Introduction to Forecasting Models for Public Transport Schemes
TAG Unit 2.10.1

June 2005

Contents

1. Introduction to Forecasting Models for Public Transport Schemes

1.1 Introduction
1.2 Purpose and Scope of the Advice
1.3 The Need to Relate the Modelling Approach to the Scheme Objectives and Appraisal Requirements
1.4 The Relationship to Other Advice on Transport Modelling
1.5 The Process of Modelling and Appraising a Public Transport Scheme
1.6 Documentation

2. Further Information

3. References

4. Document Provenance

1. Introduction to Forecasting Models for Public Transport Schemes

1.1 Introduction

1.1.1 This TAG Unit provides introductory guidance on the development and use of models suitable for forecasting the demand for major public transport schemes. This TAG Unit and Introduction to Model Structures for Public Transport Schemes (TAG Unit 2.10.2) provide advice aimed at ensuring that forecasts of the demands for new public transport schemes are made on a consistent and reliable basis and that the models used are appropriate to the type of scheme and its circumstances, and provide the information required by the appraisal. This Unit explains:

• the purpose and scope of the advice and the types of scheme to which the guidance is applicable;
• the need to relate the modelling approach to the scheme objectives and appraisal requirements;
• the relationship between the advice for modelling major public transport schemes and other advice on transport modelling;
• the process which the Department envisages will be followed in developing and using a model to appraise a public transport scheme; and
• the documentation requirements.

1.2 Purpose and Scope of the Advice

1.2.1 This advice should be applied to all major public transport schemes with a capital cost in excess of £5 million, which form part of a Local Transport Plan and for which funding, whether in part or whole, is sought from the Department for Transport.

1.2.2 This advice sets out the minimum specification of the models and forecasting procedures that the Department expects.

1.2.3 For heavy rail schemes, advice on modelling should be sought from the Strategic Rail Authority.

1.2.4 The term 'public transport model' is used in this advice to describe the whole modelling system, of demand and supply sub-models, used to appraise a public transport scheme. The term 'public transport (passenger) assignment model (or sub-model)' is used to describe the public transport supply sub-model of a public transport model.

1.2.5 The most appropriate modelling approach for any particular scheme will depend on the type of scheme, the circumstances, and its purpose or objectives. Different types of scheme may require different approaches and the same kinds of scheme in different contexts may require different approaches. However, the modelling effort for schemes needs to be related to the decisions it informs and should be proportionate to the scale of the scheme.

1.2.6 While this advice provides a minimum specification of the features which the Department expects the models to contain, it does not preclude the development and use of more sophisticated models. Where models that are more complex are used, they must be fully documented.

1.3 The Need to Relate the Modelling Approach to the Scheme Objectives and Appraisal Requirements

1.3.1 The modelling approach should be tailored to:

• the type of scheme, its context and its objectives; and
• the requirements of the appraisal.

1.3.2 The most appropriate modelling approach will depend on the type of scheme and its context. For example:

• a city-wide light rail system designed to attract car travellers will require a fully-specified, city-wide, multi-mode transport model;
• a single light rail line serving an airport will require a more sophisticated mode choice sub-model but may allow simpler techniques to be adopted for other aspects of the model; and
• a bus priority strategy aimed primarily at providing a better level of service for existing bus passengers with no affect on other modes may require only a public transport assignment model to provide the necessary inputs to a relatively simple appraisal.

1.3.3 In all cases the requirements of the appraisal should be borne in mind to ensure that the design of the model will output the information required by the appraisal process. All major public transport schemes need to be appraised in accordance with the New Approach to Appraisal.

1.3.4 Specific guidance on the appraisal of major public transport schemes can be found in:

• Major Schemes in Local Transport Plans (TAG Unit 1.4); and
• Detailed Guidance on Major Scheme Appraisal in Local Transport Plans (TAG Unit 3.9).

1.3.5 It is often tempting to use over-simplified modelling approaches, or models designed for other purposes, in the early stages of project development. However, great care should be taken to ensure that the indications from such models do not mislead.

