This paper begins with a summary of the theoretical background to the question of fixed or variable trip matrices and briefly reviews current UK advice and practice. The main bulk of the paper is a review of published evidence as to whether network improvements do or do not affect trip matrices. The conclusion is that there is strong evidence of local land use changes, particularly adjacent to major new intersections or access points. However, it is clear that any new development is dependent on the existence of a capacity for relocation or growth in the local economy and a not unfavourable stance by the local planning authority. In the absence of new land development, however, there is little published evidence of changes in trip matrices following network improvements. The conclusion drawn from this evidence, from public interest in the question of generated traffic, and from the lack of reliable forecasting models, is that sensitivity analyses should be carried out to determine the possible effects of changes in the trip matrix before any decision is taken to ignore them
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