Travel Trends Using the Puget Sound Panel Survey: A Generalized Estimating Equations Approach

This paper examines longitudinal mode use trends using four waves of the Puget Sound Transportation Panel. The analysis is conducted using generalized estimating equations for model estimation. In addition to examining mode use frequencies over time, we also consider mode use trends conditioning on household income and lifecycle stage. As expected, results indicate an overall increase in the number of worktrips made between 1989 and 1993 and these trips were marked by increasing use of single occupancy vehicles. The full parameters of the model were also to estimate the rate of increase in terms of percentage increase and their confidence intervals. Results indicate that the mean number of worktrips made by driving alone significantly increased from wave 1 to wave 4; with a 95% C.I. the rate of percent increase was estimated between 8.2 and 24.5%. The ranges for rates of change in high occupancy vehicle modes and non-motorized worktrip frequencies overlap with the range for single occupancy vehicle rate of change, and thus, it cannot be said that rate of change for the high occupancy modes was significantly different from the single occupancy rates of change. The rate of change in the mean frequency for the high occupancy¯transit mode is not only below the range for single occupancy vehicle trips but also suggests, with 95% confidence, the rate of percent decrease was between 2.88 and 44.0%.