The incremental assignment procedure models how a network continuously fills up. At the beginning, road users can use a free network for which exactly one shortest route exists for every origin/destination relation. The traffic network is then successively loaded. Every step congests the road network with additional vehicles and, in this way, increases impedance on the congested links, turns and connectors. Because of the changed impedance, alternative shortest routes may be found in every step.
The matrix is incrementally assigned to the network in the form of several parts. The total demand is distributed as a percentage across the number of iteration steps defined by the user (maximum 12). The default is an incremental assignment with three iteration steps (33 %, 33 % and 34 %).
- The first step determines lowest impedance routes for all required OD-relations of the current network for either a free network or based on a base volume.
- The defined percentage of the first incremental step of the matrix is then assigned to these routes.
- Subsequently, the new network impedances resulting from these volumes are calculated via the VD functions.
- On this basis, the next iteration step again calculates lowest impedance routes.
- This procedure is continued until the entire matrix has been assigned to the network.
If 100% is entered for the first iteration step, Visum calculates the impedances of the current network and carries out a so-called best-route assignment.
