>build all major roads in tunnels, and bring the recovered surface area back to it's original state for all to enjoy.

How deeply have you thought this idea through? What happens in places like Florida where the water table is so high you can't dig deeper than a few feet? How do you manage entrances and exits, how do you make sure there's breathable air down there in case people have to get out of their cars? How do you vent poisonous gases? How do you keep the precious animals you're trying to save from getting in there and getting trapped or run over? What about emergency access? You can't open a gate on a service road and suddenly be underground.

Those are just a couple of simple practical considerations that don't even factor in cost, which would be prohibitive.