Guidance

Forthcoming change: updates to economic and demographic data and clarified guidance on rail appraisal

Updated 17 October 2024

Description: updates to economic and demographic data, liquid fuel emissions factors and clarified guidance on rail appraisal

Unit: TAG data book, A5.3 (rail appraisal), active mode appraisal toolkit (AMAT) and appraisal workbooks

Change announced: October 2024

Expected release date: November 2024

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 TAG unit A5.3 rail appraisal. Additionally, the active mode appraisal toolkit (AMAT) and relevant appraisal workbooks will be aligned with the latest evidence and guidance in TAG.

All data book updates will become definitive in November 2024, after which an updated version of TUBA, COBALT and WITA 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 (v1.24), to be released definitively in November 2024, 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’ (expected to be released on 30 October 2024) will be incorporated into the TAG data book, subject to a review, while updated mid-year outturn population estimates from the ONS (released 15 July 2024) will also be included.

The most significant change in the OBR’s updated long-term forecasts was a change in population projection, from the ONS interim 2020-based principal population projection: year ending June 2022 estimated international migration variant (‘2020-based projection’) to the ONS interim 2021-based principal population projection (‘2021-based projection’). Driven solely by increased assumptions on future net migration (315,000 per year vs 245,000 per year previously), this results in a 7.1 million (9%) increase in total population by the end of the OBR’s forecast period (2074), compared with the 2020-based projection.

Population projections comparison

New core population projection vs current and previous core, high and low scenarios

This line chart shows 7 projections of total UK population over the period 2020 to 2100. From a single starting point of 67 million in 2020, the projections deviate in growth rate across the rest of the period. The proposed ‘TAG from November 2024’ projection is the second highest of the 7 projections by 2100, at around 85 million.

New core population projection vs current core, annual growth 2024 to 2100

Year	V1.23 annual growth (%pa)	V1.24 annual growth (%pa)
2024	0.85	0.85*
2025	0.60	0.60*
2026	0.51	0.51*
2027	0.52	0.52*
2028	0.52	0.52*
2029	0.31	0.51
2030	0.30	0.50
2031	0.28	0.48
2032	0.27	0.47
2033	0.26	0.45
2034	0.25	0.44
2035	0.24	0.43
2036	0.24	0.42
2037	0.23	0.41
2038	0.23	0.41
2039	0.23	0.40
2040	0.23	0.40
2041	0.23	0.39
2042	0.23	0.39
2043	0.23	0.38
2044	0.23	0.37
2045	0.22	0.37
2046	0.22	0.36
2047	0.21	0.35
2048	0.20	0.34
2049	0.19	0.32
2050	0.17	0.31
2051	0.16	0.30
2052	0.15	0.29
2053	0.13	0.28
2054	0.12	0.27
2055	0.11	0.26
2056	0.10	0.26
2057	0.09	0.25
2058	0.08	0.25
2059	0.08	0.25
2060	0.07	0.25
2061	0.07	0.25
2062	0.07	0.25
2063	0.07	0.25
2064	0.08	0.25
2065	0.08	0.25
2066	0.08	0.25
2067	0.09	0.25
2068	0.09	0.25
2069	0.09	0.25
2070	0.10	0.25
2071	0.10	0.24
2072	0.10	0.24
2073	0.10	0.24
2074	0.10	0.23
2075	0.10	0.22
2076	0.10	0.22
2077	0.10	0.21
2078	0.09	0.20
2079	0.09	0.19
2080	0.09	0.19
2081	0.08	0.18
2082	0.08	0.17
2083	0.07	0.16
2084	0.07	0.16
2085	0.06	0.15
2086	0.06	0.15
2087	0.05	0.14
2088	0.05	0.14
2089	0.05	0.14
2090	0.04	0.13
2091	0.04	0.13
2092	0.04	0.13
2093	0.04	0.13
2094	0.04	0.13
2095	0.03	0.13
2096	0.03	0.13
2097	0.03	0.13
2098	0.03	0.13
2099	0.03	0.13
2100	0.03	0.13

*subject to change following OBR ‘Economic and fiscal outlook’ publication

To maintain consistency with the projection that underpins the OBR’s economic forecasts, the 2021-based projection will be adopted into TAG as the new core projection. The updated population projection will be reflected in DfT’s demand driver growth (DDG) in due course. DfT’s National Trip End Model (NTEM) will incorporate updated population projections in future publications. Users are advised to make use of the Common Analytical Scenarios (CAS) to address uncertainty around future population and migration. The High Growth and Low Growth population series (based on the ONS 2018-based variant projections), that underpin the CAS High Economy and CAS Low Economy scenarios, remain unchanged.

