Each city may have one or more trade routes which give it extra commerce.
Assignment of Trade Routes
Each turn, trade routes are automatically computed by the game. All cities which a city has trade connections to are considered, with the better ones (foreign, intercontinental) being assigned to the bigger cities. A civilization gets no more than one trade route assigned to each foreign city. All other trade routes must be domestic, and there is no limit on the number of domestic trade routes that can go to any particular city.
In spite of their name, "trade" routes are not two-way, but one way. The city with a trade route gets benefit from its partner, but there is not necessarily any benefit the other way. In particular, it is possible to trade with a city that cannot trade back; this is fairly common for the first civilization to get to Astronomy. You can also blockade a civilization using privateers so that it cannot trade "out" while your trade still comes in.
Seeing Trade Routes
A city's trade routes can be seen in the city screen, on the left. They are labeled using the partner city. You can mouse over each trade route to get a tooltip, which will show how its value is computed.
Number of Trade Routes
Each city has a maximum number of trade routes, which is determined as follows:
- the base number is 1 trade route
- +1 if the city has a cothon (unique building of Carthage)
- +1 if the civilization knows the technology Currency
- +1 if the city has a castle, and the civ does not know Economics
- +1 if the civilization is running the Free Market civic
- +1 if the civilization knows the technology Corporation
- +2 if the civilization also owns the Great Lighthouse, and it has not been obsoleted by Corporation
- +1 if the city has an airport
- +1 if the United Nations resolution "Single Currency" is in effect
Usually every city will get its maximum number of routes. However, a city may not get its maximum number of routes if it does not have trade connections with at least that many cities. In particular, especially early in the game a city may have no trade routes because it is not connected to any other cities.
Value of Trade Routes
The commerce value of trade routes is computed from a base value, which then gets percentage modifiers. Trade routes values are computed down to .01 commerce, so that rounding effects will be minimal.
The base value in commerce of a trade route in city From to a partner city To on a map of size mapsize is computed as follows:
base_value = max(1.0, min(pop(To)/10, dist_factor(mapsize)*plotDistance(From, To))
where:
- min and max are the normal math minimum and maximum functions
- pop(X) is the population of city X
- plotDistance(X,Y) is an approximation of the distance in tiles from city X to city Y
- dist_factor(X) is 16%/14%/12%/10%/8%/6% for duel/tiny/small/normal/large/huge.
Put in English, what that equation encodes is as follows:
- the base value of trade routes is always at least 1
- you get more if the other city is large (population 11+), and distant
The base value is then multiplied by a factor that has a base of 100%, with the following additions:
- +5% for each point of population of the From city over 10
- +25% if the city From is connected to its capital
- +50% if the city From has a harbor
- +100% if the city From has the Temple of Artemis
- +100% if the city To is on another continent
- for foreign To city, +100% if it is on another continent and the From city has a customs house
- for foreign To city, +3% per turn (up to a max of 150%) since the last war between civs owning To and From
Vassal cities count as foreign for this purpose.