How do trade routes/roads/railroads/harbours work? (vanilla)

Introduction
Should one go by horse, carriage, train or ship?

This article goes through the basic mechanics of trade routes, roads, railroads and harbours. By virtue of explanation, some obvious strategies and clearly optimal paths begin to emerge and these are lightly explored throughout. The focus, however, is most definitely on the economic issues involved. There is some math to be had but this can be avoided if desired; the ultimate conclusion from the math is given in the basic rules of thumb that will, in the majority of the time, work.

• Key concepts
• Trade routes
• Connection Maintenance
• Effects of the Capital on Trade Routes
• Miscellaneous trade route information
• Roads
• Railroads
• Harbours
• Conclusion

Key Concepts

• Connected city: A city connected to the capital.
• Trade route: The link between a city and the capital which produces income.
• Production bonus: The 25% boost in production a city receives from having a railroad link to the capital.
• Local road/railroad network: The process of linking cities with roads/railroads to a nearby coastal city with a harbour and having this city connecting to the capital with a harbour (or through another coastal city with a harbour that is linked by road/railroad to the capital). The screen shots make this clearer.
• Trade Network: The combination of all the roads and harbours connecting your cities. If a city has a trade route, it is part of the trade network.
• gpt: gold per turn.

Trade routes
There's math here - consider yourself warned.

A trade route is a link between a city and the capital city. This link generates gold according to a set formula and, with the advent of railroads generates a production bonus under certain circumstances. The amount of trade income your cities are generating can be found by leaving the cursor idle over the gold balance along the top of the screen, although this will not factor in any of the infrastructure required to establish these routes. Tile improvements is the amount of roads/railroads in the empire (or maintained by your empire) that are costing gold and that could be providing trade route links but are not necessarily doing so. Also, if a city is connected by a harbour, this will not be shown here either and is instead found under a generic building maintenance most of which will not be of any use in calculating net trade route income.

To find the net trade income of a connected city, some math must be done. A basic rule of thumb is that a trade route from one city to the capital will become profitable provided it has one population for every tile of road connecting it to the trade network. For harbours, this is actually four population. There are a whole list of caveats to this but a city of at least four population should be able to sustain a short trade route to connect it to the capital and make a profit or be near to doing so.

As you can see in the screen shot to the right the formula for calculating trade route income is mislabelled in game. It is missing the critical capital component of the trade route formula. The exact formula for calculating trade route income is as follows:

1.1 / in the connected city + 0.15 / in the capital city - 1

To take a quick example of how that rule of thumb will work out:
A four population city connected to a 10 population capital.

4 * 1.1 + 10 * 0.15 - 1 = 4.9

Of course, this is just the income side of the equation. To find your net income, you need to remove the cost of the Harbour, or the road tiles, from the income. In a normal case, a harbour or three road tiles, you subtract 3g and get a 1.9 income from this trade route.

A more developed example, perhaps, might better illustrate the power of trade income. For the following examples, let's assume the tall and 'semi-wide' empires are of equal population. If we assume a tall empire of four cities with the capital of pop 40 and three cities of pop 20, then we can have a semi-wide empire of eight 10 pop cities and a 20 pop capital.

Tall empire

22 (20*1.1) + 6 (40*0.15) -1 = 27 gold/turn * 3 (number of connections) = 81gpt

Semi-Wide empire

11 (10*1.1) + 3 (20*0.15) -1 = 13 gold/turn * 8 (number of connections) = 104 gpt

From the above examples, it would appear that more cities equals more income. This is generally true when the populations of the empires are equal. Given that each citizen in non-capital cities generate another 1.1g for the trade route, while the capital adds 0.15g/trade route, population in other cities will generate more income for the empire than additional capital population, but only when you have a small empire.

Let us take a look at the above examples again. Let's cut each capitals population in half, and add a new city:

tall:
22 (20*1.1) + 3 (20*0.15) -1 = 24 gold/turn * 4 (number of connections) = 96 gpt

semi wide:
11 (10*1.1) + 1.5 (10*0.15) -1 = 11.5 gold/turn * 9 (number of connections) = 103.5 gpt

With the tall empire, it's obvious that another city will give you more gpt to spend. (+15gpt), but as you can see, the trade route income for the semi-wide empire has actually dropped! Worse yet, the maintenance costs of the trade network went up, so the actual net income went down by adding another city (see Connection Maintenance for more on this).

