Transit and Carpool Commuting and Household Vehicle Trip Making: Panel Data Analysis

Most of the evaluation of commute trip reduction programs centers on the effect on reducing driveway trips at the work site. Little has been done to explore the effects of such programs on overall trip making. This work is meant to help understand this relationship by determining the association between transit and carpool commuting and overall household vehicle trip making. Household activity models have indicated that an individual's travel decisions influence (and are influenced by) the decisions of other household members. It is thus postulated that one household member's choice of transit or carpool for the morning trip to work will affect the mode choices of the other household members. In addition, the choice of mode to work will affect the individual's mode choices for trips other than the morning trip to work. The analysis uses the first four waves of the Puget Sound Transportation Panel survey to construct linear econometric longitudinal trip generation models (random effects models). The results of the modeling indicate that one household member's mode choice to work does affect overall household vehicle travel. Compared with driving alone to work, if one household member chooses transit to work, overall household vehicle trips are reduced (on average). Carpooling by one household member is associated with little change in overall household travel, with average daily household vehicle trips ranging between one trip fewer and one trip greater than households where each driver in the household drives to work in a separate, single-occupant vehicle. The analysis strengthens the associations to imply causal links. The model results suggest that an improved understanding of interactions among household members is necessary to define strategies in support of commute trip reduction policies to better meet air quality and congestion-management goals.