The assignment with ICA is a static and accounts for detailed node impedances.
We do not recommend using ICA directly during an assignment is running, since the volume-delay functions are not separable. In the approach described, turn VD functions are made "roughly separable" by "freezing" the conflicting flows in equilibrium assignment. Normally, convergence is reached in this way. Simultaneously, the Turn VDFs are continuously adjusted to the wait times and capacities calculated by ICA. The HCM 2000 method used in ICA is one of the worldwide recognized analyses methods for node performance calculations and accounts for lane distribution and conflicting turns down to detail level.
However, this iterative approach requires more time and effort for junction modeling and calibration, since nodes whose impedances are be calculated by ICA have to be modeled in detail. If you do not want to model all nodes of the network in detail, you should make sure that for the other nodes volume-delay functions are used which provide impedance data in a comparable scale. Otherwise the route choice will be distorted if impedances of different magnitudes are produced by turns of ICA nodes and non-ICA nodes.
Using the iterative approach, assignment with ICA requires more computation time than a conventional equilibrium assignment. Additional spillback and ICA calculations are required. Adaptation of the turn VD functions after subordinate equilibrium assignment mostly leads to a setback in convergence, which must be offset through recalibration.
As in other assignment procedures, you can use existing assignment results as a starting solution for an assignment with ICA. The assignment results used must stem from an assignment with ICA. Only then is it guaranteed that the attributes used at turns and links have been calculated. This procedure can be used to calculate scenarios in which the network basis and demand data do not radically change. Thus, the parameters previously estimated form a good basis for recalibration.
For the assignment with ICA, the LUCE equilibrium assignment can be used as subordinate assignment procedure. The use of this assignment within the assignment with ICA has the following advantages:
- Stable route distribution, especially with option Optimization of the proportionality of route volumes at meshes.
- The calculation of the blocking back model, using so-called bushes, is considerably faster than using the paths of other assignment methods.
- Due to the stable distribution over routes, the blocking back result is also more stable and thus convergence can be reached faster.