Wide empire

Taken to a late game extreme, a wider empire with capital of pop 10 and 19 cities of pop 10 will generate:

11 (10 * 1.1) + 1.5 (10 * 0.15) -1 = 11.5 gold/turn * 19 (number of cities) = 218.5 gpt

A wide empire generates significantly more gold from trade networks than a tall empire. Given that it's easier to add more 10 pop cities than it is to add 10 population to a 30 pop city, a wide empire can have double the population of a tall empire, and therefore the 1.1/citizen in other cities will be more beneficial, even if it's less optimal (trade route income wise) to do so.

Tipping point

Eight trade route connections is the 'sweet spot' of trade route income. If you have less cities that the nine required for this point, it's better to expand to gain more gpt from trade routes. If you have more than eight cities, it's actually better to add new population to your capital than it is to add that population to existing cities (going from 10 to 20 pop in your capital is greater than going from 10 to 11 pop in 10 cities).

See below for more on this.

Connection maintenance
Sorry, no free lunch here either.

A trade route is not maintained free of charge. A harbour has an upkeep cost of 3 gold/turn for every city that has one (including the capital). A road has an upkeep cost of 1 gold/turn for every hex it occupies and a railroad has an upkeep cost of 2 gold/turn for every hex it occupies. This can rapidly add up. The game reveals some of these costs, but they are disjointed. In the screen shot to the right you can see tile maintenance, which is the total cost of all roads and railroads in your empire, and building maintenance, which includes all of your harbours (but you can't pull the harbour costs out of the global building maintenance costs).

Let's return to our initial example of a four population city connecting to a 10 population capital. That 4.9 gpt profit has to pay for a harbour (if used for the connection) in the city the trade route is coming from, which costs 3 a turn. This means it will only generate 1.9 in profit a turn. If it is connected by a road rather than a harbour, then its 4.9 gpt must pay for X hexes (where X is the number of road hexes between the capital and the city) * 1 gold/turn. A five hex road would put it into negative gpt.

The final calculation for trade routes, factoring in maintenance costs, reads as follows:

1.1 / in the connected city + 0.15 / in the capital city - 1 - trade route maintenance

Whether trade route maintenance is 1) a harbour (3 gpt cost), 2) roads (# of road hexes = gpt cost), or 3) railroads (# of railroad hexes * 2 = gpt cost).

In addition, if a harbour is used then the capital must also have a harbour which is another 3 gpt cost. This, however, is only paid once, so the more cities you connect by harbour, the lower the additional cost will affect your income.

The more developed calculation now looks something like this:
(using a harbour or 3 tile connection for simplicity)

Tall empire

22 (20 * 1.1) + 6 (40 * 0.15) -1 - 3 = 24 gold/turn * 3 (number of cities) = 72 gpt

with a harbour in the capital: 69 gpt

Semi-Wide empire

11 (10*1.1) + 3 (20*0.15) -1 - 3 = 10 gold/turn * 8 (number of connections) = 80 gpt

with a harbour in the capital: 77 gpt

So as you can see, the difference in net trade route income between the tall and semi-wide empires has dropped from 23 gpt to just 8 gpt. This is due to the extra maintenance costs required for more trade routes.

Wide empire
Let's double check the numbers for a wide empire. We'll also assume that you can't drop 20 cities neatly 3 tiles apart, so we'll estimate an average of 3.5 gpt for the road network/harbours.

11 (10 * 1.1) + 1.5 (10 * 0.15) -1 - 3.5 = 8 gold/turn * 19 (number of cities) = 152 gpt

While a wide empire will still generate more trade route income due to having more cities, it is no longer more than double the trade route income for double the population and trade routes.

Effects of the Capital on Trade Route Income

What might not be immediately obvious is that the more cities you build, the more income your capital city will provide in terms of trade routes irrespective of its size. In these examples, the capitals are generating 6 gpt/connection (40 pop tall), 3 gpt/connection (20 pop semi-wide) and 1.5 gpt/connection (10 pop wide) of the total trade route income.

In the trade route calculations, this is the disparity that provides the insight into how best to maximize your trade route income. The tall empire's capital has quadruple the population of the wide empire, and the difference in income each provides is relatively large (4.5 gpt/trade route). If you had a 40 population capital in the wide empire, that would lead to the capital providing another 85.5gpt alone. Given that each trade route in the wide empire is only worth 8 gpt, you could drop 10 cities from the wide empire and still have more net trade route income. This is another 30 population for your capital vs. 100 population from those 10 cities! Of course, there are other reasons to go wide, but from a trade route income perspective, a high population capital is very desirable as compared to a new city.

