I agree, partly. They were testing the vehicle, and having a human in the driver's seat to take over in the event of a problem is a major component of that test

I agree, partly. They were testing the vehicle, and having a human in the driver's seat to take over in the event of a problem is a major component of that testing. That person likely became complacent in their duties due though, likely because the software seemed to work well enough. The failure here was to expect someone to stay alert for a rare event for long periods of time. They probably should have had a more active task that would have kept them more alert.