Not a one-person game, but another one we learned from our mother (who had spent a lot of time travelling through Wisconsin), was "Cow Poker". I 'm curious if anyone else played this, and if the rules were the same.
It worked on 2-lane roads, but was more difficult once we started using the interstate system. One person takes the left side of the road, and the other the right.
The rules are as follows:
Establish an end point for the game (a certain time, a certain landmark, or when you make a stop).
You get 1 point for each cow you see.
if you pass a school, add 20 points.
If you pass a church, double your current score.
If you pass a cemetery, go back to 0 (all your cows died).
Whoever has the most points at the end wins.
This kept me and my sister occupied on many a road trip.