Guidance

Forthcoming change: updates to TAG uncertainty guidance

Updated 16 October 2025

Description: updates to the CAS data book, the TAG M4 unit, and the uncertainty toolkit, for clarity, to reflect the latest data, and to reflect future planned changes to the scenario guidance. Publication of a summary of the plan for refreshing the Department for Transport’s (DfT’s) scenario guidance.

Unit: common analytical scenarios data book, the TAG M4 unit and the uncertainty toolkit.

Change announced: October 2025

Expected release date: December 2025

Description 

This forthcoming change sets out updates to the common analytical scenarios (CAS) data book, the TAG M4 unit and the uncertainty toolkit, to reflect the latest data, to improve clarity and to pave the way for future planned updates. 

Alongside these updates, we will publish a high-level summary of our plan for refreshing the department’s scenario guidance. This will set out intended changes to guidance and the process we will use for the development of the refreshed guidance. 

The forthcoming changes announced in October 2025 will become definitive guidance in December 2025 in line with the TAG orderly release process. 

In December 2025, we will publish:  

• an updated CAS data book 
• an updated TAG M4 unit 
• an updated uncertainty toolkit 
• a summary of our plan for refreshing the department’s scenario guidance 

 After December 2025, at a later date, we will:  

• develop the refreshed scenario guidance for publication

Detail 

Changes to CAS data book 

Summary of changes 

The key changes are updates to some of the data sources for 5 of the 7 scenarios and some minor changes to the explanatory text. This update will provide users with more recent scenario inputs for use while our refreshed scenario guidance is developed, as well as correcting some minor errors and updating the text for clarity. 

High economy and low economy scenarios 

The data for these scenarios will be updated to reflect the latest forecasts and projections from the Office for Budgetary Responsibility (OBR) and the Office for National Statistics (ONS). This includes: 

• Population figures in sheets HE1 and LE1. These will be updated using, respectively, the high population and low population variants from the ONS 2022-based national population projections, released January 2025. This methodology is unchanged from previous use of CAS population figures. 
• Households figures in sheets HE2 and LE2. These will be based on the ONS 2022-based households projections due for release on 28 October 2025. These will be adjusted in line with the scenarios’ population figures. This methodology is unchanged from previous use of CAS households figures. 
• Employment figures in sheets HE3 and LE3. These will be based on the OBR medium-term employment forecast released in November 2025, along with the OBR’s long-term economic determinants released in June 2025. These employment forecasts are adjusted in line with the scenarios’ working-age population figures. This methodology is unchanged from previous CAS employment figures. 
• GDP figures in sheets HE4 and LE4. These will be based on the OBR medium-term GDP forecast released in November 2025, along with the OBR’s long-term economic determinants released in June 2025. GDP per capita growth is adjusted up/down by ±0.5 percentage points to give the high/low economy figures. This methodology is unchanged from previous CAS GDP figures.

Technology, vehicle-led decarbonisation and mode-balanced decarbonisation 

The data for these scenarios will be updated to reflect new fleet modelling produced by DfT analysts. 

The data updates include: 

• mileage splits in sheets T4, VL1 and MB1
• fuel efficiencies in sheets T5, VL2 and MB2
• fuel cost parameters in sheets T6, T7, VL3, VL4, MB3 and MB4

Changes to TAG M4 unit 

The key changes are to text and tables to improve clarity and understanding of the guidance, the addition of text to explain what has been reviewed and updated for this update, and changes which facilitate our plans to refresh the scenarios guidance. The most impactful changes are listed below:

• rather than use of the common analytical scenarios (CAS) being mandatory, the use of scenarios analysis will be mandatory – if users do not have their own scenarios, they will be expected to use the CAS as recommended in the guidance
• the addition of a note which explains that the 2025 update to the CAS data book will not be accompanied by new national transport model (NTM) runs for the updated scenarios and recommends a pragmatic approach using existing NTM outputs where appropriate
• the addition of a note which explains that, following a review, the guidance in appendix b on COVID-19 remains valid

Changes to the uncertainty toolkit 

This includes changes to the text and tables to improve clarity and understanding of the guidance, to align with the CAS 2025 data book and facilitate our plans to refresh the scenarios guidance. The most impactful changes are listed below:

• rather than use of the common analytical scenarios (CAS) being mandatory, the use of scenarios analysis will be mandatory – if users do not have their own scenarios, they will be expected to use the CAS as recommended in the guidance 
• table 9 has been replaced with text for improved clarity

Document summarising upcoming changes to scenario guidance 

Alongside the December 2025 update of the CAS data book and uncertainty guidance, we will be publishing a short summary of our scenario guidance refresh plans. This document will include context of why we are making the change, aims and objectives, and information on the change process.

Contact

For further information on this guidance update, please contact:

Transport Appraisal and Strategic Modelling (TASM) division
Department for Transport
Zone 2/25 Great Minster House
33 Horseferry Road
London
SW1P 4DR

tasm@dft.gov.uk