Traversing the Modelling Catwalk: Theoretical and Practical Challenges Associated with the Development of a Model Hierarchy
D Gilmour, I Pearce, Scottish Government, UK
The paper explores practical and theoretical issues in the development of a model hierarchy, with a focus on the potential distortion of scheme economic benefits arising from the treatment of strategic trips in demand and assignment stages of models.
This paper explores the practical and theoretical issues surrounding the development of a model hierarchy, with a focus on the potential distortion of scheme economic benefits arising from the treatment of strategic trips in demand and assignment stages of models and the use of a standard equity value of time.
Transport Scotland employs transport and traffic models in the appraisal and design of proposed transport interventions. The national transport model, TMfS:07 is a strategic model and is used in early appraisal stages of option generation, sifting and selection. National model outputs are used to inform a strategic business case for a programme or project. A key advantage of the national model is that projects can be appraised on a consistent basis, with a consistent model area, but the scale and distribution of project benefits can be obscured by model noise and by assumed changes to the context for appraisal or the transport system.
Lower tier, more detailed regional and local models are used for detailed project appraisal and design and draw on the higher tier national model for strategic demands. Lower tier models capture a wider range of transport user responses and, concomitantly, user benefits. However, for a number of reasons explored in this paper, the benefits of individual projects appraised using lower tier models are non-additive and rarely consistent with one another.
Furthermore, the results of higher and lower tier models for any one project may be inconsistent or even contradictory. The implications of this for appraisal and modelling is discussed, making recommendations to policy makers and model makers.
The issues are illustrated using outputs of Transport Scotland's LATIS modelling capability (national and regional models) together with experiences in traffic modelling.
Association for European Transport