Allowing for Variable Demand in Highway Scheme Assessment



Allowing for Variable Demand in Highway Scheme Assessment

Authors

BATES J J, John Bates Services, COOMBE D, MVA Ltd, PORTERS, VLIET D, University of Leeds, UK

Description

In their seminal report (SACTRA, 1994) the UK Standing Advisory Committee on Trunk Road Assessment concluded that "induced traffic" was likely to occur as a result of road improvements, and recommended that:

Abstract

In their seminal report (SACTRA, 1994) the UK Standing Advisory Committee on Trunk Road Assessment concluded that "induced traffic" was likely to occur as a result of road improvements, and recommended that:

"...variable trip matrix economic evaluations are undertaken fir schemes as the cornerstone of the appraisal in every case, except where it can be shown that the trip matrix will not vary as a result of the scheme being appraised" (para 13.49).

While the main thrust of the Committee's recommendations was that, wherever feasible and cost-effective, the components of suppressed and induced traffic should be modelled individually, using some form of multi-modal transport model, it recognised that there are circumstances when simpler techniques would be appropriate (in small and medium-sized urban areas, for example), and that in the short term, simple elasticity models might be used (paragraphs 14.1 1 and 14.14).

In response to SACTRA's Report, the (then) Department of Transport has issued two versions of a Guidance Note: the second version (February 1997) is part of the Design Manual for Roads and Bridges (DMRB, Volume 12.2.2), and contains advice on the form and application of elasticity models.

Towards the end of 1997, the Department of the Environment, Transport and the Regions (DETR) commissioned a study on Improved Elasticities and Methods, in order to be sure whether, and if so how, elasticity values and methods should be further refined. It quickly became clear that it was desirable to move the emphasis of the research away from elasticity models and to tackle more general problems currently inhibiting other forms of Variable Demand modelling.

The study aims to set out the theoretical basis for the future development of Variable Demand methods. Since this has implications for the way that data is collected and how inputs are prepared, some aspects may have to be regarded as long term aspirations, rather than matters which can and should be changed immediately.

The main part of this Paper is concerned with an exploration of assignment-based variable demand modelling. In addition, it sets out an approach to forecasting the reference case and hence establishing a realistic do-minimum case. Given the restrictions of length, detailed mathematical proofs are not presented here. The later sections describe the testing process with which the project is currently occupied.

Publisher

Association for European Transport