Introduction to Model Structures for Public Transport Schemes
TAG Unit 2.10.2

June 2005

Contents

1. Introduction

1.1 Objectives

2. Model Structure

2.1 Key Issues
2.2 Scheme Objectives and Impact on Modelling
2.3 Model Components
2.4 Resource Considerations

3. Further Information

4. References

5. Document Provenance

1. Introduction

1.1 Objectives

1.1.1 This TAG Unit provides introductory guidance on suitable model structures for major public transport schemes. It gives advice on:

• the necessary model components related to the objectives of the scheme;
• the selection of model elements where there is a choice;
• the way in which these components are structured; and
• the implications of resource constraints on model structure.

2. Model Structure

2.1 Key Issues

2.1.1 The three key issues that need to be understood when designing an appropriate model structure are:

• What it is that the model has to address. In general, the key issues are the changes in traveller behaviour, and hence in traffic flow, that the scheme is expected to produce, so that model components should be selected that can forecast those changes.
• What the component sub-models are that represent each of these behavioural responses and how these sub-models should link to other sub-models.
• Whether there are any time, data and money constraints, or lack of available skills. In many cases these may rule out some model components or even lead to the conclusion that it is not reasonable to develop a model.

2.2 Scheme Objectives and Impact on Modelling

2.2.1 Any modelling approach should be led first and foremost by the questions that need to be addressed. For more general demand forecasting tools the multitude of uses will generally lead to complex and expensive model forms, and supporting data collection. If such a model is available, its suitability in the light of the objectives and likely other consequences of the public transport scheme should be investigated.

2.2.2 If no suitable model is in existence the structure of a demand forecasting tool must be determined by the objectives and likely consequences of the scheme and by any potential for further use of a model in the near future. These objectives and consequences can vary widely, can be more or less difficult to quantify, and be mutually supportive or conflicting. The model will need to be able to address at least those objectives in the New Approach to Appraisal, and the Appraisal Summary Table:

• Economy
• Safety
• Environment
• Accessibility
• Integration

2.2.3 More detailed objectives of major public transport schemes include:

• Public transport performance
• Mode split
• Car congestion
• Accidents
• Reliability
• Quality
• Regeneration
• Access/attractiveness
• Pollution/noise

2.2.4 Public transport performance and the travel times experienced by travellers are at the heart of most public transport schemes. For very simple schemes in a single corridor it is sufficient to quantify travel times by simple processes that would not generally be described as a model, e.g. through explicit changes at route or individual stage level. For schemes with any degree of complexity it is necessary to set up a public transport network model and to make an assignment using that network. Such an assignment would yield the required travel times and other performance or level of service measures.

2.2.5 Mode split changes are an objective of many schemes. Typically, a model of mode split will take some resources to set up but its operation is straightforward. It will require input of public transport and alternative mode (e.g. highway) travel times and other costs, so in addition to a public transport network model, a network model for private transport modes will be necessary, unless the scheme is so simple that levels of service for the modes can be calculated without recourse to a network.

2.2.6 Congestion reduction by a public transport scheme will require a forecast to be made of a change in mode split, and subsequent decongestion impacts on the road. Hence, if this is one of the scheme objectives it is best to set up a highway network and calibrate and run capacity-restraint assignments on it, taking care to include the major contributors to delay, such as junctions. If decongestion is a major scheme objective, the coarseness of area-wide speed-flow curves is unlikely to do justice to the detailed issues that are relevant at junctions or where public transport and private vehicles share limited road space. The impact of decongestion on bus movements should be included in the model.

2.2.7 Accident reduction as a public transport scheme objective relies on a modal shift, and the resultant reduction in the volume of car traffic; therefore this objective requires the same modelling mechanisms for quantification as car congestion modelling.

2.2.8 Integration of the public transport systems is a benefit that is cited frequently, but it is rarely quantified and methods for quantifying integration are not immediately available. Approximate indicators may be obtained from public transport assignments (total number of transfers, wait and interchange time etc.).

2.2.9 Research interest in modelling reliability has been gaining momentum recently and a number of research teams have reported valuations of reliability changes. The two aspects requiring consideration are the valuation by travellers and the estimation (in the same terms) of reliability changes resulting from network or operational changes to the public transport system. Even before modelling reliability in detail becomes feasible with standard packages, the value of benefits resulting from increased reliability can be quantified separately.

2.2.10 If significant quality benefits are claimed as part of a scheme, then these should be quantified and valued on the basis of parameters determined from behavioural research in the target area or elsewhere.

