So I have this theory about figuring out who wants to drive when more than one persons in your group has a car and is able to drive. Here goes if you can follow...
When walking to the car if a person wants to drive they will say before you get to the cars, "Hey, I'll drive."
If they do not want to drive, they will say, "Hey, whose driving?"
I never have to guess or ask if I am driving because people give away there preference without me asking.
Works like a champ everytime.