Trip Generation Rates and Models

Trip generation is the first step in the classical four-step transport modeling sequence (trip generation, distribution, modal split, and assignment). Its objective is to predict the trips—and more generally the units of measure of movement, like tours— that will be made by the residents of the study area. Although personal trips account for the majority of urban trips (more than 80 percent), trip generation is also a part of freight models.

In principle, this step is very simple. Trip generation usually outputs two sets of trip ends: the number of trips originating in (trip productions) and the number of trips destined for (trip attractions) each of the zones of the study area (also referred to as traffic analysis zones, or TAZ). Personal trip productions are usually determined as a function of socioeconomic variables like income, family size, household structure, and car availability; attributes of TAZ like value of land and residential density; or other (nonresidential) land use information like some measure of accessibility.

Personal trip attractions are determined as a function of zonal employment and/or roofed space available for industrial, commercial, and other services. Freight trips are usually determined as a function of the number of employees, the number of sales, roofed area, total area, and so forth. It must be noted that even if it is one of the more used explanatory variables, the explanatory power of car ownership has declined over the years due mostly to its universality.

[Page 1620]In order to properly quantify the movements within a study area, a definition of the unit of measure is needed. There is no consensus regarding the definition and/or nomenclature of the unit of measure of movement.

The simplest unit of movement is a stage, which is a continuous movement with one mode (vehicle) of transport. A trip is a continuous sequence of stages between two activities. A journey is a sequence of trips that start and end at the same location. A tour is a sequence of trips that start and end at the home of the relevant person. Trip generation is a generic denomination of models that may use these different units of measure of movement.