Guidance

Forthcoming change: appraisal data update

Updated 17 April 2025

© Crown copyright 2025

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This publication is available at https://www.gov.uk/government/publications/tag-forthcoming-changes/forthcoming-change-appraisal-data-update

Description: updates to economic and demographic data, non-exhaust emission factors, greenhouse gas valuations, and the introduction of guidance on large cost changes in appraisal.

Unit: TAG data book and A1.3 (user and provider impacts)

Change announced: April 2025

Expected release date: May 2025

Description

This forthcoming change sets out updates to TAG unit A1.3 and the TAG data book, to reflect the latest evidence and guidance for appraisal and modelling.

The forthcoming changes announced in April 2025 will become definitive guidance in May 2025, alongside the forthcoming changes announced in November 2024, in line with the TAG orderly release process. We do not intend to publish forthcoming change versions of any guidance units or the TAG data book to accompany the changes announced in April 2025.

Hence between the publication of this forthcoming change notice and end of May 2025, we will:

By the end of May 2025, we will:

  • publish all definitive guidance and a TAG data book which includes the changes set out in the November 2024 and April 2025 forthcoming change notices

After May 2025, we will:

  • update TUBA, COBALT, and WITA to reflect the latest changes

Detail

Economic and demographic data

The TAG data book displays a range of economic and demographic data sets used in the appraisal of transport interventions. To maintain the accuracy of appraisals, these data sets are updated in line with new releases of data from the Office for National Statistics (ONS) and Office for Budgetary Responsibility (OBR). A new version of the TAG data book (v2.01), to be released definitively in May 2025, will incorporate outturn estimates from the ONS, as well as minor changes to medium-term forecasts from the OBR‘s economic and fiscal outlook. These changes will be implemented in the annual parameters table of the data book as follows:

In table A1.2.1 of the data book as follows:

  • construction output prices index (COPI): updated for 2024 to 2025 using the ONS COPI series (if published prior to data book release)

And in table A5.3.1 of the data book as follows:

  • GDP deflator: updated for 2010/11 to 2024/25 using the ONS L8GG series, (if published prior to data book release) and for 2025/26 to 2029/30 using forecasts from the OBR EFO March 2025, published 26/03/2025

  • CPI, RPI: updated for 2010/11 to 2024/25 using the ONS D7BT and CHAW series respectively, published 16/04/2025 and for 2025/26 to 2029/30 using forecasts from the OBR EFO March 2025, published 26/03/2025

  • average earnings: updated for 2010/11 to 2029/30 using data from the OBR EFO March 2025, published 26/03/2025

Two changes are worth noting:

1. The ONS has changed the base year of outturn GDP-related values from 2019 to 2022. This is reflected in the statistics presented in the TAG data book, but it does not change annual growth rates, indices or forecast figures. v2.01 of the TAG data book will use 2022 as the default base year for outturn GDP figures, but (as with the current version), has an in-built function that lets users change the base year of historic GDP-related values if required.

2. As detailed above, the ONS has published 2022-based population projection variants (high and low growth scenarios), which have been included in the TAG data book v2.01. As per TAG unit A1.1, these are used to derive the extrapolation of demand beyond the final modelled year under the approach to uncertainty set out in TAG unit M4. That is, population growth is considered the key driver of travel demand, and population is assumed to grow at the same rate as the ONS ‘low population’ and ‘high population’ variant projections in the low and high growth scenarios respectively.

Those appraisal workbooks that utilise the annual parameters table for price adjustments, discounting and uprating will also be updated accordingly. The relevant workbooks are:

  • greenhouse gases
  • air quality
  • landscape monetisation
  • noise
  • noise (aviation)
  • active mode appraisal toolkit
  • cost proforma

Non-exhaust emission factors

TAG unit A3 advises on how to assess air quality impacts, including how to account for non-exhaust particulate matter (PM) emissions, such as those arising from road abrasion, brake wear and tyre wear. TAG data book tables A3.5d and A3.5e provide factors for non-exhaust PM emissions.