1.3.6 Models can often take some time to develop to their full specification. The complete model should be specified at the outset of the work and consideration should be given to the order in which the various elements should be developed, taking account of, among other things, their usefulness for preliminary appraisals. For example, many schemes will derive the bulk of their patronage by extraction from existing public transport services. These effects can be estimated by a public transport assignment model and early investment in this element would provide some useful information about the potential viability of the proposal while other parts of the model are being developed.

1.4 The Relationship to Other Advice on Transport Modelling

1.4.1 This advice is concerned with models for the forecasting of demands for the use of major public transport schemes.

1.4.2 More general advice on transport models can be found in:

• Summary Advice on Modelling (TAG Unit 2.4) ; and
• Modelling (TAG Unit 3.1).

The advice in these Units on transport models may form a useful introduction for some readers. However, the advice in this introductory Unit and in The Introduction to Model Structures for Public Transport Schemes (TAG Unit 2.10.2) should be regarded as the more definitive for major public transport schemes.

1.4.3 Advice on models for the appraisal of highway schemes can be found in:

• Variable Demand Modelling - Overview (TAG Unit 2.9.1).

The advice on public transport modelling refers to this advice on variable demand modelling wherever the techniques are applicable to the modelling of public transport schemes.

1.5 The Process of Modelling and Appraising a Public Transport Scheme

1.5.1 The broad stages are:

• the model design stage, when the model structure is set out in general terms, along with the intentions under the topic headings given in the next section;
• the model validation stage, when the validation of the model is presented;
• the forecasting stage, when the detailed forecasting methodology, the results of the forecasts and associated sensitivity and scenario tests are presented; and
• the appraisal stage, when the appraisal assumptions and the results of the appraisal are presented.

1.6 Documentation

1.6.1 The documentation requirements are as follows.

1.6.2 The model design report should include:

• the structure of the model with an explanation as to why it is considered to be the most appropriate in the circumstances;
• the sources of the data required to establish the new model, including existing data and models and any proposals to collect new data;
• zoning systems and networks;
• details of the software to be used to create the model;
• an outline of the procedures for calibration and validation, along with target validation criteria;
• the forecasting parameters and the years for which forecasts are to be prepared; and
• a list of planned sensitivity and scenario tests, covering modelling, forecasting and policy issues.

1.6.3 The model validation report should include:

• a specification of the structure and details of the model, as developed, including a specification of the modelled area, zoning system and networks;
• a description of the assembly of the data required for the model, including any programmes of new data collection;
• a description of the matrix and network building processes and the validation of these elements of the model;
• a record of the software used to create the model;
• the calibration process adopted and the results of that process, including any estimated model parameters;
• the validation of the road traffic and public transport passenger assignment models against counts and travel times; and
• the validation of the overall model against empirical elasticities.

The forecasting report should include:

• the assumptions about and forecasts made of parameters which influence the underlying demand for travel, the costs of transport and the supply of transport;
• the resulting scenario forecasts;
• a specification of the public transport schemes tested;
• the resulting with-scheme forecasts and convergence criteria used where relevant; and
• the results of sensitivity tests of forecast parameters and assumptions, and scenario tests.

1.6.4 The contents of the appraisal report are specified in Detailed Guidance on Major Scheme Appraisal in Local Transport Plans (TAG Unit 3.9).

2. Further Information

The following documents provide information that follows on directly from the key topics covered in this TAG Unit.

3. References

None applicable.

4. Document Provenance

TAG Units 2.10 and 3.11 replace Major Scheme Appraisal in Local Transport Plans Part 3: Detailed Guidance on Forecasting Models for Major Public Transport Schemes.

Technical queries and comments on this TAG Unit should be referred to:

Integrated Transport Economics and Appraisal
Department for Transport
Zone 3/08, Great Minster House
76 Marsham Street
London SW1P 4DR

E-mail: itea@dft.gsi.gov.uk
Tel: 020 7944 6176
Fax: 020 7944 2198