The tipping point, as already stated, is at eight trade routes. Once you hit that mark, population in your capital is more valuable than population in your other cities:

0.15 * 8 connections = 1.2 gpt which is greater than the extra 1.1 gpt you would get from a single population in another city.

adding a new 10 pop city (18 connections to 19 connections) for a wide empire only adds 8 gpt. Instead, you could add 8 population to eight of your other cities and gain 8.8 gpt. Or you could add 3 population to your capital and gain 18 * 0.15 * 3 = 8.1 gpt. Of course, happiness is a limiting factor here, so the 2nd option is more likely than the 3rd option, unless you have excess global happiness.

Another interesting point is that a tall capital can better sustain uneconomical trade routes. A new 1 pop city added to the above examples would have the following math.

Tall empire: 1.1 + 6 - 1 - 3 = 3.1 gpt

Semi-wide empire: 1.1 + 3 - 1 - 3 = 0.1 gpt

Wide empire: 1.1 + 1.5 - 1 - 3 = -1.4 gpt

As you can see the taller the capital the more income is generated for a new one population city. This means that a tall empire which starts expanding can hook up the trade route far sooner than a wider empire. What effectively happens is that the more an empire is developed in one extreme (be it tall or wide), the greater the return exists in developing in the direction that has been neglected. The wide empire is less effective when developing a new trade route, rather than its capital, while the tall empire is more effective developing a new trade route, rather than it's capital. The semi-wide empire, having taken the middle ground, has little to gain from developing a new trade route but can still gain substantially from developing the capital and its other cities.

Miscellaneous trade route information
Sorry, no free lunch here either.

Meritocracy:
The Wide empire's answer to a Tall empires high pop capital.

Meritocracy is a social policy in the Liberty tree which gives +1 for each city connected to the capital. This can, indirectly, increase the potential profit of a trade route as the trade route now provides happiness, which translates to one more population/city. For a wide empire following Liberty, this may make it more economical to connect a city earlier than the trade route income would otherwise suggest.

Cutting a trade route:
You can't stop the signal... well actually, you kinda can.

Unfortunately, trade routes (and the production/happiness bonus that follows the trade route) can be blocked.

A road or rail link must be continuous for it to function, so severing any single hex along the route (say if an enemy unit pillage the tile) will prevent a trade route from being formed through that line. If there is another way of threading the trade route (by another continuous road or railroad) then it will link that way, but otherwise you will be notified that your trade route has been broken.

A road/railroad route can be cut by pillaging the tile or if ownership of any tile on continuous road/railroad hex link changes to a Civilization with which you do not have open borders. A harbour trade route can be blocked if an enemy ship prevents the harbour from establishing a single continuous line over water from the city to the capital (or a coastal city with a harbour connected to the capital). Unlike roads and railroads, the harbour's trade route cannot be destroyed, only blocked. Once the enemy unit has passed through or is defeated the trade route will resume.

Viewing your empire's trade routes
This is never as cool at it sounds.
The strategic overlay map has an option through which you can view your empire's trade routes. Although it does not show those that exist over water, only those that are on land. To see this, open up the strategic map (the small button to the left of the mini-map on the bottom) and then open up the strategic map options (the small button immediately above it). The very top option is a drop-down menu, and here you will find an option called Trade Routes, activating this will show the trade routes in your empire.

Trade routes from a harbour to another harbour can be difficult to understand as they do not follow a set path likes roads and railroads do. The crux of the issue is that the harbour must be able to connect to another harbour across any number of water tiles. It will travel the entirety of the map if it has to, but it must connect to another harbour either in the capital or from there to a landlocked capital. It must also do so unimpeded, meaning that it cannot cross hostile territory or pass through a naval unit's zone of control. This makes them difficult to block if you have naval supremacy, but not impossible, as the screenshots in the spoilers below show. China has effectively blocked Orleans' harbour trade route and the trade route must transfer through Thebes before it can reach London.








Analysis of Roads
Not all roads lead to Rome, but most do lead home.

Roads are built by workers and are one of the three ways in which cities can be connected to the capital. They also double the movement rate of any unit using the road and cost one a turn to maintain. There are, however, some alterations to these base facts as the game progresses. Researching Engineering will remove the penalty units face for crossing rivers, which still applies to pre-Engineering roads. Researching Machinery will increase the movement rate to triple that of the base unit, rather than the double pre-Machinery roads grant. Finally, unlocking the Trade Unions social policy from the Commerce tree will decrease the upkeep of roads to 0.66/tile.

