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Game Design Document
Traffic is a game about resource transportation. The game displays a map containing supply and drop-off points, and the goal for the players is to build an efficient transport network, moving resources from supply to drop-off points
The game is level-based. Each level is built around a map, consisting of pre-defined Supplies and Depots, a transport target (how many resources should end up in each depot) and some pre-defined zones where special rules are in effect for the grid-cells.
The game map is divided into equally-sized square cells. The cells contain the game elements: paths, supplies, depots,transhipment points and vehicle spawners.
Each cell has a zone type which specifies what rules are in effect for that cell. Such rules are maximum driving speed (such as in residential areas) maximum vehicle throughput (environmental concerns) and allowed path types.
Paths are used to transport resources. There are 3 types: roads (used by trucks), canals (used by boats) and rails (used by trains). Each cell has 4 connection points, allowing paths to connect with adjacent cells. Paths of different types can cross eachother, and paths can split and join. The player configures the resources that can be transported over a path.
Supplies are the start point for resources. An unlimited amount of resources can be extracted from any supply. These resources will all be of the same type. No Supply supplies more than one type of resource. A Supply occupies a cell, and can supply to vehicles on any adjacent cell.
Depots are the end point for resources. When resources arrive at a Depot, they're consumed. Depots occupy a cell, and consume resources from vehicles on adjacent cells. Depots are pre-set to receive only a certain kind of resource.
Transhipment points are both an end point for resources and a starting point. They allow transhipment of resources from one type of path to another. When a loaded vehicle drives by on an adjacent cell, it will drop off its resource at the transhipment point. If an unloaded vehicle drives by, it picks up the resource. Transhipment points can only contain one kind of resource, so if a resource has already been dropped off, only resources of the same type will be dropped off afterwards, until the transhipment point is empty again.
Vehicle spawners spawn vehicles. For roads, these are trucks, for canals boats and for rails trains. Each type of vehicle differs in maximum speed, maximum cargo and zone restrictions.
The vehicle spawner can spawn an unlimited amount of vehicles, but the players will be penalized if they don't return vehicles to a vehicle spawner. Each vehicle spawner has an entry and an exit point which must be connected correctly. Vehicles are supposed to either drive around in a loop back to the same spawner, or drive towards another spawner.
The players play by building paths and specifying what type of resource can travel over each path. The players have no direct control over the vehicles.
The game starts paused, allowing the players to specify whatever paths and routes they want. While paused, no vehicles will move. The players can resume the game in order to make the vehicles move.
Paths can be built by extending existing paths or splitting off from them. Each level will begin with some basics paths that can be extended.
Routes can be specified by extending from a supply or a depot. Since both supplies and depots only deliver/consume one type of resource, the kind of route this creates is always unambiguous. Routes can join together on a path and split away later when the path splits.
Vehicles follow simple rules even if it means driving off a road and crashing. The rules are as follows:
- On spawn, leave in the vehicle spawn exit direction.
- When passing by a supply or transhipment point while fully loaded, pick up cargo (if there is any.)
- When passing by a transhipment point while carrying cargo, drop the cargo if the transhipment point allows this cargo type (this is the case when the transhipment point is either empty or containing cargo of the same type).
- When passing by a depot while carrying cargo, drop cargo if it matches the depot's requirement.
- When at a split, choose the path that allows the current cargo. If multiple such paths are available, choose the left-most one.
- When a vehicle enters a vehicle spawn, it disappears.
- When a vehicle enters the end of a road, it crashes and disappears with a penalty.