In October 2024, the Department for Transport published new research from Ricardo which revisited estimates of average non-exhaust particulate matter (PM) emissions by vehicle and road type. Since the publication of the previous non-exhaust emissions research in 2019 (PDF, 1MB), a greater number of ultra-low emission vehicles (ULEV) have entered the fleet, thus offering a wider database from which it was possible to more accurately estimate PM emissions from non-exhaust processes. We have updated the non-exhaust PM emission factors in the TAG data book to incorporate the findings of the report, as the best reflection of the current evidence base on these emissions.

Compared with the estimates in TAG data book v2.0, the figures for non-exhaust emissions in TAG data book v2.01 reflect:

  • a 2022 revision of the EMEP/EEA inventory guidebook – from which they are drawn – which accounted for the impact of different vehicle weights on non-exhaust emissions, and introduced specific non-exhaust emission factors for ULEVs, given the effect of regenerative braking on brake wear emissions
  • revised, lower estimates of average traffic speed on UK roads[footnote 1], which are associated with higher emission factors, these being speed-dependent

Below are the updated tables A3.5d and A3.5e. Please note, no new estimates for motorcycles were produced, and as such these factors remain unchanged in tables A3.5d and A3.5e.

Table A3.5d: non-exhaust PM10 emissions (g/km)

Road abrasion Tyre wear Brake wear
Cars 0.00754 0.00766 0.01274
LGVs 0.00750 0.01195 0.01806
HGVs 0.03800 0.03202 0.03311
Buses 0.03800 0.03255 0.04589

Table A3.5e: non-exhaust PM10 emissions by road type (g/km)

Road type Tyre wear Brake wear
Cars Urban 0.00903 0.01936
Rural 0.00732 0.01113
Motorway 0.00582 0.00387
LGVs Urban 0.01410 0.02780
Rural 0.01170 0.01690
Motorway 0.00970 0.00790
Rigid HGVs Urban 0.02120 0.05450
Rural 0.01900 0.03660
Motorway 0.01640 0.02470
Artic HGVs Urban 0.04810 0.05450
Rural 0.04170 0.03660
Motorway 0.03700 0.02470
Buses Urban 0.03490 0.05450
Rural 0.02990 0.03620
Motorway 0.02470 0.01680

Indexation of values of time per person

TAG data book table A1.3.1, which presents information on values of time (VoT) per person, has been corrected to uprate values using the appraisal VoT growth index, rather than the modelling index as previously. The appraisal index assumes a 1.5% annual growth after the current year, as opposed to the modelling index, which follows the forecast GDP per capita growth rate after the current year. This correction will have a very minor impact on the table figures, and does not feed through into other tables in the data book, nor TUBA software.

Guidance on large cost changes in appraisal

Guidance on appraising projects with large generalised cost changes was previously provided in the user manual of Transport User Benefits Appraisal (TUBA), the department’s appraisal software. To provide ease-of-reference with the supplementary guidance unit on the appraisal of new modes, this guidance has now been updated and moved to TAG unit 1.3 – user and provider impacts, as appendix d.

Traded carbon valuation

TAG unit A3 advises that scheme impacts on greenhouse gas (GHG) emissions should be valued for appraisal purposes using cross-government carbon values published in TAG data book table A3.4, and that where such emissions are under the scope of the UK Emissions Trading Scheme (UK ETS), the applicable appraisal values should be adjusted to exclude any proportion already accounted for within UK ETS allowance prices.

In December 2024, the Department for Energy Security and Net Zero (DESNZ) published updated traded carbon values for modelling purposes. In line with the recommendations in TAG unit A3 that adjustments required for the valuation of traded emissions are informed by these values – specifically the DESNZ market carbon values series for adjustments required for central/core analysis, and the DESNZ high and low sensitivity series for adjustments required for sensitivity tests – table A3.4.2 of the TAG data book (v2.01) and TUBA will be updated to reflect the latest DESNZ series.

The updated DESNZ modelling has led to moderate decreases across all 3 series, with larger decreases present in earlier years in particular. The updated series, and the factors driving these changes are detailed in the DESNZ publication. All TAG guidance on the use of these series remains unchanged.

