Travel demand, traffic and capacity

The one-minute explanation

Settlements generate passenger and freight demand according to their population, economy, resources, development and distance from one another. The network then tries to connect each origin and destination through available routes.

Travellers and freight do not divide evenly across every link. Faster and cheaper routes attract more traffic. Tolls add to the perceived cost. Rail receives a preference advantage. If a route cannot accept all the assigned movement, some demand can move to another available route; movement with no usable capacity remains unserved.

Demand is not traffic

Demand is the desire to move between two settlements. It can exist before any connection has been built. Traffic is movement assigned to a particular route or link. Served demand is the share the available network can actually accommodate.

Where demand comes from

The game calculates demand separately for passengers and cargo. Both respond to the strength of the settlements at each end, but they represent different reasons for movement.

Passenger demand

Larger settlements tend to generate more trips, but demand does not grow one-for-one with population. This prevents the largest cities from overwhelming every other relationship. Economic strength increases the amount of travel, while distance reduces it.

Demand is directional. A large regional centre can generate a different number of outbound trips to a smaller city than the smaller city generates in return. The model therefore keeps both directions even though some displays summarize the pair.

Freight demand

Freight is shaped by the relationship between production and consumption. Resource-rich settlements create stronger outbound cargo potential, while developed settlements attract inputs and distribute finished goods. Minerals, energy and agricultural resources can strengthen particular flows.

Freight declines with distance more slowly than passenger demand. A distant resource centre may therefore remain an important cargo partner even when passenger demand between the same places is modest.

Distance, economy and policies

Distance weakens demand rather than imposing a hard maximum. GDP raises the scale of movement, but its effect is bounded. Policies assigned to a settlement can also increase or decrease the passenger or cargo demand originating there.

To keep a large world manageable, each settlement retains only its strongest outbound relationships. Very small, very distant pairs may be omitted when their expected movement is negligible. A visible straight-line relationship between two places therefore does not guarantee that it is one of the demand pairs being tracked.

How demand finds a route

For roads and railways, the network first looks for ways to connect the origin and destination. A journey may use a direct link, a continuous named corridor or an indirect path through intermediate settlements. This is why traffic between A and C can load the A–B and B–C links even when B is not the journey’s destination.

When several surface routes can serve the same pair, they compete through a generalized cost:

A quicker route is more attractive. A toll makes a route less attractive, but the effect depends on travellers’ value of time: wealthier endpoints are generally less sensitive to paying for time savings. Rail receives an additional preference advantage when it competes with road.

Route choice is therefore relative. Improving one corridor can draw traffic away from another even when total demand between the settlements has barely changed.

Diversion is different from suppression

A charge can produce two different responses:

1. Diversion: movement shifts to a cheaper available route.
2. Suppression: some movement is no longer made because every useful option is charged too heavily for the time it saves.

If a free surface path exists, the model can redirect demand rather than suppressing it. When all useful options are tolled, the cheapest generalized-cost option determines how much of the origin–destination demand remains. Suppression is applied once to the pair, so adding several parallel tolled links does not repeatedly erase the same demand.

This creates the basic toll trade-off: a higher rate earns more per vehicle but can reduce the number of vehicles willing to use the charged network.

What capacity means

Road and rail capacity is stored as a one-direction design-hour capacity. The game converts that rate into a monthly, peak-equivalent allowance before comparing it with assigned flow. It is not the theoretical number of vehicles that could pass if the link ran at full design capacity for every hour of the month.

Passenger and cargo movement also consume capacity differently. Cargo is converted into passenger-car-equivalent units, with each cargo unit occupying more capacity than a passenger movement.

These quantities should not be expected to match. A highly directional commuter corridor may be close to capacity during its busy direction while its billable total includes journeys in both directions.

Overflow and unserved demand

When parallel direct routes or continuous corridors compete for the same demand, the assignment first follows route attractiveness. If one route exceeds its available capacity, excess passenger and cargo demand is redistributed toward alternatives that still have room.

Redistribution cannot create capacity. If every practical option is full—or if no connection exists—some demand remains unserved. Adding capacity at the wrong place may not solve the problem: every segment of a corridor has to support the movement passing through it, and the narrowest or slowest part can remain the constraint.

Rail does not use road-style congestion in the same way. Its carried load is capped at capacity rather than being shown as indefinitely accumulating on the railway. Excess movement is served only where the route assignment can place it on another available choice.

How congestion develops

Road congestion appears when directional peak-equivalent traffic becomes large relative to capacity. As saturation rises, travel time increases. The slower time then raises the route’s generalized cost, making alternatives more competitive in later assignments.

The published route load is adjusted progressively toward the current assignment rather than snapping completely between competing paths every month. This helps traffic settle when two similar routes would otherwise alternate between “empty” and “overloaded.”

Congestion matters beyond the traffic display. Poor road service can reduce transport efficiency, weaken the service associated with affected settlements and create development or migration pressure. Capacity is therefore an economic constraint as well as an engineering number.

Capacity is not the same as service

A route can have high nominal capacity and still provide weak service. Capacity answers how much can pass; service also depends on whether the link is in the right place, connects the right settlements, offers competitive travel time and avoids bottlenecks along the full journey.

Coverage

Can demand reach the network at both ends?

Continuity

Does the entire path connect, including every intermediate segment?

Attractiveness

Is the route competitive in time and price?

Capacity

Can its busiest direction carry the assigned passenger and freight load?

Reliability

Will congestion or a narrow segment undermine the promised journey?

This is why widening an uncongested feeder can achieve little while a short saturated segment continues to constrain the corridor.

Air and water services

Air and water routes draw on the same underlying demand matrix, but their service and revenue are constrained by each scheduled route’s own daily capacity. Passenger and cargo suitability also differs by mode and service type.

Unlike a road or rail corridor, an air or water connection requires an active route rather than only the presence of endpoint infrastructure. An airport or port can therefore exist without carrying the potential demand between its settlement and another place.

When comparing modes, do not read a large origin–destination demand number as a promise that every mode can capture it. Availability, capacity, preference, charges and mode-specific eligibility determine the part that becomes actual traffic.

How to diagnose a corridor

Use this sequence when a route is empty, congested or less profitable than expected:

1. Confirm that demand exists. Check passenger and cargo movement for the origin–destination pair.
2. Confirm the connection. Look for a complete path, not merely infrastructure near each endpoint.
3. Inspect through traffic. A link may be busy because longer journeys pass through it.
4. Compare alternatives. Faster, cheaper or rail-preferred routes may be taking most of the flow.
5. Check charges. Tolls can divert traffic or suppress it when no free option exists.
6. Find the constrained segment. Capacity added elsewhere will not remove the active bottleneck.
7. Separate directions and purposes. Peak saturation, billable traffic, passengers and cargo answer different questions.
8. Watch more than one month. Route loads move progressively toward equilibrium after network, demand or toll changes.

Common misconceptions

“Building a road creates its own traffic immediately”

The road connects and distributes existing demand. Longer-term economic and population changes can later alter demand, but the initial traffic still depends on the places and journeys already present.

“A link only carries demand between its two endpoints”

Indirect journeys can pass through intermediate links. A seemingly modest local segment may be carrying an important national corridor.

“Capacity is the sum of both directions for the month”

The displayed saturation test uses the busier direction against directional peak-equivalent capacity. Billing can still count movement in both directions.

“An empty route proves there is no demand”

Demand may exist but choose a faster or cheaper alternative, fail to find a continuous path, be suppressed by charges or exceed the service’s usable capacity.

Continue the collection

The next part will connect network performance back to settlements: coverage, congestion, transport service and the wider economy.