Diary Three ended with a small tease: engines, plural.
Then the community voted for the next diary, and simulation won.
So this is the first proper look beneath Regions—not just at the cars or people you can see, but at the systems deciding who those people are, where they are going, what they own, what actually happened to them, and how all of that survives when the camera moves away or the game is saved.
This is Simulation: Part One.
And the best place to begin is not with a citizen, a car, or a traffic jam. It is with a question:
Who owns the truth?
Regions uses several simulation engines, but they are not all allowed to decide the same things.
The Regions Simulation Host owns the authoritative state of the Region: the clock, households, people, jobs, vehicles, journeys, parking custody, money, government accounts, external visitors, utility providers, and the current state of every accepted plan.
MATSim handles strategic travel planning. Regions gives it the approved people, places, available vehicles, services, schedules, and road network. MATSim proposes how those activities connect: when someone should leave, which mode they can use, and which route or service makes sense.
SUMO handles detailed road execution. Once Regions has accepted a personal-vehicle journey, SUMO executes that demand against the current road network and the traffic-signal states owned by Regions.
Unreal Engine owns the interactive world. It renders the Region, responds to the player’s tools, and gives nearby people and vehicles a physical presence.
In simpler terms:
- Regions decides what is true and what is allowed.
- MATSim plans the journey.
- SUMO executes the road traffic.
- Unreal shows the result and handles the physical moments you can see.
That division is not just an organizational preference. It prevents several systems from quietly disagreeing about the city.
If MATSim is unavailable, Regions does not invent a secret fallback plan and pretend everything is fine. If SUMO returns traffic results for an old road revision, those results are rejected. If Unreal loses a visible pedestrian because the camera moved away, that does not erase the citizen.
Every important exchange carries stable identities, revisions, and fingerprints so both sides can prove they are talking about the same version of the Region. The rule is simple: one authoritative writer per fact.
A person is not a pedestrian
This distinction is probably the most important one in the whole system.
A visible pedestrian is not the citizen.
A visible car is not the vehicle record.
They are temporary physical representations of something persistent.
Close to the camera, a person may exist as a fully physical Unreal entity walking along a real sidewalk. Farther away, the simulation may only need their location. While they are inside a building, it may only need to know which activity they are performing and when their next decision is due. At larger scales, some work can be processed in groups.
Changing that level does not change who the person is.
Their household, home, job, money, vehicle, plan, and history remain attached to the same persistent identity. Entering a building removes the walking body; it does not remove the citizen.
When a physical action is required, Unreal receives a temporary representation lease. That lease names the exact person, journey, stage, destination, and generation Unreal is allowed to represent. When the action finishes, Unreal reports the result to the host.
It cannot simply say, “someone arrived.”
It must say which person arrived, where they arrived, which journey they were completing, and which generation of permission produced the observation.
That may sound strict, but it closes a whole category of bugs where a visual result accidentally rewrites the simulation.
Moving into the Region without teleporting
The clearest way to show the system working is to follow a household moving into the Region.
A new household does not appear inside a residential building the moment capacity becomes available. It begins outside the Region as a prospective household.
Regions chooses a legitimate outside connection and reserves an eligible destination. MATSim plans the journey from that gateway. If the household owns a car, SUMO executes the road portion using that exact persistent vehicle.
At the destination, the vehicle must occupy a real parking space. The occupants then get out and physically walk from that space to the building’s actual entrance.
Only when the final arrival is accepted does the household become settled and present.
Before that moment, assigning a home is only a reservation. It is not proof that anyone moved in.
We have now run this entire chain in a live authored city: entry through the map edge, real road execution, exact parking, physical pedestrian egress, arrival at the facade door, and the final presence transition. We have also saved during these systems, restarted the simulation host, restored the active state, and continued into the next day’s journeys.
Households move together
That first settlement path exposed a problem with treating everyone as an independent commuter.
Households do not move into a Region one employed adult at a time.
Children, retirees, unemployed adults, and other non-workers still need a legitimate arrival. Settlement is committed for the household as a whole.
For a household with a car, one person drives while the other members travel as passengers. SUMO receives one physical vehicle, not a duplicate car for every occupant.
A carless household follows a different path. Regions creates an outside-owned arrival carrier that enters through the gateway, transports the household to its destination, completes the drop-off, and exits the Region again.
The household does not magically gain ownership of that vehicle. The temporary driver does not become a resident. The whole carrier lifecycle can survive a save made after entry and complete its drop-off and exit after loading into a new host process.
This admission boundary is deliberately mode-neutral. Road vehicles are the working entry mode today, but future passenger rail, marine, and air arrivals can use the same household settlement transaction without inventing another definition of residency.
The exact parked car matters
Parking in Regions is not just a capacity number attached to a building.
Vehicles have persistent identities. Parking spaces have stable identities. Custody matters.
If a resident leaves work to drive home, they should walk back to the same vehicle they previously parked—not whichever decorative car happens to be nearby.
A complete personal-car journey now includes:
- Walking from the current activity to the exact parked vehicle.
- Physically reaching the car and pulling out.
