You've got it backward. You have to provide justfication to make someing illegal, not to make it legal.
You avoided the question. How about this? Separate the concept of car and driver for a moment.
You have two entities, a car and a driver. Which one do you really care about? The car. What do you care what the driver does in the car as long as the car does what it's supposed to? Why do you care if there even is a driver as long as the car does what it's supposed to? Perhaps there is a robot in the car driving it, or the person at the wheel has the ability to "sense" the road ahead and can blindfold himself. Perhaps the person in the car is able to drive well and talk on the phone or smoke or read a book.
Why do you care what the person is doing or what state the person was in as long as the car does what it's supposed to? You can take the mental state of the driver into account after an accident to determine if the driver knowingly decreased his own ability to control the car. But what you really care about is what the car does, not the driver.