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 The following excerpt was taken from the Transportation Planning Handbook published in 1992 by the Institute of Transportation Engineers (pp. 108-112). Trip Generation Models (p. 110) There are two kinds of trip generation models: production models and attraction models. Trip production models estimate the number of home-based trips to and from zones where trip makers reside. Trip attraction models estimate the number of home-based trips to and from each zone at the non-home end of the trip. Different production and attraction models are used for each trip purpose. Special generation models are used to estimate nonhome-based, truck, taxi, and external trips. Cross-Classification Over time the profession has come to understand that considerable predictive power and accuracy can be gained by disaggregate analysis of influential variables. . . . This means that the models use factors describing individual sample units (e.g., persons, households or workplaces) rather than an average value of each factor for each analysis zone. The result is trip generation models with trip rates for sample units having specific characteristics, such as households of one, two, or more family members, owning one, two, or more vehicles. These models are based on the trip rates for individual sample households having those particular discrete characteristics. . . . (p. 112) Most trip production models are two- or three-way cross-classification tables with the dependent variable being trips per household or trips per person. The independent variables are most often income, auto ownership, and household size. . . . Virtually all of the trip attraction models use employment and an identifier of location as independent variables. Multiple Regression (p. 110) Early trip generation models were commonly developed by regression analysis because of its power and simplicity. The independent variables in such models were usually zonal averages of the various factors of influence. Trip generation equations developed by regression are still used by some planning agencies, more commonly for attraction models than for production models. This is because only zonal averages of trip attracting characteristics are usually available since most travel surveys do not survey at trip destinations. Obtaining more detailed data for individual attraction zones requires a survey of trip attractors, such as a workplace survey. Experience Based (p. 108) Early travel forecasting used extrapolation of past trends to estimate future travel. Such an approach is still used occasionally for estimating future traffic on a single facility, in a relatively isolated area, where only moderate and uniform growth or change in development pattern is anticipated. One level of sophistication that can be added to trend analysis to respond to anticipated growth is comparing the past traffic trend to the trend of development during the same period. This provides understanding of how traffic on the subject facility will respond to expected development changes. That relationship between the two trends is incorporated subjectively in the trend forecast.
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