3.13 Feedback

Feedback is used in two primary ways in the NERPM activity-based model system: between iterations within the highway assignment process, and between “system” iterations of the model system in which both the Daysim activity-based demand components and the Cube Voyager-based network supply components are executed.

The Method of Successive Averages (MSA) method is a common means of achieving overall stable model system results. The basic idea of the MSA method is to combine the link flows from the current system iteration with the best estimate of link flows from prior system iterations to produce the current best estimate of link flows. This is then used to compute congested link travel times using the volume delay function and these congested times are input to the skimming procedure.

Feedback during “system” iterations in the model is achieved using network skims. It has been mentioned earlier that the skims from the current iteration of the model are used in the next iteration. . In order to facilitate efficient system convergence, the model system is set up to average the highway skims from the current iteration with the skims from the previous iterations. The skims that are used in an iteration after the first iteration are an average of skims at the beginning and end of the previous iteration. For the first iteration of the mode, free flow skims are used. For the second iteration of the model, skim values used are an average of those at the beginning (free flow skims) and at the end (congested skims) of the first iteration. This goes on during subsequent iterations. Currently, three such iterations of the model are run. As noted in the transit skimming section, transit times are function of the highway times and so transit skims are also fed back from one iteration to the other.