Trip generation models strive to predict the number of trips generated by a zone. These models try to mathematically describe the decision-to-travel phase of the sequential demand analysis procedure. It may be mentioned here that typically the term trip generation is used to mean trip production -- generally the trips made from households, and trip attraction -- the trips made to a particular urban location or activity. However, it is felt that analysis of trip attractions should not be in the purview of trip generation models which attempt to quantify a populations urge or propensity to travel. Rather, trip attractions are an outcome of the destination choice phase of travel behaviour. Similar concerns about trip attractions being part of the trip generation phase of urban demand analysis have been also voiced in Kanafani [#!kan1!#]. In keeping with this, the present section discusses trip generation primarily in the context of trip productions. Trip attractions are assumed to be an outcome of the destination choice phase and are discussed in the section on trip distribution models.
There are basically two different model structures for trip generation models -- the cross-classification models and the regression models. However, both these model structures incorporate the same basic factors which affect the trip generation of a zone; the models only differ in their characterization of these factors.
The factors (for any given trip purpose) which affect the trip generation of a zone are: