Guidance

Forthcoming change: vehicle emissions carbon tool

Updated 16 October 2025

Description: vehicle emissions carbon tool 

Unit: A3 environmental impact appraisal 

Change announced: October 2025 

Expected release date: December 2025 

Description 

This forthcoming change presents the newly developed vehicle emissions carbon tool (VECAT), which has been designed to assess road user carbon. Road user carbon is a significant contributor to whole-life carbon and carbon assessment is an impact that is routinely appraised in business cases submitted to the Department for Transport (DfT) due to the government commitment to achieve net zero targets. 

The reason for the development of the tool was to improve evidence on the carbon impact of transport policies and interventions. While there are existing tools that can be used, there are issues associated with those, as summarised below. 

Alongside the introduction of VECAT, changes are made to the A3 unit. Previously, the transport users benefit appraisal (TUBA) software was the only carbon assessment tool specifically mentioned in TAG, which could result in an incorrect perception that this is the sole tool endorsed by DfT to be used for this purpose. Unit A3 will now also refer to VECAT as the preferred tool and the Department for Environment, Food & Rural Affairs’ emissions factors toolkit (EFT).

Unit A3 will also explain the difference between matrix-based (such as TUBA) and link-based (such as VECAT and EFT) methods and clarify that link-based methods are considered more robust. Unit A3 will not mandate any specific software for road user carbon assessment, but will clearly signal the advantages of link-based approaches, which are generally preferred to matrix-based carbon assessment. 

Detail 

While the TAG data book provides formulae and parameters for calculating road carbon emissions, until now, there has been no publicly available, link-based tool or software that is consistent with TAG to automate the road user carbon assessment over the appraisal period.

Users would need to apply these formulae and parameters manually or develop their own tools, leading to inconsistencies and inefficiencies across the industry. Alternatively, they could have used matrix-based TUBA, but this is less robust (for road user carbon emissions assessment) than link-based methods. 

VECAT follows the TAG and the TAG data book methodology and parameters, following link-by-link calculation of emissions. VECAT uses the same inputs as TAG data book formulae (flows and speeds). 

The tool will be regularly updated in accordance with the TAG orderly release process to be consistent with the latest TAG data book parameters regarding vehicle-kilometres splits to petrol/diesel/electric (primary) source of energy, fuel/energy consumption parameters, emissions per litre of fuel or kWh electricity used and minimum and maximum speeds.  

Additionally, it also allows the use of common analytical scenarios (CAS) technology and decarbonisation parameters instead of TAG core parameters. (Other CAS scenarios use the same parameters as TAG core.) Users also have the option to provide their own parameters where appropriate. 

VECAT’s inputs are flows and speeds for each combination of modelled year, period without scheme and period with a scheme. These inputs are normally supplied by a traffic model. 

VECAT carries out the following process for each combination of time period, with and without case and year. 

Chart: how VECAT transforms inputs

The diagram shows how VECAT transforms inputs and what measurement units are used at each stage. Inputs and units are as follows:

• link flow multiplied by link distance, given in vehicle kilometres
• vehicle kilometres split by fleet mix, given in vehicle kilometres by petrol, diesel, and electric
• fuel consumption factors applied by fuel type, given in litres of fuel per kilowatt hour of energy per kilometre
• emission factors applied by fuel type, given in kilogrammes of CO2e emissions per litre of fuel burnt or kilowatt hour used
• after an annualization factor is applied, carbon emissions by link, given in annual kilogrammes of CO2e emissions per litre of fuel burnt or kilowatt hour used

The main outputs from VECAT are annual traded and non-traded net carbon emissions (user carbon emissions, which are the result of a scheme or policy implementation). These are produced in an identical format to the input sheet in the TAG greenhouse gases workbook, so the data can be copied and pasted in.

VECAT is supported by the user guide, which explains in detail how VECAT should be used and what assumptions are made. 

VECAT is registered on the list of DfT’s business-critical models. 

VECAT’s key features are summarised below: 

• uses DfT Excel templates and standard structure 
• allows up to 10 future (model) years 
• allows up to 10 time periods 
• allows up to 100 years appraisal 
• if default setting is used, then no change in flows and speeds is assumed after the last modelled year – this approach is consistent with TUBA
• calculates emissions in each year based on interpolated and extrapolated flows and speeds and emission parameters relevant for the particular year 
• designed to consider 8,760 hours a year 
• user classes car, LGV, HGV, PSV (consistent with TAG data book)

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