I don't know why this occurred to me, except that when I'm laying in bed waiting to fall asleep my mind has a tendency to wander in unusual directions. I realized that while my dad's three elder sisters always referred to their parents as "Papa" and "Mama" ("Papa taught me to drive" or "Mama painted that vase"), my father always called them "Dad" and "Mom"--or, actually "My Dad" and "My Mom" ("My Dad took me fishing there" or "My Mom scolded me for that").
I wonder if the decade between my dad and his next-oldest sister was more significant than I thought...
My brother has some advice for drivers. I suspect he has been stuck behind one too many snails on the road...