• Daniels, J.D. and L.H. Radebaugh (2000) International Business: Environments and Operations, 9th Edition, New York: Prentice Hall.
• Ullman, E.L. (1956) "The Role of Transportation and the Bases for Interaction", in W.L. Thomas jr. et al. (eds) Man's Role in Changing the Face of the Earth, Chicago: University of Chicago Press.