Forthcoming change: appraisal updates
Updated 16 April 2026
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This publication is available at https://www.gov.uk/government/publications/tag-forthcoming-changes/forthcoming-change-appraisal-updates
Description: we are updating the economic and demographic parameters for appraisal data, and improving the way we communicate changes to users
Unit: TAG data book, TAG unit A3 (environmental impact appraisal)
Change announced: April 2026
Expected release date: May 2026
Updates to economic and demographic data
The TAG data book provides a range of economic and demographic datasets 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 Budget Responsibility (OBR).
A new version of the TAG data book (v2.03), to be released definitively in May 2026, will incorporate the latest 5-year medium-term forecasts from the OBR’s March 2026 economic and fiscal outlook (EFO), as well as 2025 outturn data from the ONS.
The figures from the ONS and the EFO will be used to update the parameters in the data book’s annual parameters tab, table A1.2.1 (cost inflation series) and table A5.3.1 (inflation measures and real average earnings index).
The OBR’s economic forecasts have changed minimally since November 2025, so we expect little impact from this update.
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
Clarification to values of time per person in the data book
In the notes section of table A1.3.1 (values of time per person) of the TAG data book, we will clarify that the values of travel time per person in that table are uplifted in future years at a constant 1.5% per annum (real terms) in appraisal. The note reflects current guidance in TAG unit A1.1.
Updating the marginal external cost (MEC) of congestion
As we announced in the road freight values of travel time forthcoming change, we are planning to update tables A5.4.2 and A5.4.4 (marginal external cost of congestion) in the data book, using updated NTM runs. While we were hoping to release these in May 2026, these changes are now delayed to allow for greater assurance around the updates.
Updates to air quality damage costs
The Department for Environment, Food and Rural Affairs (Defra) has updated the appraisal values of air quality damage. Some of these metrics are reported in table A3.2 (air quality) of the TAG data book.
The national central values (best estimates) for NOx and PM2.5 damage costs have increased by 9% and 34%, respectively. The updated damage costs reflect changes in the underlying methodology and inputs used to calculate them:
- emission and ambient measurement data
- the dispersion modelling for NOx and PM
- underlying trends in mortality rates, population and inflation
We are also updating the factors to convert PM10 emissions to PM2.5 emissions.
The tables below show the updated damage costs for NOx and PM2.5 (in 2023 prices and 2023 values) and the updated factors to convert PM10 emissions to PM2.5 emissions.
Table: damage cost values for NOx in £ per tonne
| Central value sensitivities | Low value sensitivities | High value sensitivities | |
|---|---|---|---|
| NOx national | 9,337 | 1,531 | 31,644 |
| NOx aircraft | 15,729 | 2,149 | 54,544 |
| NOx rail | 12,129 | 1,801 | 41,645 |
| NOx ships | 3,319 | 949 | 10,086 |
| NOx road transport | 12,486 | 1,836 | 42,926 |
| NOx road transport central London | 66,626 | 7,072 | 236,880 |
| NOx road transport inner London | 65,601 | 6,973 | 233,209 |
| NOx road transport outer London | 36,684 | 4,176 | 129,613 |
| NOx road transport inner conurbation | 26,497 | 3,190 | 93,118 |
| NOx road transport outer conurbation | 16,822 | 2,255 | 58,461 |
| NOx road transport urban big | 16,172 | 2,192 | 56,131 |
| NOx road transport urban large | 13,105 | 1,895 | 45,145 |
| NOx road transport urban medium | 10,653 | 1,658 | 36,360 |
| NOx road transport urban small | 9,171 | 1,515 | 31,051 |
| NOx road transport rural | 5,445 | 1,154 | 17,700 |
| NOx rail transport central London | 76,493 | 8,028 | 272,228 |
| NOx rail transport inner London | 73,820 | 7,770 | 262,649 |
| NOx rail transport outer London | 47,275 | 5,204 | 167,549 |
| NOx rail transport inner conurbation | 28,861 | 3,423 | 101,581 |
| NOx rail transport outer conurbation | 15,785 | 2,159 | 54,732 |
| NOx rail transport urban big | 16,556 | 2,234 | 57,490 |
| NOx rail transport urban large | 13,210 | 1,912 | 45,504 |
| NOx rail transport urban medium | 10,018 | 1,603 | 34,067 |
| NOx rail transport urban small | 8,122 | 1,421 | 27,273 |
| NOx rail transport rural | 5,459 | 1,164 | 17,729 |
Table: damage cost values for PM2.5 in £ per tonne
| Central value sensitivities | Low value sensitivities | High value sensitivities | |
|---|---|---|---|
| PM2.5 national | 104,799 | 36,663 | 340,431 |
| PM2.5 aircraft | 134,415 | 46,792 | 407,144 |
| PM2.5 rail | 102,366 | 35,440 | 285,142 |
| PM2.5 ships | 60,174 | 20,814 | 165,268 |
