Traffic congestion

Trips generated by zones or agents travel over a road network; each segment has a capacity, and travel time rises as volume approaches or exceeds it. Micropolis, the released SimCity engine, has no vehicles: a zone tries a random walk of up to 30 steps along roads until it is adjacent to a destination zone of the required type, and success adds 50 to the traffic density at saved points on the path. Density decays each scan by 34 above 200 and by 24 above 24, and is otherwise reset to zero. Cities: Skylines simulates individual vehicles on lanes. Cities: Skylines II agents choose routes by a combined cost of time, comfort, money, and behavior, spread across lanes, and can be stopped by accidents checked per segment. Inputs: trips, network capacity, route choice. Outputs: per-segment volume, travel times, and delays to services.

Watching gridlock form. Players see jams, trace them to bad intersections or missing routes, and redesign. Congestion is the most visible symptom of a city's structure, and fixing one jam often moves it elsewhere.

• capacity per lane or road type
• route choice model: random walk, shortest path, or cost-based with re-routing
• trip generation rate and timing
• consequences: lower desirability, delayed services, or lost deliveries
• stuck-vehicle handling such as despawning

• Vehicles that crowd into one lane while parallel lanes sit empty, a common complaint about the first Cities: Skylines, produce jams that better road design cannot fix.
• Despawning stuck vehicles, as the first Cities: Skylines does by default, hides real congestion problems.
• Opaque route choices make traffic feel random.

• Induced demand: new capacity attracts more trips.
• Congestion delays emergency services, so fires spread and crime rises.
• Players build hierarchies and transit to shift demand.

• Could learned driving policies trained on real traffic data make in-game congestion match real-world patterns at city scale?
• What level of traffic fidelity improves design decisions rather than just spectacle?