There are plenty of area that could support mass transit, especially in cities.

Even suburbia can be made more walkable.

For example, walking to my local gas station isn't really that practical due to a lack of continuous sidewalks. It isn't that far but the path to the gas station can get pretty muddy at time.

I once tried to get food at a fast food joint by walking, the lack of sidewalks is even more acute. Crossing several lanes of road felt pretty unsafe even when the traffic light is red.