[arjie]

For what it's worth, I remember reading two things:

1. Driving is one of those activities where people actually use a lot of their brains functions to do. This is partly why it can tire you out so much.

2. People speaking to a passenger drive as poorly as people otherwise impaired but the fact that they're both in the car (grounding them in the fact that they're in the vehicle) and that both of them pay some attention to the road helps mitigate the negative effects.

All I can recall is that I read #1 in a BBC News article and the latter on something I found on Google Scholar. I feel they're relevant. If you're interested, I hope it helps.