Our minds are such complex and funny things. Moments of forced idleness, such as riding in a car, allow are minds to wander and make the wonderful connections that we would love to reach for when we try to write. Yet, when forced to actually do the work, our minds come up with every possible excuse to avoid the heavy lifting of writing.
I know this is why many writers carry around a writer's journal with them. That way they can jot down their inspired thoughts soon after they occur. I keep one next to my bed for this reason. I find, however, that when I attempt to carry one around, the ideas tend to be even more illusive, waiting for a moment when I don't have a pen or can't use a pen to show themselves. This game of mental hide-and-seek is not that amusing for me anymore.
Oh well ... they probably weren't that great of ideas anyway ...