Guidance

Forthcoming changes: uncertainty guidance updates

Updated 16 April 2026

Description: updates include:

• addition of an explanatory box to the uncertainty toolkit and TAG unit M4 containing information on interim arrangements for sharing advice on uncertainty analysis (including climate-related uncertainty), and clarification on the status of certain references within TAG guidance following recent updates
• changing how the exogenous demand growth estimator (EDGE) is accessed in TAG unit M4
• amendments to table 1, table 2 and section 3 of the uncertainty toolkit to improve clarity following user feedback
• changing text in section 3 of the uncertainty toolkit and section 4 of TAG unit M4 to remove errors (similar changes to be made across both, as the errors are the same)
• minor updates to correct typographical errors in the uncertainty toolkit

Unit: TAG M4 unit and the uncertainty toolkit

Change announced: April 2026

Expected release date: May 2026

Description

This forthcoming change introduces the temporary approach by which updates to uncertainty guidance will be communicated ahead of the full release of refreshed guidance. This includes interim climate-change-related advice and clarification on the status of certain references within TAG guidance following recent updates to uncertainty guidance. It does this via the addition of an explanatory textbox to the TAG M4 unit and the uncertainty toolkit.

It also reflects a change to how the exogenous demand growth estimator (EDGE) within TAG unit M4 will be made available.

In response to user feedback, the update additionally includes changes to table 1, table 2 and section 3 of the uncertainty toolkit to improve clarity and usability.

We are also changing some text in section 3 of the uncertainty toolkit to remove errors. It will be changed from:

The NRTP 22 [note 6] suggests a range using the CAS of a 50% increase or decrease in travel demand from the core scenario. While scenarios developed by schemes need to centre the core, they should use this as an approximate benchmark for appropriate stretch.

To:

Using the traffic growth rates across all the scenarios in the CAS as published in NRTP 22 [note 6] the typical lowest growth rate across all scenarios and years is around 50% less than the core. For scenarios with positive traffic growth, these values change with respect to time from 39% in 2030 [high scenario] to 149% in 2060 [technology scenario]. To ensure your scenarios have sufficient ambition and stretch, the growth rates in your scenarios should be compared to the growth rates in the core scenario. The growth rates in either traffic or trips should be at least plus or minus 50% of the growth rate in the core when considering the change from your base year to your forecast year. The balance of plus or minus 50% treats positive and negative scenarios equally and it should be noted these are minimum variations, thus enabling the widest choice of scenarios.

A similar edit will be made in TAG unit M4, to rectify the same errors.

Minor updates will also be made to correct typographical errors.

Detail

An explanatory textbox will be added to the TAG M4 unit and the uncertainty toolkit. This textbox will outline how updates to uncertainty guidance will be communicated ahead of the full release of refreshed uncertainty guidance.

The textbox will specify that interim updates – including advice on the application of uncertainty approaches, scenario development, and relevant use cases – will be shared via the Summary of upcoming changes to uncertainty scenario guidance.

Any additions, clarifications, or updated advice during the transition period to the refreshed guidance will be posted on this page to ensure users have access to the latest information before formal guidance is published.

The textbox will note that any guidance or resources shared this way are provided to support users and will not become definitive until the formal release of refreshed uncertainty guidance. Interim advice relating to climate-change considerations, reflecting paragraphs 8.85 and 8.86 in the recent Green Book update, will also be communicated via this page as it becomes available.

The textbox will also note that some references to TAG unit M4 and the uncertainty toolkit in other TAG units may be outdated and will provide advice on how users should navigate this during the transition period to refreshed guidance.

In addition, TAG unit M4 (footnote 5) will be updated to reflect a change in the way users can access the exogenous demand growth estimator (EDGE). EDGE itself will no longer be available to the public, but the department will continue to provide users with EDGE output on request.

Table 1, table 2 and section 3 of the uncertainty toolkit will be updated to provide clarifications and improve wording, ensuring the guidance is easier to follow and reflects feedback from users.

We are also changing some text in section 3 to remove errors. It will be changed from:

The NRTP 22 [note 6] suggests a range using the CAS of a 50% increase or decrease in travel demand from the core scenario. While scenarios developed by schemes need to centre the core, they should use this as an approximate benchmark for appropriate stretch.

To:

Using the traffic growth rates across all the scenarios in the CAS as published in NRTP 22 [note 6] the typical lowest growth rate across all scenarios and years is around 50% less than the core. For scenarios with positive traffic growth, these values change with respect to time from 39% in 2030 [high scenario] to 149% in 2060 [technology scenario]. To ensure your scenarios have sufficient ambition and stretch, the growth rates in your scenarios should be compared to the growth rates in the core scenario. The growth rates in either traffic or trips should be at least plus or minus 50% of the growth rate in the core when considering the change from your base year to your forecast year. The balance of plus or minus 50% treats positive and negative scenarios equally and it should be noted these are minimum variations, thus enabling the widest choice of scenarios.

Equivalent text in TAG unit M4 will also be updated to ensure consistency between TAG unit M4 and the uncertainty toolkit and accuracy in both documents.

Minor updates will also be made to the uncertainty toolkit to correct typographical errors.

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

Email: tasm@dft.gov.uk