- Receiving host approval for the departure.
- Executing the road journey through SUMO.
- Parking in an eligible destination space.
- Walking from that space to the destination’s actual entrance.
The person, vehicle, journey, parking custody, and destination remain linked through the whole chain.
This is also where the difference between background life and authoritative citizens becomes important. Regions can generate ambient cars and walkers to keep the visible world busy, but they are scenery unless bound to an authoritative journey. Players can now disable those ambient agents entirely without stopping the physical citizens required by the real simulation.
The simulation does not promote a background car into a citizen’s permanent vehicle just because it looks convenient.
Roads can change while people are using them
Roads in Regions are not static scenery, which means the simulation cannot assume the network will stay still while people are travelling.
When the player edits a road, Unreal publishes a revisioned network change. The host validates the candidate network, reconciles its facilities and parking, and identifies which journeys were actually affected.
For localized disruptions, only those people need new MATSim plans. Unrelated citizens keep their existing plans and identities.
The larger goal is to keep the current SUMO world running while a replacement network and its affected plans are prepared in parallel, then switch at a safe simulation tick. That complete hot-swap system is still planned. Major topology changes can still require the external traffic world to be replaced.
But the architecture is already built around revisions instead of resets. Changing one intersection should not become an excuse to forget the entire city.
Travel is more than cars
The transport system is built around complete journeys, not isolated vehicle spawns.
The shared mode vocabulary includes walking, private cars, bicycles, taxis, buses, light rail, subway, trains, boats, and airplanes.
That does not mean every physical mode is finished today.
Walking and personal road travel have working physical execution paths. Passenger services now have the deeper authority needed for scheduled runs: stops, queues, boarding order, seating and standing capacity, accessibility, transfers, fares, delays, missed runs, and cancellations.
Passengers and service vehicles remain separate records. A person waiting for a train does not become a property of the train actor. If capacity is full, that individual can be denied boarding and receive a new plan instead of disappearing. A transfer between different stations can include a real walking leg between the platforms.
Regional rail is the first physical service adapter. The host owns the scheduled train’s progress; Unreal projects that progress onto the visible train and the current track geometry. The same passenger contracts can later support buses, subways, ferries, and air services, but those physical adapters still need to be built.
Buildings are becoming actual places
The parcel system from Diary Three knows where land and buildings exist, but deep simulation needs more than “this parcel contains residential capacity.”
People need persistent relationships with actual places.
Regions now creates stable identities for buildings, home units, establishments, and primary entrances beneath the larger parcel or facility. Those identities are never based on a temporary actor or an array position that might change after an edit.
A household can be bound to a specific home unit. A job can belong to a specific establishment. A pedestrian can finish a journey at the actual front door instead of disappearing at the property line. Removed identities are retired rather than recycled onto some unrelated future building.
The complete property system is not finished. Tenancy, leases, ownership, rent, mortgages, household moves between exact units, and the deeper business lifecycle still need to be connected.
That work also changes demolition. An occupied home or establishment cannot eventually be treated like an empty prop. The planned eminent-domain system requires appraisal, compensation, notice, relocation or closure, vacancy confirmation, and host authorization before demolition can complete.
Persistent identity means the game has to deal with what deletion actually does to people.
An economy where money comes from somewhere
The economy follows the same authority rule.
Regions is not using one unexplained city-wide money counter. It has a Region-level government, counties, incorporated settlements, and unincorporated areas, each able to hold responsibilities, accounts, revenue, and expenses.
Transactions use a balanced ledger. Money leaving one account must arrive somewhere else.
The current foundation includes household cash, payroll, tax withholding, government transfers, road-construction identity, and recurring road-maintenance responsibility. Recurring household obligations can also carry a payer, responsible household member, counterparty, amount, due date, grace period, and payment history.
If a household can pay, the ledger records the balanced transaction. If it cannot, the obligation can move through grace, delinquency, and default.
Those states do not currently trigger an eviction, foreclosure, or family collapse. The evidence exists first. Later property and social systems can respond to that evidence without inventing the financial history afterward.
Business ownership, production, inventory, prices, rents, loans, savings, property markets, and deeper taxes remain future layers.
The edge of the map is not a magic spawn line
The Region also has an outside world.
External workers, business visitors, shoppers, tourists, students, medical visitors, social visitors, through-travellers, imports, exports, and through-freight are represented as different kinds of boundary demand.
Temporary people remain nonresidents. Freight remains trade, not a pretend person.
A road-based tourist can enter through a real gateway, walk to a destination, perform the scheduled activity, walk back to the boundary, and leave without ever joining the resident population.
Completed visits can create external spending and sales-tax transactions. They can also produce reputation evidence tied to the visitor’s source market and the Region, County, settlement, and destination they experienced.
Later demand from that same market can respond to those recorded outcomes.
The original visit is never rewritten to justify the new demand. An event happens, the economic result is recorded, reputation evidence is derived, and the next decision reads that history.
Production demand sources, external payroll, fares, tolls, parking charges, detailed freight settlement, and non-road gateways are still in progress.
