About 7 minutes
What drives the numbers
Every other reference page lists values. This one lists causes: which factors move demand, population and output, how much each one counts for, and — usually the part that matters — what shape each relationship has.
The three quantities are one loop
Demand, population and output are not independent systems. Each is an input to the next, which is why a change anywhere takes years to show its full size.
GDP
Rises with development, resources and how well the settlement is served. It sets the income band that everything else reads.
Population
Natural increase follows income; migration follows how a place compares with the national average.
Demand
Generated from the population and wealth at both ends, decayed by distance, lifted by the service you provide.
Serving demand raises a settlement’s service score, which raises output and attracts migration, which generates more demand. That is the whole engine — see How the economy fits together for it in prose.
Demand between two settlements
Demand is generated per ordered pair, separately for passengers and cargo, before any of your network is considered. What you build changes how much of it you capture — and, through the service term below, how much of it exists at all.
The factors
| Factor | Shape | Notes |
|---|---|---|
| Population at the origin | Sub-linear power | Doubling a city does not double the trips it generates. |
| Population at the destination | Sub-linear power, weaker than the origin | Origin size counts for slightly more than destination size. |
| Income (GDP per person) | Power curve, clamped both ends | Richer places travel more, but the effect is bounded — it cannot run away at either extreme. |
| Distance | Inverse power, relative to a reference distance | The dominant term at range. Cargo decays more slowly than passengers, so freight sustains long corridors that passengers will not. |
| Transport service at the destination | Linear, from a positive base | Even an unserved pair generates demand; good service adds to it rather than unlocking it. |
| Administrative status | Flat multiplier | National capital most, then province capitals, then regional centres. |
The administrative multipliers are the one set of figures published elsewhere, in What to aim at next, because they are small enough to steer by without being an optimisation target.
Why the pair list is finite
Every settlement keeps a bounded number of trading partners — a larger allowance for local pairs than for long-distance passenger pairs, and a smaller one again for cargo. Ordinary settlements share a further overall budget. Pairs beyond the budget are dropped rather than computed at negligible values, which keeps the model tractable and means very distant, very small pairs simply do not appear.
Population growth
A settlement’s population changes by two independent mechanisms each month. They behave completely differently, and confusing them is the usual reason a city does not grow as expected.
Natural increase follows a demographic transition
The base rate is decided by the settlement’s own GDP per person, in bands. The curve is not monotonic — it is the standard demographic transition:
High mortality holds the rate down. The poorest settlements grow slowest of all.
The fastest natural increase in the game happens on the way out of poverty, not at the top.
The rate falls with every band above the peak.
An ageing, wealthy nation barely grows on its own.
Two development fields adjust this: healthcare raises it slightly, and education lowers it slightly. Both effects are small next to the income band itself.
Migration is entirely relative
Net migration is not driven by how good a settlement is, but by how it compares with the national average. Every pull term is a difference from that average, clamped so no single factor can dominate:
| Pull factor | Measured against | Relative weight |
|---|---|---|
| Wealth | National average GDP per person | Strongest |
| Connectivity | National average transport service | Strong |
| Jobs (industry and commerce) | National average of the same fields | Strong |
| Passenger accessibility | National completed-trip rate | Moderate |
| Climate | A fixed comfortable range | Weakest |
Output and development
A settlement’s GDP is its population times a per-person figure, and that figure is a product of three factors on a common base. Their ranges are wildly unequal, and that inequality is the single most useful thing on this page:
| Factor | Shape | How much it can multiply output |
|---|---|---|
| Development score | Exponential in the score | By far the largest — tens of times over the full range |
| Transport service | Square-root — steep at first, flattening | Under twice, from nothing to perfect service |
| Natural resources | Linear in the resource score | Well under twice, and fixed at generation |
How development itself grows
Development is nine separate fields — agriculture, industry, commerce, tourism, healthcare, education, administration, logistics and military — each growing every month from a common base rate, shaped by four things:
| Modifier | Shape | Effect |
|---|---|---|
| Early-stage boost | Tapers to nothing at a low development score | Undeveloped settlements grow markedly faster; the boost is gone well before mid-game. |
| Headroom to 100 | Diminishing as the field approaches its ceiling | The final points are genuinely slow. A maxed field stops contributing growth. |
| Field-specific resources | Logarithmic in the resource value | Each field is lifted by its own resources — minerals and energy for industry, human capital for education, and so on. |
| Accessibility | Linear exposure, per field | Unmet passenger demand holds back the passenger-facing fields; unmet cargo demand holds back the freight-facing ones; commerce is exposed to both. |
National effects multiply on top: the development-speed rewards from economy milestones, active policies, and the city completeness feedback described in Land, allowance and no construction fund.
The National Income band
The wording shown in the HUD — Least developed through High income — is decided by national GDP per capita alone. It is not the development score, and a nation can hold a high development score while sitting in a low income band.
The band sets your loan interest rate, your government grant rate and the income-sensitivity of several costs and recommended charges. The exact thresholds and rates are tabulated in Transport mode parameters.
Practical readings
- Short strong pairs beat long strong pairs. Distance enters demand as an inverse power; it enters cost roughly linearly.
- Freight tolerates distance that passengers will not. On a long corridor, check the cargo figure before dismissing it.
- The first connection is worth more than the fourth. The service term is a flattening curve.
- Improving everything equally moves no one. Migration reads differences from the national average.
- Judge investments over decades. The exponential term is development, and development is slow.
Related reading
- How the economy fits together — the same loop in prose.
- Travel demand, traffic and capacity — how generated demand becomes routed traffic.
- GDP, population and development — the concept article behind this page.
- Transport mode parameters — the exact values this page deliberately omits.