| PM2.5 road transport | 111,038 | 38,751 | 348,559 |
| PM2.5 road transport central London | 593,662 | 207,281 | 1,876,312 |
| PM2.5 road transport inner London | 575,727 | 201,136 | 1,834,442 |
| PM2.5 road transport outer London | 323,550 | 113,017 | 1,028,711 |
| PM2.5 road transport inner conurbation | 232,306 | 81,093 | 731,798 |
| PM2.5 road transport outer conurbation | 145,733 | 50,859 | 457,566 |
| PM2.5 road transport urban big | 140,026 | 48,874 | 440,333 |
| PM2.5 road transport urban large | 112,560 | 39,285 | 353,678 |
| PM2.5 road transport urban medium | 91,268 | 31,854 | 286,765 |
| PM2.5 road transport urban small | 77,857 | 27,172 | 244,497 |
| PM2.5 road transport rural | 45,059 | 15,721 | 140,982 |
| PM2.5 rail transport central London | 511,526 | 176,889 | 1,399,229 |
| PM2.5 rail transport inner London | 650,313 | 224,882 | 1,778,675 |
| PM2.5 rail transport outer London | 418,324 | 144,661 | 1,144,447 |
| PM2.5 rail transport inner conurbation | 243,780 | 84,301 | 666,841 |
| PM2.5 rail transport outer conurbation | 132,907 | 45,965 | 364,223 |
| PM2.5 rail transport urban big | 145,454 | 50,302 | 398,313 |
| PM2.5 rail transport urban large | 109,698 | 37,967 | 304,178 |
| PM2.5 rail transport urban medium | 81,946 | 28,349 | 225,554 |
| PM2.5 rail transport urban small | 67,154 | 23,256 | 188,015 |
| PM2.5 rail transport rural | 42,015 | 14,557 | 118,539 |
| PM10 to PM2.5 conversion factors | |
|---|---|
| National (all sectors) | 0.506 |
| Aircraft | 0.636 |
| Rail (RaT) | 0.888 |
| Ships | 0.947 |
| Road transport (RoT) | 0.564 |
| RoT central London | 0.551 |
| RoT inner London | 0.537 |
| RoT outer London | 0.541 |
| RoT inner conurbation | 0.557 |
| RoT outer conurbation | 0.563 |
| RoT urban big | 0.561 |
| RoT urban large | 0.562 |
| RoT urban medium | 0.562 |
| RoT urban small | 0.563 |
| RoT rural | 0.570 |
| RoT exhaust | 1.000 |
| RoT brake | 0.400 |
| RoT tyre | 0.710 |
| RoT road abrasion | 0.545 |
| RaT central London | 0.966 |
| RaT inner London | 0.966 |
| RaT outer London | 0.965 |
| RaT inner conurbation | 0.966 |
| RaT outer conurbation | 0.957 |
| RaT urban big | 0.961 |
| RaT urban large | 0.906 |
| RaT urban medium | 0.938 |
| RaT urban small | 0.868 |
| RaT rural | 0.839 |
We are also updating table A3.2.2 by presenting concentration based damage cost values for NOx and PM2.5 from table 7-9 of the research report underlying the damage cost update.
Table: costs per µgm-3 concentration change (2025 prices, 2025 values) – central damage costs sensitivity
| Primary pollutant | A) Direct impacts (£ per popwm 1 µgm-3 change per person per year) | B) Secondary PM2.5 impacts (£ per tonne) | C) Other impacts (£ per tonne) |
|---|---|---|---|
| NOx | 8.58 | 2,780 | -12 |
| PM2.5 | 65.95 | - | - |
We will add the following methodological note to accompany table A3.2.2:
Damage costs are typically expressed per tonne of emission. However, costs can also be expressed for many of the damage cost pathways per µgm-3, which is the unit of concentration. The costs per change in concentration need to be multiplied by the population weighted change in concentration, and the total population in the appraisal domain to estimate the total cost.
Not all impacts which make up the damage costs are calculated based on concentration exposure. As such, where the costs per change in concentration are deployed to estimate effects, analysts will also need to calculate the costs for additional ‘other impacts’ that are expressed per tonne. These ‘other impacts’ costs include pathways to assess environmental, productivity, and building impacts, and are separate to, and thus do not overlap with those included as ‘direct impacts’.
The ‘direct impacts’ (column A) and ‘other impacts’ (column C) cost estimates can be added together to generate the total cost estimate if the formation of secondary PM2.5 has been included in the air pollution modelling (A+C).
If the formation of secondary PM2.5 has not been included in the air pollution modelling, then the ‘direct impacts’ (column A), ‘secondary PM2.5 impacts’ (column B) and ‘other impacts’ (column C) cost estimates should be added together to generate the total cost estimate (A+B+C).
We will add the following sentence to paragraph 3.2.5 of TAG unit A3 (environmental impact appraisal):
If you use this table, please contact Defra at igcb@defra.gov.uk to receive further guidance.
Improving how we communicate changes to users
We are making it easier for users to understand what has changed in TAG after each publication cycle by:
- including links to the archived forthcoming changes when we email users about the definitive changes being available
- publishing online an overview spreadsheet with links to the previous versions of TAG
- ensuring the ‘provenance’ section of each TAG unit is kept up to date for any future changes
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