I, too, am a closet car singer. Of course it's pretty crowded in my closet what with the car in there and all -)

Seriously, I don't have much problem memorizing (my wife calls me Memorex) but I do have problems projecting my voice and also enunciating properly. So I take my five-minute commute home for lunch, back to work, and back home in the evening and sing away at full blast (if I feel like it). I don't usually sing on the way to work in the morning. It's too early, besides I have kids in the car to drop off at school and it inhibits me a little.