I think you have meditation confused with automation.
In fact, if you were truly meditating on your daily drives, you would have the opposite effect of waking up from your daydream in the middle of the route and wondering how you got that far. No, you would actually know perfectly where you are, where you were, and where you are headed.
It's simple to explain: meditation is the state of being mindful. "Mindfulness," the complete awareness of your surroundings, is meditation.
Driving a car or a jet plane on automatic is hardly a meditative state of mind. This daydreaming occurs because you are so used to driving a car that your brain can run it on automatic without fully being there in the present. This opposes meditative qualities, and really gets further from mindfulness than anyone who is a practicioner would want to be.
Now, for something you think about a lot, such as your lovely lady friend... I agree that it can put you in a meditative state of mind; a contemplation. But daydreaming is not meditation! Making up scenes in your head and playing them out is not meditation. The autopilot your mind switches into is a lower form of human functioning, whereas meditative aspects strive to function at a higher level.
If you were to meditate upon driving a car, you would be aware of your body touching the seat, your foot on the pedals, the cars around you, the road (and the bumps in the road), the wind as it passes your car, your hands on the steering wheel, the steering ratio, and every other little thing involved in driving a car. When sleeping, you may be aware of your body against the bed, your breathing, whatever is behind your eyes, this is helpful during sleep and lucidity because it trains you to be aware of the transition from waking body to sleeping body.
You may enjoy reading the first few chapters of this online book, entitled "Mindfulness in Plain English." It may give you a greater understanding of mindfulness and meditation.
http://www.urbandharma.org/udharma4/mpe.html
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