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Description: updates to economic and demographic data, values of travel time and guidance on the forecasting and appraisal of rail performance impacts, clarified guidance on marginal external costs.

±«²Ô¾±³Ù:ÌýTAG data book, A5.3 (rail appraisal), A5.4 (marginal external costs), M4 (forecasting and uncertainty), active mode appraisal toolkit (AMAT) and appraisal workbooks.

Change announced: October 2025

Expected release date: December 2025

Description

This forthcoming change sets out updates to the TAG data book, to reflect the latest evidence and guidance for appraisal and modelling, plus minor clarifications to guidance on the forecasting and appraisal of rail performance impact and TAG unit A5.4 – marginal external costs. Additionally, the active mode appraisal toolkit (AMAT) and relevant appraisal workbooks will be aligned with the latest evidence and guidance in TAG.ÌýÌý

All data book updates will become definitive in December 2025, after which an updated version of TUBA,ÌýCOBALT ²¹²Ô»åÌýWITA will be made available to reflect the updated appraisal values.

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 (v2.02), to be released definitively in December 2025, will incorporate the latest long-term economic forecasts from the OBR’s fiscal risks and sustainability (FRS) report. Five-year medium-term forecasts from the OBR’s forthcoming economic and fiscal outlook (EFO) (expected to be released on 26 November 2025) will also be incorporated into the TAG data book, subject to a review.Ìý

The figures from the FRS and the EFO will be used to update the parameters in the data book’s annual parameters tab and table A5.3.1 (inflation measures and real average earnings index).Ìý

Furthermore, outturn household estimates for 2024 from the ONS (released 23 July 2025), and outturn estimates of the construction output prices index (COPI), covering the 12 months to March 2025, will be incorporated into the annual parameters and cost inflation tables, respectively. This data was unavailable at the time of the May data book release, and this update supersedes the previously published data, including the March 2024 to December 2024 average previously used in the interim for the 2024 to 2025 COPI.Ìý

The most significant change in the OBR’s updated long-term forecasts was a change in population projection, from the ONS interim 2021-based principal population projection (‘2021-based projection’) to the ONS 2022-based migration category variant population projection (‘2022-based projection’). This variant reflects the use of improved UK-wide migration data to inform a more accurate reflection of short-term population change in the UK. This results in a 2.4 million (3%) decrease in total population by the end of the OBR’s forecast period (2073), compared with the 2021-based projection (further detail in the population projections comparison).

To maintain consistency with the projection that underpins the OBR’s economic forecasts, the 2022-based projection will be adopted into TAG as the new core projection. The updated population projection is already reflected in the DfT’s demand driver growth (DDG), April 2025 release.ÌýÌý 

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 
• ²Ô´Ç¾±²õ±ðÌý
• noise (aviation) 
• active mode appraisal toolkit 
• cost proforma

Population projections comparison 

The 2022-based projection sits in the middle of the previous population projections that were included in the TAG updates in November 2023 and 2024. The 2022-based projection also sits at the centre of the common analytical scenarios (CAS) fan – the range of projections implied by the CAS high economy and CAS low economy scenarios, which are being updated.

Table: new core population projection vs current core, annual growth 2024 to 2100 

