We are different than other animals because we have the ability to reason as sapient beings and we all have the human capacity to act equally moral as judged in being equal in the eyes of the law.
It can and has to be objectively measured inorder for eminent domain to be justified.
It is not hard to define. It is called “owned in common as an individual equal right”.
You actually can by requiring an obligation to share the economic rent that results from exclusive use of land in a scarcity market because the landowner’s rights remain intact as they contribute no labor towards the creation of the economic rent. It by definition is unimproved land value.
Just because eminent domain requires it to be justified, does not mean it can be justified, or even measured. How is it to be measured?
Individuals who do not use and did not help create the road should have no ownership of it. So the owners are the creators of the road, a rather difficult thing to define. Otherwise, how can you define exclusive use? Indians and Chinese should not have any claim to ownership over a path made by Native Americans, for example. I agree there is a common right of way and all that, but that comes from the creator’s (and thus owner’s) use of the road. The right of way in England is different than the United States because of the customs of the people who homesteaded the roads. The roads are owned by many, many people, but are still owned (and I appologize if this is what you ment by “owned in common”).
Example: Robinson Curuso is living alone on a desert island, obvilious to everything else around him. He is discovered by a fisherman, and a Georgian governor travels to his island to collect rent due. How is this not coercive?