Ibisch, P.L. Hoffmann, M. Selva, N., Pe’er, G. Kati, V. Biber-Freudenberger, L. DellaSala, D.A. Vale, M. Kreft, S., Hobson, P.R. A global map of roadless areas and their conservation status. Science 354(6318), 1423-1427.
http://science.sciencemag.org/content/354/6318/1423