In general, roads are efficient connections when the cities are tightly packed (3 tiles apart). As soon as you start settling cities 4 or more tiles away from the trade network (any city or road tile already connected to the capital), roads are less efficient than harbours. This can't be helped in most cases, but when possible, use of harbours will help more than roads (as long as the 3 gpt from the capitals harbour is equal to the excess roads needed).

The military benefits from roads should not be overlooked. The advantage in doubling and tripling movement, as well as removing the movement penalty from crossing a river can rapidly increase the speed with which an army can respond to an attack. A single road improvement to provide a river crossing point post-Engineering should be considered to avoid the river crossing movement penalty. In addition, if your cities occupy rough terrain then roads may be the only way in which your units have any hope of moving at any speed.

A note here should be made about the naval Civilizations of England and Denmark. Their increased embarked movement rate can mean that roads can be postponed, possibly even indefinitely. While this does restrict their movement somewhat, and places an increased premium on dominance of local waters at the least, it can save a tidy sum of gold per turn. See their respective guides for more details. The Roman unique unit, the Legion, also deserves a mention. It has the ability to create roads just as a worker can. Having legions develop a road network from which to funnel units along can be an attractive option during the build up to a large offensive while still allowing your workers to continue vital tile improvements. These roads, however, should be pillaged after use to save on road maintenance, see the Rome Civilization guide for more details.

The mechanics of road ownership need a small explanation, especially if your road maintenance is unexpectedly high (or low). If a road is in your territory, you pay its upkeep. If a road is in another civs territory, they pay for its upkeep. It does not matter who built the road, only whose territory it is in. If a road exists outside of anyone's territory then the builder of that road pays its upkeep.

One minor point is that on Warlord the cost of a road is reduced to 0.75 gold a tile, on Chieftain to 0.5 gold a tile and on Settler to 0.33 gold a tile.

Reducing road costs

There is a certain type of settlement strategy that can reduce road maintenance costs by taking advantage of the ability to "reuse" existing roads for new trade routes. By placing cities within close proximity to another city the number of roads you have to build and maintain can be reduced substantially.

A "Y" pattern of city placement can be achieved through careful management and allow three cities to be connected with a total of four roads. More likely though, you'll be able to 'share' a road connection with another city, therefore requiring five total roads to connect two cities to the trade network. This drops the cost of each connection to 2.5 gpt, instead of 3 gpt. A wide empire that can save 0.5 to 1 gpt on each trade route is going to stack up a lot of extra gpt.



Analysis of Railroads
War by timetable.

Railroads are also built by workers and can form trade routes from cities to the capital. More importantly though, they increase the production of any city fully connected by a railroad to a capital (or through a harbour) by 25%. On top of this, the speed of a unit travelling on a railroad is multiplied by ten. The price for a single railroad hex, however, is two gold per turn. No technologies affect any of these advantages, but as with roads, Trade Unions reduces upkeep by a third, to 1.44 gold a turn.

In order to gain the +25% production bonus a complete rail link must be present between the city and the capital. It can, however, utilize harbours just as roads do to carry the trade route. This means that all coastal cities, assuming the capital is also a coastal city with a harbour, gain a +25% production bonus once the Railroad technology has been discovered. A road connecting a landlocked capital to a coastal city with a harbour is not sufficient to transfer this bonus. Once a railroad is built connecting the capital to this coastal city (with a harbour) though, the bonus applies to all coastal cities with a harbour and to all cities that have a railroad link to a coastal city with a harbour.

Although railroads do have a high upkeep cost, the benefit from the +25% production often makes it worth the cost. Obviously, however, adding +25% to a city that has no base production value is a waste of a railroad. Once again a harbour and a localized railroad network to landlocked cities is the more cost efficient way in which to ensure cities have access to the +25% bonus. Notably, the railroad does not grant the +25% production boost to the capital.

Railroads have significantly more military potential than roads, as the speed increase is sufficiently large that even the embarked water units of naval powers with Naval Tradition and the Great Lighthouse will be unable to match the speed of a unit moving on a railroad. Again, however, the cost upkeep can be quite high for the construction of railroads solely for military purposes. This can be offset with Commerce's Trade Unions, which reduce railroads costs fairly significantly (effectively giving you one free hex of railroad maintenance for every two maintained), but it is still a significant investment.

