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Trip generation is traditionally the first step in 4-step travel models. Trip generation estimates for an individual traffic analysis zone (TAZs) the number of trip ends produced or attracted by that zone. Trip ends produced are called "trip productions", and trip ends attracted are called "trip attractions". A trip is produced where the need (or desire) for travel is located; and a trip is attracted where the need (or desire) is satisfied. A trip production can be either an origin or a destination, and a trip attraction can be either an origin or a destination. Productions and attractions are separately tabulated for each trip purpose (e.g., HBW, HBO, NHB).
One of the more popular methods of trip production calculation is cross-classification. Cross-classification is described in NCHRP Report 716, among many sources. A popular method of trip attraction calculation is by linear equations. Neither of these two methods use measures of mobility or accessibility. Inputs consists entirely of forecasted socioeconomic or demographic characteristics of zones.
A subsequent step, trip distribution, very often requires that the total of all productions equal the total of all attractions for each trip purpose. Consequently, an important part of trip generation is "balancing", where inconsistencies between productions and attractions are reconciled. Consistency can be achieved by holding productions constant and varying attractions or vice versa.