2.2.11 Quantification of the impact of walk and cycle enhancements relies first on the quantification of the improvements that users of these modes might experience, after which a model could be used to predict increased use of the modes. Sometimes walk and cycle times and distances are skimmed from a highway network model, but alternatively the impacts of the public transport schemes on slow mode levels of service may be calculated externally. To incorporate the impact of any improvements on the pedestrian and cyclist modal share requires the inclusion of relevant variables in the mode choice model.

2.2.12 Quantification of success in meeting the regeneration objective is complex. Methods exist that estimate the impact of (public) transport improvements on land use development, but these require specialist skills, and significant resource inputs in terms of data, model development and application. The valuation of the benefits of land-use changes brought about by transport schemes is a separate issue, which has not been solved yet by the profession.

2.2.13 Accessibility models have been developed on behalf of the Department for Transport which can quantify changes in the accessibility to or from key locations. Alternatively, the accessibility measures which form the basis of land-use modelling could also represent accessibility. Both approaches require network models of both private and public transport as input.

2.2.14 Attractiveness may be related to accessibility but is not necessarily a description of the same effects. Attractiveness is likely to be a qualitative measure related to the physical design of facilities, similar to quality and equally difficult to quantify.

2.2.15 A quantified assessment of pollution and noise can be made on the basis of vehicle kilometres, using the same methods as are used for car congestion modelling. If external environmental models are used, these are fed by these same network models, making use of more local estimates of speeds, flow and traffic composition.

2.2.16 Other issues such as improved parking conditions and improved access for the mobility impaired do not lend themselves to conventional modelling in the present state of the art. Quantification may be possible on an ad-hoc and external basis.

2.2.17 The structure of a model for appraising a major public transport scheme could conceivably contain the following components:

• car ownership forecasting;
• trip end forecasting and trip frequency;
• transport and land use interactions;
• mode choice;
• destination choice;
• time-of-day choice (i.e., in principle, departure time choice);
• public transport service and route choice, including the potential use of access modes such as park-and-ride and the use of multiple public transport sub-modes; and
• highway route choice (i.e. assignment), together with a model of emissions when necessary.

2.2.18 In making a forecast of the situation in a future year under do-minimum or reference case assumptions, all eight stages could play a role. In forecasting the effects of a scheme, however, only the last five stages would normally come into play, and not all of these would be always necessary. Based on the scheme's objectives, it is advised the following will be needed:

Note: car ownership models, trip frequency (generation) and time of day models are likely to be required for reference case forecasting, but not specifically for assessing the scheme's impacts.

Footnote:
1. In this case the mode choice model and, particularly, the trip frequency model must reflect the trips made by ALL modes.

2.3 Model Components

2.3.1 Within each of the above components there are decisions to be made, in terms of their need, the form of model, and linkages. Each of the components is described in turn.

2.3.2 A choice can be made by the model developers between trip-based and tour-based approaches. In general, a tour-based approach can be considered to give higher quality representation of behaviour in several of the components of the model system, but they are, at the present time, generally restricted to large scale strategic models. If available they could be used to provide inputs to more locally based models.

2.3.3 For land use and transport interaction modelling it is recommended:

• that full land-use transport interaction modelling be undertaken only when it is such an important part of the justification of the scheme that the substantial cost and time requirements are justified;
• that the use of accessibility measures to inform expert judgement be considered as an alternative;
• that accessibility measures of the same type be used to quantify accessibility improvements when these are part of the case for a scheme.

2.3.4 In terms of car ownership forecasting it is recommended:

• that car ownership forecasting be included in the model system when forecasting over any lengthy period (say 5 years or more);
• that the DfT's TEMPRO forecasts be used for this purpose, unless there are good reasons to depart from these;
• that local adjustments are made if good quality local data are available;
• that if local data are used, a sensitivity test with TEMPRO data is also carried out.

2.3.5 For trip end forecasting and trip frequency forecasting it is recommended that:

• TEMPRO information should be considered first as the source for information on trip generation (use the all-modes figures);
• where possible total generations, i.e. including walk and cycle trips, should be predicted;
• induced traffic is unlikely to be required as part of the model unless very large improvements are made, and public transport carries a large share of total traffic;
• attractions can also be taken from TEMPRO;
• a local model of trip attractions, if required, should be integrated with the model of destination choice.

2.3.6 For models of mode and submode choice it is recommended that:

• proper linkages must be established between mode and destination choice models;
• linkages are more easily established if both models are, or are transformed to, the logit form;
• a logit form should also be used if it is necessary to model park-and-ride;
• the appropriate logsum variable should then be used to input from the park-and-ride model to the main mode choice model;
• the decision between choice modelling or assignment for the choice among public transport submodes depends on the local circumstances and software; when a choice model is used it should be of the logit form and a proper logsum linkage should be made with the main mode choice model;
• choice among public transport routes should be carried out using the assignment software and not in a separate choice model;
• the specification of composite cost for the models based on assignment is difficult and depends on software capability.

