Source: adapted from EPA420-R-97-007.
The four stages (or four steps) transportation/land use model follows a sequential procedure:
- Trip Generation. For each discrete spatial unit, it is estimated the extent to which it is an origin and destination for movements. The output is usually the number of trips generated and attracted by a given spatial unit.
- Trip Distribution. Commonly a spatial interaction model estimates movements (flows) between origins and destinations and which can consider constraints such as distance. The output is a flow matrix between spatial units.
- Modal Split. Movements between origins and destination are then disaggregated by modes. This function depends on the availability of each mode, their respective costs, and also social preferences.
- Traffic Assignment. All the estimated trips by origin, destination and mode and then “loaded” on the transportation network, mainly with the consideration that users want to minimize their travel time or have to flow through existing transit networks. If the traffic exceeds the capacity of specific transport segments (which is often the case), congestion occurs and negatively affects travel time. This in turn, through a feedback process, may influence trip generation and distribution.
This procedure is consequently iterative and converges towards a solution, often measured as the minimal transportation cost considering a given travel demand and the characteristics of the existing transportation network. It relies on an extensive array of data that can be obtained through census information, surveys and estimates.