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Each change in data, parameters, or assumptions represents a refinement or upgrade of the rules. Each refinement must be backed by a reason. No changes are made to simply get better volumes. To apply the model to alternate scenarios, especially future year forecasts, each justification must be questioned for its continued application. If the rule still holds for the scenario, then it can be applied. If the rule is not applicable, then adjustments must be made in rules for that scenario. Finally, it must be emphasized that the simulation being run with the model is one of human interaction with the transportation system. To do this, the program uses the gravity model to simulate the distribution of trips between zones and selects "shortest paths" for the assignment of trips. Human behavior is equated to a series of mathematical formulas that assume that all humans behave logically. While people do not always behave in a logical and rational manner, under most situations these assumptions are valid. Keeping this in mind, the calibration process is carried out. Model Calibration Process Essentially, calibration is comprised of three stages. First, working from outside to inside and large to small, all volumes that lead to the outside world through external zones are calibrated. Analyzing the model for general trends of trips is the next step. The third step is to evaluate the individual count locations and individual routes. Changes at any level may affect operations at another stage in calibration. That is, a proper allocation of trips to the right route may affect the general trends. Therefore, the calibration process is one of always looking back and continually monitoring each step until the procedure is complete. External Zones In VISUM, zones are differentiated between "internal" and "external." Internal zones are those in which all the land use is known and all generated trips will go to and arrive from other zones in the modeled system. An external zone interacts with other zones in the modeled system and with the external world that surrounds the network. (Traffic count data, collected on the roadway leading in or out of an external zone, is used.) It is impossible to describe fully the land uses outside the modeled area that interact with the internal zones. Therefore, an external zone is described in the model as having origins and destinations to produce the appropriate volume of traffic on the roadways that connect it with the rest of the network. Kittitas County, Washington Transportation Model-May 2009 Page 37