Nanaki, E.A., Koroneos, C.J., Roset, J., Susca, T., Christensen, T.H., De Gregorio Hurtado, S., Rybka, A., Kopitovic, J., Heidrich, O., López-Jiménez, P. Amparo, "Environmental assessment of 9 European public bus transportation systems", Sustainable Cities and Society, 2017