Fuel Use and Optimality of Assignments in Multivehicle Households in 2001 and 2009

Multivehicle household fleets often consist of vehicles with a wide range of attributes, including passenger and cargo capacities, towing capability, and fuel consumption. Decisions for how these vehicles are assigned to trips can have a significant impact on a household's total fuel use. In this paper, actual vehicle assignments from the 2001 and 2009 National Household Travel Surveys data are compared with the decisions for minimizing fuel use by using CTRAM, a model that determines optimal vehicle-to-trip assignments while considering constraints on vehicle availability and capability. Results show that the average potential reduction in fuel use through optimal assignment for multivehicle households was 10.9% in 2001 and 10.1% in 2009. However, the corresponding increase in assignment optimality seen in this period does not appear to be the result of a greater prevalence of active, short-term vehicle assignment decisions, such as a driver's voluntary use of a nonpreferred vehicle or switching of vehicles midday. This finding provides evidence that the higher level of assignment optimality in 2009 was influenced by other, possibly longer-term, decisions such as consideration of fuel consumption in purchase decisions in accordance with the primary driver's expected usage of the vehicle (e.g., a small, efficient vehicle for long-distance work commuting). The significance of this conclusion is reinforced by the finding that increases in assignment optimality are smaller in lower-income households, possibly because of the lack of efficient vehicles in the secondary market in the years preceding the 2009 survey.