Railroads follow the same mechanics for ownership as roads, and the difficulty level scales in the same manner. A railroad on Warlord will cost 1.5 gold a tile, Chieftain 1 gold a tile and Settler 0.66 gold a tile.

Analysis of Harbours
Harbour n. A place where ships shelter from storms and are exposed to the fury of customs.

Harbours are constructed in cities and cost 180 production to build and 3 gold per turn to maintain (available at Compass). They form trade routes between cities, and also transfer the production boost that cities can get post-Railroad technology. They also give +1 per sea resource in the city radius. They can be boosted with Commerce's Trade Unions, increasing gold output by 1 gold per turn.

Harbours form a trade route over water to the capital, and can utilize harbours in other cities to do so. A harbour effectively views every water tile as a road tile, including those water tiles covered by ice, for the purposes of connecting cities to the capital. Given that they only cost three gold per turn to maintain (less with Trade Unions because of the gold output boost, a boost which benefits from things like markets and banks reducing maintenance even further), they are the most cost-effective means of creating trade routes between cities and the capital, if you use a lot of them. For a single harbour trade route, you pay 6 gpt (2 harbours). So you have to ensure that the 6 gpt cost of those two harbours is less than the road cost to connect the cities. If you have sea based resources in both cities, then it can be worth the extra cost anyways.

The trade routes formed between harbours, however, can be cut. An enemy naval unit can break a trade route between a city and the capital if its zone of control (one hex in a circle around the unit) blocks the continuous (invisible) trade route line between two harbours. In a similar manner, a trade route cannot pass through enemy (or even neutral) territory. So a city surrounded by water owned by a Civilization with which you do not have open borders, will be unable to create a trade route between it and the capital by way of the harbour. This makes control of coastal waterways and choke points very important if many of your trade routes rely upon harbours. The good news is that harbours will use any possible water connection to make the trade route, even if it has to go 100 tiles in the opposite direction to do so.

Mixed Trade Routes
Some cities, however, will not have access to a harbour and quite possibly the capital will be unable to build a harbour. This does not mean that a road network is the only option available through which cities can be connected to the capital.

Connecting the capital to a city on the coast with a harbour will allow other coastal cities with a harbour to connect to the capital. Their trade routes are routed through the coastal city to the capital. A combination of roads linking landlocked cities to coastal cities, and utilizing harbours for trade routes is an effective and efficient manner in which cities can be connected to the capital when you have an empire that is spread all over the map, or on continent/island maps. A common example of this situation is on a pangea map where you take cities from another civilization which happens to be eight or more tiles away from your borders. If you aren't planning on settling the middle area, then connecting trade routes via harbours is far more effective than dropping an eight tile road.

Conclusion
The roads may be clogged, the trains run late and the ships slow but it beats walking!

There are a few clear conclusions to be made from the mechanics of trade routes, roads, railroads and harbours. The following is a brief list of the most critical points in understanding how the trade routes/roads/railroads/harbour mechanics work.

• Harbours also provide the railroad production bonus of +25%.
  
  
• Trade routes (and the production bonus) can route through a harbour in another city, even multiple cities to reach the capital.
  
  
• Roads cost 1 gold/hex, Railroads 2 gold/hex and harbours 3 gold/city. Ownership of roads and railroads is dependent on tile ownership or (if none) the builder of the road/rail.
  
  
• Roads increase movement x2 (x3 with Machinery). Railroads increase movement x10.
  
  
• Trade unions (Commerce tree) reduces the gold per turn cost of roads, railroads and harbours by 33%.
  
  
• As a rule of thumb, a city should be connected with a harbour if it is five or more hexes away from the nearest city/capital. If it is fewer, then a road is preferred.
  
  
• As a rule of thumb, if you use one harbour trade route, you'll need two more to help overcome the cost of the capitals harbour.
  
  
• As a rule of thumb, a city needs at least four population before it becomes profitable to put a harbour there and create a trade route.
  
  
• As a rule of thumb, a city needs one population plus one for every hex tile of road between it and the capital before it becomes profitable. Until your capital reaches size 7, at which point one population for every hex tile of road is needed.
  
  
• As a rule of thumb, a city needs two population for every hex tile of railroad between it and the capital before it becomes profitable. This may be ignored if the paramount concern is increasing the production capability of the city in question.

Patch version of this article: 1.0.1.383