to-face interaction progresses, the officer will eventually decide whether the driver is sober, whether to go to the third phase and ask the driver to exit the vehicle so that field sobriety tests can be administered, or to arrest the driver based on probable cause of DUI.
When the officer begins the personal contact with the driver, he or she will be relying on sensory cues to assess the driver’s state of sobriety. These cues include: