Curiously, the California Vehicle Code does not provide a definition of what constitutes driving for the purposes of proving that a person drove a vehicle while impaired or over the limit.
In the DMV case of Mercer v Dept of Motor Vehicles 53 Cal. 3d 753, 280 Cal. Rptr. 745, 809 P.2d 404 (1991) the Court held that for the purpose of license suspension hearings, the DMV must establish evidence of volitional movement of the vehicle, however slight.
Most Prosecutors accept that proof of driving requires proof of some movement of the vehicle. Even taking the hand brake off and rolling forward, with the engine switched off would constitute “driving”.
Some States make it an offense to be in care and control of a vehicle while impaired, so that a person could be guilty by being in the drivers seat of a stationary vehicle. California has no such law, although it is theoretically possible to be guiltly of an attempt to drive a vehicle while impaired.
Circumstantial evidence of driving is admissable to prove driving. Prosecutors may sometimes seek to prove driving by the surrounding circumstances, such as who the vehicle is registered to, who was in the drivers seat, whether the engine was warm or running, who had the keys and who was in the car and how did it get there etc.
I recently had a DUI case in Orange County, where my client was observed urinating on the side of the freeway. He had the keys to the car which was parked on the shoulder. However, the registered owner of the car was also with him although he was in the passenger seat and alleged that my client drove. The case was never filed by the DA's office due to lack of proof of driving. At the DMV hearing in Irvine Driver Safety Office, the DMV set aside the suspension based on my argument that it was impossible to establish who drove the vehicle.