Joint model of vehicle type choice and tour length

Tour-based microsimulation model systems are increasingly being applied to the forecasting of travel demand. This paper examines the relationship between two dimensions of tours: the type of vehicle (in a household that owns multiple vehicles of different types) chosen to undertake the tour and the overall length (distance traveled) of the tour. These two dimensions are of much interest in the current planning context, in which concerns about energy sustainability and greenhouse gas emissions are motivating planners to seek ways to mitigate the adverse impacts of automotive travel. Moreover, virtually all tour-based models currently used do not explicitly account for choice of vehicle type in the modeling of tour attributes, despite the critical importance of the choice of vehicle type for energy and emissions analysis. This paper presents a joint discrete-continuous model of choice of vehicle type and length of tours. Estimation results suggested that significant common unobserved factors affected vehicle type choice and length of tours. These factors justified the use of modeling approaches with joint simultaneous equations to model tour attributes. The model specification in which vehicle type choice affected tour length performed better than the specification in which tour length affected vehicle type choice. This outcome suggested that choice of vehicle type (and allocation to household members) was a longer-term choice that influenced shorter-term tour-length choices.