Hour 4. This is the point at which, if traveling with a familiar friend or family member, a long and happy silence would kick in. Music volume increases; minds shut down; scenery passes. The conversation has an opportunity to recharge, possibly restarting with an off-the-wall statement or question related to some shared experience.
With a stranger, no matter how skilled at conversation, this typically does not happen. Firstly, a pause in the conversation becomes awkward, as though we are telling each other: "I have entirely lost interest in speaking with you." To avoid a potential breach of etiquette, we both draw up meaningless topics not worthy of discussion just to avoid the pause, or we rehash bits of conversation from previous hours. Secondly, we have no shared experiences to discuss, not really anyway. Despite both being parents, I don't know his kids and he doesn't know mine. I don't want to hear more than a handful of stories about his kids and he doesn't want to hear more than a handful about mine. We both eat food. We both have favorite foods. But beyond knowing he likes an ethnically diverse menu and I prefer Italian cuisine, we really don't care too much about each other's dietary preferences. Besides, for a guy that says he enjoys foods from all continents, so far he's eaten only chicken caesar salad two days running for lunch.
By the time I leave him at his front door I am spent. I have nothing left mentally for the wife and kids and it shows.