The varying proportions of different transport modes which may be used at any one time. The choices of modes may be determined by the costs, destinations, capacities, and frequencies of the modes together with the nature of the goods carried and their destinations; Buchanan et al. (2006) J. Transp. Geog. 14, 5 find that, in commuter behaviour, the key variable determining modal split and trip length was the distance the residence was located from the central business district. Titheridge and Hall (2006) J. Transp. Geog. 14, 1 find that location, urban form, levels of car ownership, and the mixture of different social classes in the ward of residence are important determinants of modal split.