Guidance

Summary of upcoming changes to uncertainty scenario guidance

Published 11 December 2025

This document summarises plans to refresh the Department for Transport’s (DfT’s) uncertainty scenario guidance. It explains why we are making changes, outlines our aims and provides information on the change process.

Why we are making the change

The Department for Transport’s transport analysis guidance (TAG) provides advice and tools for best practice to model and appraise transport schemes, drawing on evidence, research, industry expertise and other government guidance. It includes practical guidance and tools, such as an Uncertainty toolkit which looks at:

• documenting and communicating uncertainty
• different sources of uncertainty
• ways to test and quantify uncertainty

We want to ensure that uncertainty remains central to business cases and that schemes gain meaningful insights from the analysis they undertake and can use this to effectively communicate the uncertainty to decision makers.

We want decision makers to understand the choices they face in an uncertain future and feel confident that the analysis clarifies when schemes are a worthwhile investment.

To continue to achieve this, we need to understand:

• how the guidance works in practice
• what decision makers find most useful
• what users find is not working

We have completed some engagement exercises, and we regularly receive feedback, which we have reviewed. Based on the feedback we have received, wider contextual changes over the last 2 or 3 years and evolving best practice, we have decided to refresh our scenarios guidance.

Aims of making the changes

Our refreshed scenario guidance is an evolution. It keeps what works and focuses where improvements are needed most.

We want our scenario guidance to:

• be aligned with wider strategy, for example the Appraisal, Modelling and Evaluation Strategy (AMES), which is due to be published next year and the Integrated National Transport Strategy (INTS), which is due to be published early 2026
• evolve in response to feedback we have received, for example, feedback on how the common analytical scenarios^{[footnote 1]} (CAS) do not work for all users in their current format and feedback on how we could improve scenario analysis guidance
• be aligned with evolving best practice, which we want to better understand through work with academic experts
• be resilient to upcoming improvements to modelling capability and to the emergence of new and developing areas of uncertainty
• equip schemes to consider more bespoke and more relevant scenarios
• provide guidance on currently underexplored areas, such as active travel, new modes and freight

We will carry out extensive engagement before publishing the refreshed scenario guidance, using the TAG orderly release process. More detailed objectives for the refreshed guidance will be considered in due course.

Information on the change process

Short-term plans for the change process

Before we make any decisions on what the refreshed guidance will look like, we need some time to engage more fully with stakeholders to better understand:

• how the current guidance works well for them
• what they think could be improved
• what areas they believe are not sufficiently covered

There will then be some time required to transition to the refreshed guidance.

Feedback from users is that the 2022 CAS publication is no longer up to date because more input data has become available (for example, updated OBR forecasts). Therefore, we have published:

• a minimal CAS 2025 update
• an update to TAG unit M4
• an update to the Uncertainty toolkit

This will be the final time that we publish a set of common analytical scenarios in this format.

Medium-term plans for the change process

In the medium term, during the development of the refreshed guidance, we will ensure that our guidance is as effective as possible by:

• continuing to publish a core scenario, which will likely be defined as it is now^{[footnote 2]}, as well as guidance on some form of high and low scenarios – this is our current intention and details of where these will be published will be shared in due course
• considering how we could maintain the benefits of the CAS for users who find them helpful
• considering how uncertainty that has previously been part of the CAS can be developed further within the wider scenario guidance refresh
• considering how uncertainties which have not been included previously can be accounted for in the refreshed guidance
• thinking about the work we can do with users and academic experts during the guidance development process

Long term plans for the change process

We will implement the refreshed guidance as part of the TAG orderly release process and have published updates to the CAS databook to support users in the interim.

The development of the refreshed guidance will take place over the next few years and therefore, users will have ample time to shape and understand the changes before they become definitive.

The refreshed guidance is not the conclusion of our work on uncertainty scenarios. We will continue to welcome feedback and to iterate the guidance to support users.

Additional information on the updated guidance

Developing the refreshed guidance will take time. We want it to be as collaborative as possible and for users to be comfortable that, as much as possible, we are supporting them and hearing their feedback.

As this will not be a short process, there will be no immediate substantial changes to the scenario guidance. Changes will be communicated ahead of their implementation and there will be leniency in transition periods. We encourage users to get in touch, as they do now, if they have any questions about what is expected of them by the department.

We are aware there are risks to making changes to guidance and we will continue to try to predict and mitigate them, but we welcome feedback to help us do this.

In terms of uncertainty guidance beyond scenarios, we have made some changes in TAG unit M4 and the uncertainty toolkit to make the guidance clearer and easier to understand. These will be published alongside this document. Beyond that, we have no changes planned currently, but we are open to reviewing this in future.

All changes to guidance, at all stages of this process, will follow the TAG forthcoming change process.

Contact

For further information on this guidance update, please contact:

Transport Appraisal and Strategic Modelling (TASM) division
Department for Transport
Zone 1/3 Great Minster House
33 Horseferry Road
London
SW1P 4DR

tasm@dft.gov.uk

1. A set of 7 ‘off-the-shelf’ scenarios provided within our uncertainty guidance. ↩

2. See National Road Traffic Projections 2022 – the definition of the core scenarios is in paragraph 3.1 of the PDF. ↩