"The backup driver" is the driver.
Until everyone is in a fully autonomous car and they are interoperable with each other on the road, if your ass is in the seat with the steering wheel, you're the only driver.
What we have now with "autonomous" driving is slightly more useful than power steering but ultimately, you have to be aware that it can fail, it will eventually fail, and you need to be ready to immediately take over to get to safety.