Some very good points.
First let me say that yes, 'little trucks' would be kind of cool (as long as they didn't clog up the map too much), especially if you could assign them to continuously re-supply specific units. But if you think the turns are long now just think how long they would be with thousands of supply trucks running around!
Your right, realism definitely has its limits.
The 'Supply Depot' idea is good (I think snowmelk mentioned something about this as well). Just like Air Bases essentially extend the range of air units by allowing them to operate further a field, Supply Depots would extend the range of ground units beyond the supply area provided by cities.
I figure that cities provide should automatically re-supply (i.e. you dont have to build a Supply Depot city improvement). That simplifies things and keeps with the air unit analogy. There should however be a Fuel Depot city improvement. This allows motorized units to re-supply. Supply Depots MUST be connected to a city with a Fuel Depot if it is to provide motorized unit with range. Likewise the supplying city MUST be connected to the required resource. Note: this should also apply to Air Bases so that Air units cant just re-supply there at no cost, thus allowing for an obvious exploit. (See below.)
The Fuel Depots maintenance represents the costs associated with transferring the resource to the supply area. Although
technically you could just connect all your Supply Depots to the only city with a Fuel Depot. The easiest way past this that I can think of is to limit the number of Supply Depots that are covered by a single resource.
Now that Im on the subject, there should be a limit to how many cities can use a single resource. The limit means that although you may connect the resource to a higher number of cities than the limit, only the first cities up to the limit will be able to use the resource. For example:
1 Oil resource can be used by the first 10 cities to be connected to it.
If the limit is 10 then an 11th city will not be able to use the resource even if it is connected to the resource by Road.
That means you will need a second Oil resource to add the 11th city.
The problem mentioned about overlapping range limits can be easily dealt with by having the Supply Limit Line (that would be the dotted line) warp to meet the units range and taking terrain into account when a particular unit is selected if no unit is selected, then the supply limit line doesnt appear at all. In other words, the same effect you get when you select an air unit only in the case of ground units, the line may be further away or closer based on the terrain movement cost.
The problem with that is what happens if you have Railroad? Then the limit line would appear all along the length of the Railroad (which could be half the map). Not that this would be a problem, but it wouldnt look as aesthetic as the air unit range limit line.
The simplest way would be to have the supply limit line appear exactly the same way it does for air units thus ignoring terrain movement cost. In other words, if a unit with Range=8 is selected the supply limit line will appear at 8 squares from the unit in every direction regardless of differences in terrain (i.e. just like it does for air units).
The only difference between the air unit range limit and the ground unit supply limit line is that the latter can mesh with the lines extending from other Cities/Supply Depots/Fortresses (i.e. just like Cultural Borders do when they touch).
This wouldnt be as realistic in that it doesnt take the benefits of Roads giving greater range or difficult terrain into account, but it really simplifies things a lot and makes the system more user-friendly. Just to make it a little realistic, I think Roads and Railroads should be immune to the Range limitations (i.e. a unit use RR to travel beyond the supply limit line but cannot move away from the Railroad unless it enters another supply area). This ensures that RRs dont become an exploit they are only used to move unit between supply areas. It also means that theres far less work for designers to modify the limit line system used for borders and air units it just means that the air unit range gets applied to land units, is dependent on cities and functions like borders, nothing more. If you dont mind, I think Ill go with this simpler version. It sells better, and I would really like to see supplies implemented into Civ3 at some point.
Ill go over the basics of this concept as envisioned thus far:
Units:
Ground units are limited to moving within the area set by the Range field in the Editors Units window. Units cannot move beyond the lines outlining this area.
When a unit with the Land/Sea domain is selected, its supply limits appear. Note: the limit lines appear extending from every City, Fortress and Supply Depot on the map simultaneously (a unit travelling along Road can move anywhere within those supply limits within a turn as long as the various lines are connected; if there is a gap, the unit cannot move to the other square unless Road connects them).
This is an example of Road connecting two supply areas:
000000000000000000
011111000000111110
011111000000111110
011C1U-->---11C110
011111000000111110
011111000000111110
000000000000000000
C = City/Fortress/Supply Depot
U = Unit (range = 2)
1 = Supply Area
0 = Beyond Supply area
-->-- = RR/Road
Some units (usually ships) have the ability to go beyond the supply area without the use of Railroads. If a unit goes beyond the supply area or the supplying city/fort./depot is cut off, the unit will gradually lose health until finally disbanding. In the case of motorized units, they will also lose movement. For instance, if a Foot unit is airdropped onto a square that is outside its supply area, it will begin to lose health from the moment it is dropped air units usually have a greater range than ground unit so this would be something you would want to keep an eye on.
Note: unit supply areas that overlap enemy supply areas will be unaffected --will not stop at enemy borders either.
Additionally, a unit without the ability to go beyond its supply area cannot attack an enemy unit that is outside the area even if the enemy unit is right next to the line. However, it can bombard the enemy unit.
Cities/Fortresses/Supply Depots:
A city always provides supply to units. A Fortress must be connected via Road to a city in order to extend the supply area. If the selected unit requires a fuel resource, the connected city must in turn be connected to the required resource. The only difference with the Supply Depot is that it can provide supply to non-motorized units without being connected to a city. Although a Supply Depot must be connected to a city with access to the required resource in order to provide supply to motorized units, it can continue to supply those units for a certain number of turns even after the resource has been cut off (i.e. stores resources). The Fuel Depot city improvement has the same effect (i.e. city functions as if resource were present for certain number of turns after city has been disconnected from resource).
If this concept were applied to Civ3, a neat addition would be to give the Guerrilla unit no range limitation thus imitating its independence on supplies (i.e. lives off the land). Would give the unit a special kind of uniqueness. Making the unit weaker would be required so as not to make Infantry redundant. (What would really be cool is having Civ2s Guerrilla Warfare effect in Civ3
but thats another story.)
Thanks snowmelk for bringing this system to my attention. Its simple and its highly applicable to the already existing Civ3 program. The best part is that the AI could be set to adapt to this far more easily than my more realistic system.