After they went out of the base, the obvious destruction outside was even more noticeable than inside the base, as they had only cleaned up the road enough for most of their cars to pass through but just left the buildings and other debris outside of it. So, a lot of buildings outside were destroyed again and in a worse state than before.
And when they drove far away from the base, the roads weren't clean and were still full of debris, so they had to clean it up again.
Because of the zombie birds and the following earthquakes, most of the people in the base hadn't gone out, as they had been busy cleaning their things as well as their buildings.
Those who went out also probably didn't go very far from the base because they only had their first hour of driving very easy, as it wasn't blocked with anything and was without a lot of zombies, so they managed to drive and covered a lot of distance in just an hour.
