There wasn't one. It was a maze of twisty, tiny two-lane roads, all alike.
The vast majority of the routes would eventually lead us through Carbondale where my dad (and much later I) went to college at Southern Illinois University. On Illinois 13, there was the Family Fun Drive-In Restaurant. My mom really liked this place, possibly because we were finally far enough south to be able to find a BBQ pork sandwich with the cole slaw piled on top of the meat in the Southern style. (That's easier to find now that barbecue has become a staple in most towns.)
We kids hated the place. I'm not exactly sure I remember why. It's possible that it was simply because it wasn't McDonald's. And the result of this was that the vast majority of the meals that we ate in Carbondale while traveling were consumed in the car sitting outside of the tiny original-style McDonald's, as opposed to the Family Fun Drive-In which had food my mom liked and indoor seating.
I'm sorry, Mom.