From my book with Bill Garrison The Transportation Experience:
An important thing we did learn was not to think of ourselves as transportation geographers, or transportation engineers, or transportation planners, or transportation policy analysts, or transportation economists, but rather, to coin a term, “transportationists”. The study of transportation is sufficiently interdisciplinary to warrant a discipline of its own. The movement of people and goods across networks over time and space is the unifying object of study. The central research questions in transportation concern what moves, why and how people and goods move, how networks operate, how the interaction of travelers and shippers and carriers and networks shape behaviors, how networks are (or should be) built and paid for and so on.
While in our forthcoming book Place and Plexus Kevin Krizek and I write:
We are transportationists. This means we are interested in understanding the transportation system holistically. While we both have training as transportation planners, transportation policy analysts, transportation engineers, and transportation economists, it is the subject of transportation (and in this book, its inter-relationship with location or land use) that is of interest.
The key is thus what has traditionally been called “interdisciplinarity” in transportation, but may alternatively be viewed as redefining the discipline to be transportation-centered.
— dml