3.14 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 Visum-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. The skims generated from the MSA averaged link flows and resulting congested times are used for the auxiliary models and DaySim in the next iteration. Transit skims are updated based on congested link time for the next iteration. Currently, the model is set up to run up to 7 such iterations. At the end of each iteration, the overall model convergence is checked. The model run is terminated if convergence is achieved before 7 iterations. The convergence is achieved when all of the following conditions are satisfied between the current and previous iteration outputs:

• The % change in Vehicle Miles Travelled (VMT) is less than 5% for both AM and PM periods
• The % of links with under 5% change in link volumes is less than 5% for both AM and PM periods