Trade Routes

Trade Routes allow a city to trade with another connected city, generating Dust and  Science income.

Creating Trade Routes
The first step in creating trade routes is to research Imperial Highways or Cargo Docks. These technologies allow the Right of Way or Cargo Docks improvements to be built, which allow cities to be connected by land or by sea. Two cities are connected by land if they are in adjacent regions and have both built the Right of Way improvement. Two cities are connected by sea if they have both built the Cargo Docks improvement in the same body of water. Note that this does not require the Docks to be built adjacent to the same named Oceanic region. Additionally, if two cities are connected by land or by sea to a third city, then they are also connected themselves. This means that two cities may be connected via a complicated series of alternating land and sea routes passing through multiple cities.

Note that only cities within your empire and friendly cities (that is, cities belonging to empires that are either at peace or in an alliance) may be connected. Alternatively, if a city is governed by a Roving Clans hero with the "Black Marketeer" skill, then that city may be connected to any other valid city, regardless of diplomatic status.

Once two cities are connected, then trade routes are established between them. By default, each city may create one trade route, and each capital may create two trade routes due to the Palace, though this number can be increased by some city improvements. If a city is able to create multiple possible trade routes, then the one(s) that generate the most Dust and  Science will automatically be selected.

The city creating the trade route is called the origin, and the city which it connects to is called the destination. Empires only receive income from trade routes originating from its own cities, but not from trade routes originating in other empires and connecting to its cities. Additionally, most trade route bonuses apply only to the origin city or empire. The base Dust and  Science generated by a trade route is determined by the distance between the two cities and the population of the destination city. However, trade route bonuses generally only apply to the origin city; for example, having a Caravanserai in a city only improves the trade routes originating in that city, but not the trade routes having that city as a destination. See the Calculation section for more details.

You can check the status of all trade routes originating in your empire in the Empire Management Screen (default keybind F1).

Improving Trade Routes
Once trade routes have been established, there are a number of ways to increase a Trade Route's output, as well as the number of Trade Routes a city can have. All of these methods are listed below, but it should be noted that some of these methods have effects beyond what is described here - such as additional Upkeep costs from City Improvements.

Secondary Trade Routes
When a trade route passes through multiple cities between the origin and destination, all of the cities in between also receive bonus income based on the main trade route. This is called a Secondary Trade Route. While a city may be limited on how many primary trade routes it can create, it can have an unlimited amount of secondary trade routes passing through it. Secondary trade route income is determined only from the base income of the primary trade routes passing through a city. Additionally, none of the trade route modifiers affect secondary trade routes.

Calculation
The Dust and  Science yields for a primary trade route are computed using the following formulae:

 yield = (1 + 0.6 &times; (DestinationPopulation + 0.3 &times; TradeRouteDistance)) &times; (1 + TradeRouteBonus) &times; (1 + TradeRouteDustBonus)

yield = (1 + 0.6 &times; (DestinationPopulation + 0.3 &times; TradeRouteDistance)) &times; (1 + TradeRouteBonus) &times; (1 + TradeRouteScienceBonus)