In the US, concerns about dependence on oil (foreign oil in particular) have intensified and the transportation sector has garnered increased attention for its contribution to global warming. In response to these concerns, there is an increasing focus on the efficiency of our transportation systems. The objectives of this paper are to provide a framework for defining transportation efficiency (TE), highlight the ambiguities present in the transportation-research literature and in public policy, and describe the challenges that come with attempts to comprehensively measure TE. Of particular concern are the many studies which simply describe policy strategies associated with TE, without attempting to comprehensively define or measure it. Defining “transportation efficiency” as the maximization of services at the lowest possible cost will require that a variety of costs and service variables be assimilated into a comprehensive measurement tool. Such a tool could be used to find a TE index for temporal and operation TE comparisons. However, temporal applications are susceptible to rebound effects and operational comparisons are susceptible to shifting effects. Few assimilative measures of TE have been found which even attempt to deal with these potential sources of error. Better models are needed to adequately assimilate a wide variety of economic, environmental, human, energy, and operational variables on TE, and to assess rebound and shifting effects.