I’m just remembering a line from the 3rd or 4th episode, where the military man in charge said “we’re target free, 6 miles out”. That’s a pretty big perimeter, when you think about it. Let’s call the “safe zone” a 2 x 2 mile block (it’s bigger, but meh). That means the total cleared area is 14 x 14 miles. That’s 196 square miles of zombie free territory. I assume the army also cleared out the abandoned cars from the main roads in the same area to make their patrols easier and to eliminate areas a walker could be hiding.
I’m going to give them the benefit of the doubt and say that, when they left the military complex, they took the roads that had been cleared by the military, which is why there were only a few scattered walkers and almost no abandoned cars.