EFFECTS OF INCOME AND CAR OWNERSHIP ON TRIP GENERATION: A STRUCTURAL EQUATIONS MODEL AND ITS TEMPORAL STABILITY

A simultaneous equations model is developed to explain car ownership and trip generation by mode at the household level. The data are from three years of a panel survey conducted in the Netherlands. The variables are household income, treated in three dummy variables to represent four income classes; number of cars; and trips per person by three modes: car, public transport, and bicycle. Each equation in the simultaneous system represents the direct effect of one variable upon another, and all variables with the exception of the income variables are endogenous. The application of the model to a panel data set allows testing of the temporal stabilty pf the structure of cause and effect. It was found that the structure can be considered stable, but its details change over the years 1984, 1985, and 1986. The principal results are that income effects on car demand are channeled exclusively through car ownership, and that car ownership and car travel explain a significant portion of public transport and bicycle demand. However, there are also direct links from income to public transport and bicycle demand. In particular, public transport trip making is shown to be an increasing function of income, controlling for differences in car ownership.