It's hard to enjoy a book when you're constantly scanning the highway for things crossing the road, or slowly inching up a lane of traffic. The concentration required for driving detracts from the audiobook experience. It's often better than driving in silence, but I'd hardly call it a replacement for leisure time. I reckon most people would rather be home.

I've found non-fiction audiobooks to be good when you can't devote full attention. I do find that fiction audiobooks are ok on interstates, though, for especially long drives.