The FAA don't really care where you land as long as you don't do something naughty passing through controlled airspace on the way.
It then depends on who owns the land. Places like National Parks are a big no no because the Feds say so. For roads and the likes, it depends on the State. Lets say you make a precautionary landing on some highway and determine you can safely depart. In some states you can get a friendly cop to close the road and take off, in other states you need to crate the plane up and put it on a truck.
On private land, I think you can do whatever the land owner lets you. I guess zoning might play a part if you want to build an airstrip.
Well, that's my understanding of it.