This paper considers how two responses to congestion - modal shift and trip redistribution - might be incorporated into a variable trip matrix modelling approach to highway scheme appraisal. In December 1994 the Department of Transport accepted SACTRA's conclusion that induced traffic - flow changes resulting from behavioural responses other than reassignment - may have a significant impact for many more schemes than was previously thought. The Department commissioned studies to identify and evaluate alternative methods of including such responses in standard scheme assessment procedures. This paper discusses the findings of one study; these will contribute to TAM (Traffic Appraisal Manual) advice on best practice. Following a review of alternative model forms, the study focussed on issues that are likely to be of greatest interest for practitioners, and undertook wide-ranging tests on three "live" networks. Its main aims were to address the following questions: (1) Do alternative trip distribution and mode choice modelling approaches produce significantly different traffic forecasts from simpler fixed trip matrix or elastic assignment methods? (2) How much public transport data collection is really necessary? and (3) What validation standards are required for successful use of these methods? This paper summarises the findings of the study and draws some preliminary conclusions on the need to incorporate trip distribution and mode choice within the Department's standard highway scheme assessment procedures.
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