Addition of CORSIA price series to TAG data book

TAG unit A5.2 contains guidance on the appraisal of GHG emissions arising from the aviation sector, including accounting for emissions which fall within different carbon pricing schemes, such as the Carbon Offsetting and Reduction Scheme for International Aviation (CORSIA). DfT previously published future CORSIA price assumptions for modelling purposes in the Jet Zero strategy further technical consultation – annex b. DfT has now updated these price assumptions and will publish them in a new table A5.2.1 of the TAG data book (v2.01). TAG unit A5.2 (aviation appraisal) will be updated accordingly

These assumptions are designed to illustrate the potential range of carbon prices faced by airline operators on routes in scope of CORSIA in the future for use in scenario analysis. The assumptions do not represent the UK government’s view on the most likely evolution of market prices under the scheme.

DfT has utilised the Department of Energy Security and Net Zero (DESNZ) Global Carbon Finance (GloCaf) model and evidence on the current price of CORSIA credits to develop 3 series of prices. As summarised in the table below, these price series make alternative use assumptions about the level of abatement in: 1) the global International aviation sector and 2) all other non-international aviation global sectors.

CORSIA price series assumptions

Scenario GloCaf regions International aviation assumptions
Low – used in DfT’s ‘current trends’ aviation scenario Stated emission policies – the least cost abatement to achieve industry emissions target corresponding to IEA WEO 2024 STEPs scenario 85% of 2019 emissions until 2050
Medium – used in DfT’s ‘high ambition’ aviation scenario The median abatement required to remain below 2°C temperature increases by 2100, as per the Paris agreement (IPCC scenario C3a) 85% of 2019 emissions until 2036 then interpolate to NZ 2050
High – not currently used in DfT’s aviation scenario The median abatement required to remain below 1.5°C temperature increase by 2100, with low overshoot, as per the Paris agreement (IPCC scenario C1a) 85% of 2019 emissions until 2031 then interpolate to NZ 2050

These prices supersede the prices in the Jet Zero strategy further technical consultation and are being used within DfT modelling from the publication of this forthcoming change notice.

CORSIA price series (£ per tonne of CO2e; 2023 prices)

Factor cost unit of account – low Factor cost unit of account – medium Factor cost unit of account – high Market price unit of account – low Market price unit of account – medium Market price unit of account – high
2024 16.56 16.56 16.56 19.71 19.71 19.71
2025 17.14 17.14 17.14 20.40 20.40 20.40
2026 17.73 26.33 49.11 21.10 31.33 58.44
2027 18.31 35.51 81.09 21.79 42.26 96.50
2028 18.90 44.70 113.06 22.49 53.19 134.54
2029 19.49 53.89 145.03 23.19 64.13 172.59
2030 20.07 63.07 177.00 23.88 75.05 210.63
2031 20.66 72.26 190.40 24.59 85.99 226.58
2032 21.24 81.44 203.80 25.28 96.91 242.52
2033 21.83 90.63 217.20 25.98 107.85 258.47
2034 22.41 99.81 230.60 26.67 118.77 274.41
2035 23.00 109.00 244.00 27.37 129.71 290.36
2036 24.00 114.20 251.80 28.56 135.90 299.64
2037 25.00 119.40 259.60 29.75 142.09 308.92
2038 26.00 124.60 267.40 30.94 148.27 318.21
2039 27.00 129.80 275.20 32.13 154.46 327.49
2040 28.00 135.00 283.00 33.32 160.65 336.77
2041 28.10 139.90 292.60 33.44 166.48 348.19
2042 28.20 144.80 302.20 33.56 172.31 359.62
2043 28.30 149.70 311.80 33.68 178.14 371.04
2044 28.40 154.60 321.40 33.80 183.97 382.47
2045 28.50 159.50 331.00 33.92 189.81 393.89
2046 28.60 164.40 340.60 34.03 195.64 405.31
2047 28.70 169.30 350.20 34.15 201.47 416.74
2048 28.80 174.20 359.80 34.27 207.30 428.16
2049 28.90 179.10 369.40 34.39 213.13 439.59
2050 29.00 184.00 379.00 34.51 218.96 451.01

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

  1. Produced for the National Atmospheric Emission Inventory 2020 ↩

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