2.3.7 It is recommended for destination choice (distribution) modelling:

• that destination choice changes be forecast when a scheme is of major impact on accessibility or is appraised over a period of years;
• the connections of this model with the attractions and mode choice models need to be considered carefully to ensure consistency.

2.3.8 It is recommended for departure time or time-of-day modelling:

• that time-of-day choice should not form part of models for most current scheme appraisals;
• that matrices should be split over time periods in any case, so that a model may actually be required for base case forecasts;
• that if crowding is of importance, time-of-day choice modelling should be considered;
• time-of-day choice modelling should be considered if the scheme includes changing the differences between peak and off-peak fares;
• that where feasible in models designed for long-term use, the possibility should be retained for the future incorporation of this component.

2.3.9 For public transport assignment it is recommended that:

• Schedule-based methods are most appropriate for low frequency services, higher frequency services with high levels of passenger information, where station or service crowding is a serious issue, or where consistency with operational planning is essential; but run times can be high for large networks, whilst it can be difficult to forecast time tables for future years and alternative scenarios; and that economic appraisal will be sensitive to the precise timetable used (and therefore sensitivity testing to alternative timetables is recommended).
• Fares need not be included in the assignment as matrices of fares can be added to the generalised costs afterwards; where fares influence choice they should be included; and that travel pass owners for whom fares are not important may need assigning as a separate user class.
• Crowding should only be modelled where it is likely to have a significant effect on traveller behaviour or where an impact on crowding is one of the scheme's objectives; even where crowding is not modelled volume to capacity ratios should be monitored.
• For submodes that run on-street the journey times in the PT assignment model are consistent with the level of traffic congestion, which will require some linkage with the highway assignment model.
• The modeller should distinguish between the wait at the first stop and at subsequent stops, as the traveller has less control over arrival times at the latter; wait time can be estimated as half the headway for service headways up to 10-15 minutes, but for longer headways a wait curve should be applied; if services are irregular, half the mean headway will be an underestimate.
• The skimming of costs from the PT assignment model must be considered carefully to ensure consistency with the assumptions made to split flow between routes, particularly where a new route is introduced.
• Where biased networks need to be used to obtain consistency between the submode choice in the demand and assignment models, the amount of bias should be as little as possible, and properly recorded. Particular care is required for forecasting, where the same amount of bias must be retained.

2.3.10 For highway assignment it is recommended:

• that an explicit quantification of decongestion benefits should be made for every scheme where these benefits are claimed as part of the justification;
• that where a local assignment model exists, it, or an enhanced model, should be used;
• otherwise, a new assignment model should be set up to provide the required quantification;
• that the use of area-wide speed flow curves should be regarded as a departure from best practice and as such would require careful justification.

2.4 Resource Considerations

2.4.1 The main limitations imposed on modelling for major public transport scheme appraisal are those of the resources available to the analysis team:

• existing models,
• data,
• time,
• money and
• skills available.

Before commencing the model design for the appraisal of a major public transport scheme an inventory of existing models should be made; and that each model is assessed on the basis of: the structure of the overall model and its components; the age and quality of the underlying data; and the model's adherence to quality criteria for calibration and validation set out in Road Traffic and Public Transport Assignment Modelling (TAG 3.11.2).

2.4.2 Before any data collection is undertaken a thorough inventory should be made of existing data sources, their applicability and limitations; that if car ownership or trip frequency is a necessary component in the model structure, household interviews are considered; that stated preference surveys are considered only if new modes or new variables are part of the scheme under consideration; that SP surveys are always supported by RP data (existing or new); that en-route surveys are most suitable for the modelling of destination choice, and for base year matrix estimation; and finally, the data collection required for a study may also impose requirements in terms of time, money and skills.

2.4.3 Time and money constraints both may lead to a necessary reduction in model specification. Here, a reduced scope of modelling, e.g. omitting expensive components such as land use interaction modelling, is preferable over inaccurate modelling of specific model components. Naturally, the incompleteness of the model should be acknowledged, and this is easier done when clear elements are omitted.

2.4.4 Where skills requirements exceed those available to the study, it is prudent to restrict the ambition of the model system in terms of complexity, but not necessarily in terms of scope. Both the conduct of specialist surveys (such as stated preference) and the increased linkages where multiple aspects of behaviour are modelled require specialist input. Simpler model structures can at least quantify the scheme aspects considered relevant in the light of the scheme's objectives - but sensitivity testing will be required.

3. Further Information

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

4. References

None applicable.

5. 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