that trip destination rates would not change over time. Revisions to the analysis As was true for land use analysis, the approach developed at CATS was considerably modified in later studies. The conventional four-step paradigm evolved as follows: Types of trips are considered. Home-based (residential) trips are divided into work and other, with major attention given to work trips. Movement associated with the home end of a trip is called trip production, whether the trip is leaving or coming to the home. Non-home-based or non-residential trips are those a home base is not involved. In this case, the term production is given to the origin of a trip and the term attraction refers to the destination of the trip. Residential trip generation analysis is often undertaken using statistical regression. Person, transit, walking, and auto trips per unit of time are regressed on variables thought to be explanatory, such as: household size, number of workers in the household, persons in an age group, type of residence (single family, apartment, etc.), and so on. Usually, measures on five to seven independent variables are available; additive causality is assumed. Regressions are also made at the aggregate/zone level. Variability among households within a zone isn’t measured when data are aggregated. High correlation coefficients are found when regressions are run on aggregate data, about 0.90, but lower coefficients, about 0.25, are found when regressions are made on observation units such as households. In short, there is much variability that is hidden by aggregation. Sometimes cross-classification techniques are applied to residential trip generation problems. The CATS procedure described above is a cross-classification procedure. Classification techniques are often used for non-residential trip generation. First, the type of land use is a factor influencing travel, it is regarded as a causal factor. A list of land uses and associated trip rates illustrated a simple version of the use of this technique: Such a list can be improved by adding information. Large, medium, and small might be defined for each activity and rates given by size. Number of employees might be used: for example, <10, 10-20, etc. Also, floor space is used to refine estimates. In other cases, regressions, usually of the form trip rate = f(number of employees, floor area of establishment), are made for land use types. Special treatment is often given major trip generators: large shopping centers, airports, large manufacturing plants, and recreation facilities. The theoretical