You would still tend to spam roads.
If you had extra worker time, they would build extra roads... as Backup
Sure, if you had extra worker time, but you'd still be less likely to end up with roads on every tile than in Civ4, particularly if you doubled or tripled the build time for roads. The benefits from building more roads would be very small. So the real problem is that there is nothing else/better for workers to do, and that their upkeep costs are trivial.
So maybe just increase worker upkeep cost, to make you better off by disbanding the workers if there is nothing valuable to build than by sending them on road building duty?
Maknig roads unpillageable as you imply would also tend to reduce any incentive to build more roads at all.
1. "roads" in a realism sense are part of Every developed tile, or every tile in your culture
Yes, this is fine, this is implied in my proposal.
There's something nice about having it in every developed tile, but then there's the possibility of needing to build wasted improvements in tiles that aren't in the BFH of any city in order to get the mvoement bonus.
2. "roads" in the sense of a major transport network have gameplay effects that prevent other gameplay effects like having the tile totally devoted to food production from taking place(especially if they 'connect' tiles that are adjacent to them.
This just makes no sense given the scales we're dealing with. A massive 16 lane superhighway is still maybe 50 meters wide, in a tile that represents many km.
Building a superhighway doesn't have any real impact on food production, or commerce, or industry.
I agree that these are the most obvious options:
1. Roads Replace other improvements
2. Roads worsen tile output (in some other way)
These just fail the basic logic test.
3 is possible, but the problem is if they can be blocked or pillaged I will build roads in every tile, particularly in vulnerable areas, if I have excess workers (which I probably will at various points)
I realize this problem, but I think its probably better to attack the issue of excess workers.
4. Roads have an actual significant cost besides worker time (it costs 20 gold to put a road on a tile
A small gold construction cost seems reasonable. 20 seems high, but its hard to know given we don't have a feel for the overall economy size.
Again, we need to be sure that the cure isn't worse than the problem. Roadspam really isn't that big a problem.