Edinburgh Napier University

  Sustainable transport is a malleable and changing concept which needs addressing before the questions of shortfalls in current modelling practice in addressing these issues can reasonably be assessed. It is tempting to take one of the simplest definitions available: In a global context any economic or social development should improve, not harm, the environment (Newman and Kenworthy, 1998). This does of course beg the question as to what comprises improvement and indeed what comprises harm, let alone the other interrelated issues not deemed to be either economic or social. Exploring the implied platform of assumed agreement on these issues is a productive and useful way of communicating what is at stake and why. Conflicting and inconsistent perspectives abound, as do assertions made in the spirit of hope rather than substantiable fact. It should be noted that transport, communications and mobility are rarely (if ever) used or provided for their own sake. This is the classic 'transport is a derived demand' stance, which evades such issues as travel tourism, leisure activities that involve exercise or scenery changes and even the revealed preferences of travellers. In communications the enormous response to text messaging as a means of interchange with intrinsic pleasure is demonstrated daily by younger and younger people. The pure derived demand stance can be adjusted to reconcile these and many other points, but it is more constructive to accept that aspects of mobility, communications and transport are indeed undertaken for their own sake in a range of areas. This is important, as the playing down of these three concepts as simply derived demands leads to the assumption that substitution is far more likely to occur than has yet proved to be thecase in practice. It also means that the assumptions on which modelling is based may not be entirely appropriate. There are many more examples where implied assumptions are made that affect what is built in to models of urban and activity systems. The scope of a model is determined by the scaleof representation, the complexity of the actors and processes, and the mechanisms deployed to represent interactions between these parties. In essence a model may be regarded from a top down view as a process of radical simplification to allow these interactions to be represented and exercised,and from a bottom up view where the entities modeled (people, vehicles, land use parcels, etc) are progressively represented in greater and greaterdetail. The top down view leads directly to scenario building to represent the interactions at the broadest level of different instruments. Any form of simplification and synthesis of models pertaining to sustainability will have strong influences from the cultural or disciplinary background(s)of the proponents, as differences that are not understood, valued, or appreciated are more readily collapsed if specialist understanding is not to hand.