Year	V2.01 annual growth (%pa)	V2.02 annual growth (%pa)
2024	1.06	1.06*
2025	0.65	0.65*
2026	0.39	0.39*
2027	0.37	0.37*
2028	0.41	0.41*
2029	0.44	0.44*
2030	0.50	0.45
2031	0.48	0.43
2032	0.47	0.42
2033	0.45	0.41
2034	0.44	0.40
2035	0.43	0.39
2036	0.42	0.38
2037	0.41	0.37
2038	0.41	0.36
2039	0.40	0.35
2040	0.40	0.35
2041	0.39	0.34
2042	0.39	0.34
2043	0.38	0.33
2044	0.37	0.32
2045	0.37	0.32
2046	0.36	0.31
2047	0.35	0.30
2048	0.34	0.29
2049	0.32	0.28
2050	0.31	0.27
2051	0.30	0.25
2052	0.29	0.24
2053	0.28	0.22
2054	0.27	0.21
2055	0.26	0.20
2056	0.26	0.19
2057	0.25	0.18
2058	0.25	0.17
2059	0.25	0.16
2060	0.25	0.16
2061	0.25	0.16
2062	0.25	0.15
2063	0.25	0.15
2064	0.25	0.15
2065	0.25	0.15
2066	0.25	0.15
2067	0.25	0.15
2068	0.25	0.15
2069	0.25	0.15
2070	0.25	0.15
2071	0.24	0.15
2072	0.24	0.15
2073	0.24	0.14
2074	0.23	0.14
2075	0.22	0.13
2076	0.22	0.12
2077	0.21	0.11
2078	0.20	0.11
2079	0.19	0.10
2080	0.19	0.09
2081	0.18	0.08
2082	0.17	0.07
2083	0.16	0.06
2084	0.16	0.05
2085	0.15	0.05
2086	0.15	0.04
2087	0.14	0.03
2088	0.14	0.03
2089	0.14	0.02
2090	0.13	0.02
2091	0.13	0.01
2092	0.13	0.01
2093	0.13	0.01
2094	0.13	0.00
2095	0.13	0.00
2096	0.13	0.00
2097	0.13	0.00
2098	0.13	0.00
2099	0.13	-0.01
2100	0.13	-0.01

*subject to change following OBR economic and fiscal outlook publication

Values of travel time 

Table A1.3.1 of the TAG data book contains information on values of time for different modes, purposes and distances, while tables M2.1 and M2.2 contain values of time by income band for non-work and work purposes respectively.

An updated version of A1.3.1, to be included in v2.02 of the TAG data book, will see very minor corrections to all values derived from the 2014 to 2015 national study into the value of travel time savings and reliability. The data book now uses the study’s 2014 values, which are then adjusted depending on the price and value years chosen by the user. Up to v2.01 of the data book, the study’s 2014 values were first converted to a 2010 base and then integrated into the data book.

This approach will result in changes <0.2% to values and will ensure the figures remain robust to any revisions to outturn inflation statistics. We are notifying users of this minor change given the important role of values of travel time in modelling and appraisal.Ìý

Tables M2.1 and M2.2 will also be updated to reflect the use of 2014-based figures as source data. Furthermore, the income bands presented in both tables have been amended to move in line with the choice of user parameters in the data book, while 2 further corrections have been made to the values of time:

• the values presented in table M2.1 now relate to car VTT only
• the values presented in table M2.2 for public service vehicle passengers are now drawn from the 0 to 50km distance-band employers business segment, correcting a previous error

These changes lead to minor (<10%) changes to the values in M2.1, and more significant changes (40 to 70% decreases) to the PSV passenger values in M2.2. Annex b of TAG unit M2.1 – variable demand modelling will additionally be updated to reflect the clarifications to the income-banded tables.

Update to guidance on the forecasting and appraisal of performance impacts 

DfT’s approach to forecasting the demand impacts of changes in rail performance is based on the passenger demand forecasting handbook (PDFH), which has recently been updated.Ìý

So far, the relationship between changes in demand and changes in rail performance has been estimated through a standard elasticity framework, which relates demand changes to relative changes to performance. However, the latest research evidence shows that absolute changes to performance should be considered instead, as they represent a better fit to the data and the approach benefits from some practical implementation considerations.Ìý

The update to PDFH introduces a semi-elasticity approach, which estimates the relationship between changes in demand and absolute changes in rail performance. We have made minor corrections in TAG units A5.3 and M4 to recommend users refer to the updated version of PDFH (v6.1). Note that PDFH 6.1 values cannot be used to measure benefits per passenger and instead, the ratios in PDFH 5.1 should continue to be used for that purpose.

Clarified guidance on marginal external costs 

TAG unit A5.4 (marginal external costs) provides advice on the appraisal of decongestion benefits with the marginal external costs method. An updated version of unit A5.4, to be published in December 2025, will include a minor clarification to guidance on analysis relating to changes in car kilometres by time of day, or day of week.

Where this is the primary focus of the analysis or scheme, for instance, with rail interventions targeted at a particular time of day, the updated wording will recommend the use of tables A5.4.3 and A5.4.4 in the TAG data book. Analysis of average impacts across the week should use the values in table A5.4.2.

Contact

For further information on this guidance update, please contact:

tasm@dft.gov.uk