There is plenty of land that could be used for well-managed forest gardens. Bali is an example of a modern economy where people can forage a lot of their calories that way.

It would be real nice if our neighborhoods and streets were lined with edible plants. It seems like it could/would be a lot of work to maintain, but with the appropriate tools and planning it might be pretty simple. Non-edible (as far as I know) street trees are already watered by hand (with water trucks) here and they only seem to be used for decoration and shade.

edit: one issue to be careful of is the plants collecting "pollution"