Trip Distribution Submodule of SMARTPLANS
Once the total number of trips for all specified categories are estimated using the Trip Generation Submodule, the Trip Distribution Submodule is engaged in SMARTPLANS to calculate trips Tij|p from zone i to zone j for each trip segment p. This submodule utilizes a gravity model formulation to estimate the origin-destination (OD) matrices containing the trips Tij|p. More specifically, the program allows the user to choose one of the following gravity models: (1) production constrained, (2) attraction constrained, or (3) doubly constrained.
The default Trip Distribution model used in SMARTPLANS is the doubly constrained model. Here, trips between zones i and j are to calculate as follows:
where t_ij is the inter-zonal travel time between origin zone i and destination zone j and beta is an impedance parameter depicting the friction over space. Elements Ai|p and Bj|p are balancing terms which are calculated in the program through an iterative procedure using the following formulas: