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The decision to travel for a given purpose is called trip generation.
These generated trips from each zone is then distributed to all other zones
based on the choice of destination.
This is called trip distribution which forms the second stage of travel demand
modeling.
There are a number of methods to distribute trips among destinations; and two
such methods are growth factor model and gravity model.
Growth factor model is a method which respond only to relative growth rates at
origins and destinations and this is suitable for short-term trend
extrapolation.
In gravity model, we start from assumptions about trip making behavior and the
way it is influenced by external factors.
An important aspect of the use of gravity models is their calibration, that is
the task of fixing their parameters so that the base year travel pattern is
well represented by the model.
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