If you could reliably work from your autonomous vehicle for two hours you might be able to include that as part of your work day, e.g. 2 hours working on the ride in, 4 hours in the office, 2 hours on the ride home for the full 8 hours. If not, you could fall asleep in the backseat, watch some TV shows, or... you know. Just make sure your windows are tinted. In any case, I'd definitely take a 2 hour "do whatever in the backseat" commute over an hour-long "pay attention to the road at all times" commute.

This is all predicated on true autonomous vehicles though, not the half-baked non-solutions that exist today.