Ignoring undesirable outputs: This refers to the traditional DEA model which neglects undesirable outputs (Nakashima et al, 2006; Lu and Lo, 2007a, 2007b). However, nowadays it is not only inappropriate to ignore the presence of CO2 emissions during transportation, but this could result in erroneous decisions, since undesirable and desirable outputs are produced at the same time. As the undesirable output is of a different, that is ‘negative’, nature, it should be clearly distinguished in modelling the problem.