1. 3. W. Bunge, "The Location of Population, Demand, Purchasing Power and Services on Highways" (unpub. study, Dept. of Geog., Univ. of Washington, 1958 ).
2. 4. D. F. Marble, Symposium Discussion inProceedings of the IGU Symposium in Urban Geography Lund 1960(Lund Stud, in Geog., ser. B, Human Geography no. 24, Lund, 1962 ), p.159 .