4.1 Base Gravity Model Application
The output of the Task 3 Trip Generation procedure serves as one of two basic inputs to the Task 4 Trip Distribution procedure. The conventional Gravity Model distributes trips from production zones to attraction zones as a function of zonal productions and zonal attractions, both outputs of trip generation, and of some function of travel impedance. This measure of spatial separation is typically taken as uncongested automobile network travel times, although most model applications feed back congested travel times from traffic assignment (and often reflect alternate modal travel times). The gravity model can be singly-constrained (followed by attraction balancing), or doubly constrained, where all productions and attractions remain balanced in the estimated trip matrix. transcad can either calibrate a Gravity Model (i.e., estimate the function of travel impedance, or "friction factors") or simply validate or forecast with an existing calibrated model. A set of estimated friction factors for Miasma Beach will be used for this exercise.
4.1.1 Open Input data
To execute trip distribution analysis in transcad, the Travel Impedance Matrix (skim tree matrix) that was created in Task 2 must be openned. Go to File / Open and select the drive and directory where the Task 2 results were saved. Select Matrix as the file type; a list of matrix files created in Task 2 should appear. Select skimtree.mtx and click OK. Also open the TAZ dataview by first opening the TAZ Geographic file and then clicking on the New Dataview button. This table contains the production and attraction values that you estimated for each TA
