Who is in the driver's seat!
The whole notion of driving a car goes back to the days of the horse and carriage and the bullock wagon where the person in charge of the team would encourage the horses or bullocks to move forward. Drive them on. So the sense of forward movement brought about in accordance with the mastery of the person in charge over the animals, and then over the vehicle, is pretty basic.
The definition reads:
3. to cause and guide the movement of (an animal, vehicle, etc.).
This is followed by drive in the sense of I’ll drive you to the station.
4. to convey in a vehicle.
From this there is an extended meaning:
14. to be in control of a motor vehicle.
The person who is nominated or accepted as the one who is or who will be in charge of the vehicle and ultimately cause it to move on its way, is the person who drives the vehicle.
Thus the driver is:
the person who is causing the vehicle to move in a certain direction.
The person who is nominated or accepted as the one who will make this movement happen.
But the second definition is one of those shorthand definitions and is dependent on the first and primary definition.
These are Macquarie Dictionary definitions but they are backed up by the OED which follows the same train of thought.
So it follows that you are only driving a car when that car is in motion.
There have been various accounts recently of police giving tickets to people sitting in the driver’s seat in cars in car parks and using their mobile phones. The police defence was that the car’s engine was switched on. In one case this was because the occupant wanted the aircon on which could only be done with the engine on. There have been cries of revenue-raising which I think are perfectly justified. The driver’s defence should be that you become the driver when you drive the car, that is, cause it to move it in some direction.
The police definition of driver – the person sitting in a stationary car in the driver’s seat with the engine switched on, is so wilfully tortured and twisted that it is ridiculous. It doesn’t pass the pub test.