Hi Halfstack...
I've been thinking about this as well as I'm currently determining if I should go drive through only, standard walk in, or possibly a mobile truck.
For convenience sake, if I go for a walk in, I am considering a 'fast lane' type set up. I've seen places that have rewards cards that you can load up and do a pre-paid type setup. This would allow for customers to pay rapidly and load up the card on-line. You could even set up a special counter for these people so they won't have to wait too long.
Good luck...