Utilities need providers before they need pretty pipes
Utilities are being built foundation-first too.
Before drawing a power line or water pipe, the simulation needs to know who provides the service, which territory they serve, what supply exists, which customers are connected, how shortages are prioritized, what the tariff is, and what happens during an outage.
The current authority covers electricity, potable water, wastewater, solid waste, fixed internet, and cellular service. Providers can be public, private, cooperative, or contracted without receiving an arbitrary ownership bonus.
Capacity can be allocated by priority. Shortages can curtail or interrupt connections. Metered usage can produce an exact bill. Outages and restoration preserve the same provider and customer identities.
The Region Overview can already read this authority. Physical utility networks, coverage tools, customer contracts, balanced bill settlement, and the consequences of losing a service are later work.
Saving the city without rebuilding it from actors
All of this only matters if it survives saving and loading.
The Regions host snapshot preserves the authoritative identities, plans, journeys, parking custody, economy, jurisdictions, utilities, and external flows.
Unreal separately stores the temporary physical state needed to resume nearby moving cars and walkers.
On load, the system validates those layers before simulation continues. It does not inspect the scene and guess that whatever actors happen to be visible must be the truth.
We have cold-restored a city into a new host process with dozens of travelling vehicles, active walkers, parked identities, door-to-door journeys, and the clock continuing from the saved tick. We have also deliberately killed the external host and proven the client can relaunch and re-establish the simulation connection without restarting the Unreal session. Recovery of every populated failure case is still being hardened, but failure is treated as a supervised state—not permission to fabricate a replacement city.
Things that broke
No Regions diary is complete without the part where a technically reasonable system does something completely unreasonable.
The pedestrian who stole somebody’s car. Ambient walkers originally reclaimed parked cars by matching the parking space. That worked until the space contained a persistent citizen-owned vehicle. The ambient walker found the right stall, ignored the owner, and drove away in someone else’s authoritative car. Reclaim now checks vehicle ownership and identity, not just location.
The timer that beat the person to the door. The host had a duration fallback for parking and building-entry stages. During a large SUMO advance, that timer could complete the stage before Unreal had time to create the physical pedestrian. The simulation would claim the person had arrived while the visible person had not even started walking. Runtime completion now waits for physical evidence; the duration shortcut is confined to self-tests.
The parking spaces on the wrong side of the street. Splitting a road at a new junction can reverse the direction used to bake one half of the road. One mapping path failed to reverse its lane list with it, producing 46 parking spaces bound to the opposite carriageway. Cars were not randomly crossing the centerline—the simulation was faithfully following a bad lane-to-space relationship. A map-wide audit caught it, and the single-span mapping now uses the same direction correction as the rest of the road baker.
These are annoying bugs, but they are also why the authority boundaries exist. When the city disagrees with itself, we can trace which system wrote the wrong fact instead of adjusting random traffic values until the symptom goes away.
What is working, and what is not finished
The foundation working today includes:
- An external authoritative simulation host with a deterministic clock.
- Mandatory MATSim planning and SUMO road execution.
- Persistent households, people, jobs, vehicles, plans, journeys, parking custody, homes, establishments, and entrances.
- Physical household entry, exact parking, door-to-door walking, and mixed car/walk commutes.
- Save and restore across authoritative state and nearby physical agents.
- Regional, County, and settlement government accounts with a balanced ledger.
- External visitors, trade identities, first commerce and tax postings, and reputation feedback.
- Utility-provider, territory, capacity, billing, and outage authority.
- A Region Overview that exposes this state without pretending ambient scenery is the population.
Still ahead are the layers that turn that foundation into a deeper society: tenancy and property ownership, richer businesses and markets, genuine job search and unemployment, household relocation, physical utility networks, more transport executors, temporary resident travel outside the Region, seamless large network replacement, and optional dedicated-server scale.
The first deep-social work is now being shaped around continuous, need-driven decisions rather than one fixed morning schedule. People should be able to decide during the day that they need food, shopping, a service, or leisure, then create a new activity demand using the same planning and physical journey systems described above.
Eventually that expands into relationships, memories, beliefs, health, education, institutions, family changes, and complete life histories.
But those systems have to affect the same persistent people.
A job loss should change one person’s employment history, household finances, schedule, and travel—not spawn a generic unemployment icon.
A move should preserve the household and its history.
A missed payment, difficult commute, or successful visit should become evidence that other systems can respond to later.
That will be Simulation: Part Two.
It will not be the next diary—we have other parts of Regions to return to first—but we will come back to show how these foundations become a deeper living society.
For now, Part One is about establishing who exists, who owns each decision, and how several simulation engines can work together without losing the person at the center of it.
That is the simulation foundation being built beneath Regions.
You can follow the live work on the Regions roadmap, join the Discord, and wishlist Regions on Steam.
Current simulation status: Regions host ✓ mandatory MATSim ✓ SUMO road execution ✓ persistent identities and save/restore ✓ whole-household entry ✓ exact homes and establishments ✓ economy and utility foundations ✓. In progress: resident vehicle recovery, external demand, household finance, utilities, passenger services, and the first deep-social layer.