University of California Transportation Center

The product of this research is a dynamic simultaneous equations model of car ownership and modal travel distances as a function of income. The data are from the Dutch National Mobility Panel (1984-1987); and four modes are encompassed: car driver, car passenger, train, and bus-tram-subway. A novel feature of the simultaneous equation system is the consistent treatment of the measurement scales of the variables: ordered probit functions for income and car ownership and tobit functions for distances. The dynamics are expressed in terms of pooled panel survey measurements of the variables at two points in time one year apart. This allows the identification of lagged responses and serial correlations over a one-year time-horizon. Results indicate that increased car ownership and car kilometers at time T2 is influenced by heavy usage of other modes at time T1. This indicates there are significant noninstantaneous adjustments of car ownership and usage that represent modal substitutions.