WebTAG provides information on the role of transport modelling and appraisal.
This content contains guidance on the conduct of transport studies. The guidance includes or provides links to advice on how to:
- set objectives and identify problems
- develop potential solutions
- create a transport model for the appraisal of the alternative solutions
- how to conduct an appraisal which meets the department’s requirements
Projects or studies that require government approval are expected to make use of this guidance in a manner appropriate for that project or study. For projects or studies that do not require government approval, TAG should serve as a best practice guide.
The department endeavours to keep the guidance up-to-date in light of new evidence and developments in best practice modelling and appraisal methodologies. Changes are released through an orderly release process, giving users early sight of changes and when they are to be released definitively. Users working on longer-term business cases should refer to the proportionate update process when considering incorporating these changes into their analyses.
For users wishing to understand how the new structure relates to previous version of WebTAG a correspondence table (MS Excel Spreadsheet, 24.7KB) is available. An archived version of the previous WebTAG site is also available.
This overview provides general introductory information on the role of transport modelling and appraisal, and how the transport appraisal process supports the development of investment decisions to support a business case.
Guidance for the senior responsible officer (SRO)
This guidance is for project promoters and sponsors who manage and oversee those projects as a whole.
It provides knowledge on how the evidence derived from the transport appraisal process can be used to support the transport business case and inform the decision-making process. It introduces the high-level concepts of transport appraisal and modelling, the important outcomes required and the main risks to monitor and manage.
Guidance for the technical project manager (TPM)
This guidance is for technical managers that oversee the detailed modelling and appraisal work.
It provides more detailed knowledge on the key components of the transport appraisal process – including options development analyses and appraisal – describing how the concepts of transparency and proportionality should be applied. Overall, this should enable those managers to programme and manage the deliverables and resources more effectively. This identifies the main outputs expected in transport business cases and provides checklists of those expectations in more detail.
Transport appraisal process
This guidance gives a more detailed description of the transport appraisal process, including more detail on the 3-stage process of option development, further appraisal and implementation/evaluation.
Proportionate update process
This guidance provides an overview of the department’s process for updating guidance, when guidance is intended to become definitive and how analysts should go about adopting changes to guidance in a proportionate manner.
Guidance for the appraisal practitioner
These guidance documents inform practitioners of best practice in preparing the necessary outputs for the appraisal of transport schemes and policies.
A1 cost-benefit analysis
These guidance documents give advice on the principles of cost-benefit analysis in transport appraisal, the estimation of scheme costs and the calculation of direct impacts on transport users and providers.
- TAG unit A1-1 cost-benefit analysis, November 2014
- TAG unit A1-2 scheme costs, July 2017
- TAG unit A1-3 user and provider impacts, March 2017
A2 economic impacts
These guidance documents give advice on the appraisal of the wider impacts, regeneration impacts, and dependent development impacts, of transport interventions.
- TAG unit A2-1 wider impacts, January 2014
- TAG unit A2-2 regeneration impacts, January 2014
- TAG unit A2-3 transport appraisal in the context of dependent development, July 2016
Forthcoming changes to wider impact guidance released 18 December 2017.
A3 environmental impacts
This guidance provides advice on the appraisal of the environmental impacts of transport interventions. It deals with impacts on both the built and natural environment and on people. The environmental impacts covered in this manual are noise, air quality, greenhouse gases, landscape, townscape, the historic environment, biodiversity and the water environment. The guidance discusses the relationship between environmental impact appraisal (as set out in this unit) and environmental impact assessment and the need to tailor the level of appraisal to the stage of development of the proposal.
A4 social and distributional impacts
These guidance documents give advice on the appraisal of the social impacts and distributional impacts of transport interventions.
- TAG unit A4-1 social impact appraisal, November 2014
- TAG unit A4-2 distributional impact appraisal, December 2015
A5 uni-modal appraisal
These guidance documents give advice on issues specifically relating to cases where only one mode of transport is explicitly modelled. Guidance is provided for active modes (eg walking and cycling), aviation, rail and highway interventions and on the use of marginal external congestion costs to estimate decongestion benefits resulting from mode switch away from car use.
- TAG unit A5-1 active mode appraisal, January 2014
- TAG unit A5-2 aviation appraisal, December 2015
- TAG unit A5-3 rail appraisal, July 2017
- TAG unit A5-4 marginal external costs, July 2017
- TAG unit A5-5 highway appraisal, January 2014
Worksheets used to present evidence for each impact:
- Appraisal tables
- Economic impacts worksheets
- Environmental impacts worksheets
- Social and distributional impacts worksheets
Guidance for the modelling practitioner
This guidance informs practitioners of best practice in transport models that provide evidence for use in the appraisal of transport schemes and policies.
M1 modelling principles
This guidance provides an introduction to transport models, modelling data and forecasting. It aims to help practitioners understand the general principles of transport modelling and the department’s forecasting approach. It also identifies the main sources of transport data that are available to practitioners developing transport models and address the methods used for gathering data including survey methodology.
- TAG unit M1-1 principles of modelling and forecasting, January 2014
- TAG unit M1-2 data sources and surveys, January 2014
M2 demand modelling
This guidance provides advice on the development of variable demand models. This covers scoping the need for such models, their specification, development, calibration and validation, and ensuring fitness for purpose in forecasting.
M3 assignment modelling
These guidance documents give advice on developing, calibrating and validating highway and public transport assignment models. It provides advice on aggregate highway assignment models, for both general and specific purposes, which represent average conditions over the modelled period. It also provides detailed advice on methods for public transport passenger assignment models.
- TAG unit M3-1 highway assignment modelling, January 2014
- TAG unit M3-2 public transport assignment modelling, January 2014
This guidance provides advice on using transport models to prepare future forecasts of demand and supply. This includes guidance on the use of the department’s demand projections in the NTEM dataset. Furthermore, forecasting is, by nature, uncertain, and this unit gives guidance on how various sources of uncertainty should be taken into account throughout the forecasting process.
M5 advanced modelling techniques
This guidance provides advice on using advanced modelling techniques. These techniques include modelling the impacts of parking policies and park and ride schemes and incorporating the impacts of smarter choice initiatives in existing modelling tools.
- TAG unit M5-1 modelling parking and park-and-ride, January 2014
- TAG unit M5-2 modelling smarter choices, January 2014
WebTAG data book
The TAG data book includes all of the appraisal and modelling values referred to in the guidance in one workbook.
To support transport business case development, the department has produced a suite of tools to enable the appraisal of transport policy and interventions. The purpose of these tools is to ensure a consistent approach in transport appraisal, as well as reducing the cost of carrying out the analysis.
- TEMPRO (Trip End Model Presentation Program)
- DIADEM (Dynamic Integrated Assignment and DEmand Modelling)
- COBALT (COst and Benefit to Accidents - Light Touch)
- TUBA (Transport Users Benefit Appraisal)
This guidance provides further information on more specific areas of modelling and appraisal methods that WebTAG users may find useful.
- SI NTEM sub-models, July 2016
- SI mixed logit models, January 2014
- SI bespoke mode choice models, January 2014
- SI land use/transport interaction models, January 2014
The forthcoming changes document provides a list of all the approved alterations to guidance and values, where to find them and when they are planned to be released.
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Published: 29 October 2013
Updated: 18 December 2017
- Forthcoming changes to wider impact guidance released.
- TAG units A1-2, A5-3, A5-4, M4, TAG data book and 'Forthcoming changes' updated.
- TAG unit A1-3, TAG unit M2 and TAG data book updates.
- Updated forthcoming changes, unit 2-3 and SI NTEM sub models.
- Autumn 2014 updates released.
- First published.