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

Note the updated landscape monetisation workbook will also include an updated approach to discounting, discounting to the current default base year in TAG (2010). This is a correction to the current methodology, which discounts to the (locally) specified appraisal year.

Emissions factors

TAG unit A3 (environmental impact appraisal) advises that the impact of transport schemes on greenhouse gas emissions can be estimated by converting the estimated fuel/electricity consumption into carbon dioxide equivalent (CO2e) emissions using emissions factors published in the TAG data book (table A3.3).

The DfT Low Carbon Fuels team have calculated an updated set of emissions factors for liquid fuels (petrol, diesel and gas oil), for the period 2021 to 2100. This maintains the current methodology (as detailed in TAG unit A3) but updates the underlying forecast biofuel blend based upon the latest Renewable Transport Fuel Obligation (RTFO) policies. The result, which will be incorporated into v1.24 of the TAG data book, is marginally lower diesel emissions factors (~5% over long-run), while petrol and gas oil factors see changes of less than 1%. Electricity emissions factors remain unchanged.

Current vs new emissions factors for liquid fuels (KgCO2e/litre)

Year	Current factors – petrol	Current factors – diesel	Current factors – gas oil	New factors – petrol	New factors – diesel	New factors – gas oil	% difference – petrol	% difference – diesel	% difference – gas oil
2010	2.230	2.562	2.926	2.230	2.562	2.926	0.0%	0.0%	0.0%
2011	2.256	2.611	2.926	2.256	2.611	2.926	0.0%	0.0%	0.0%
2012	2.233	2.583	2.926	2.233	2.583	2.926	0.0%	0.0%	0.0%
2013	2.226	2.705	2.758	2.226	2.705	2.758	0.0%	0.0%	0.0%
2014	2.229	2.705	2.758	2.229	2.705	2.758	0.0%	0.0%	0.0%
2015	2.229	2.704	2.757	2.229	2.704	2.757	0.0%	0.0%	0.0%
2016	2.228	2.704	2.757	2.228	2.704	2.757	0.0%	0.0%	0.0%
2017	2.226	2.705	2.758	2.226	2.705	2.758	0.0%	0.0%	0.0%
2018	2.229	2.698	2.750	2.229	2.698	2.750	0.0%	0.0%	0.0%
2019	2.229	2.657	2.709	2.229	2.657	2.709	0.0%	0.0%	0.0%
2020	2.230	2.537	2.694	2.230	2.537	2.694	0.0%	0.0%	0.0%
2021	2.227	2.523	2.694	2.187	2.530	2.708	-1.8%	0.3%	0.5%
2022	2.110	2.530	2.694	2.136	2.503	2.703	1.2%	-1.1%	0.4%
2023	2.110	2.523	2.694	2.114	2.472	2.703	0.2%	-2.0%	0.4%
2024	2.110	2.519	2.694	2.114	2.462	2.703	0.2%	-2.2%	0.4%
2025	2.110	2.515	2.694	2.114	2.454	2.703	0.2%	-2.4%	0.4%
2026	2.110	2.514	2.694	2.114	2.443	2.703	0.2%	-2.8%	0.4%
2027	2.110	2.513	2.694	2.114	2.432	2.703	0.2%	-3.2%	0.4%
2028	2.110	2.512	2.694	2.114	2.421	2.703	0.2%	-3.6%	0.4%
2029	2.110	2.511	2.694	2.114	2.409	2.703	0.2%	-4.1%	0.4%
2030	2.110	2.510	2.694	2.114	2.398	2.703	0.2%	-4.5%	0.4%
2031	2.110	2.509	2.694	2.114	2.388	2.703	0.2%	-4.8%	0.4%
2032	2.110	2.509	2.694	2.114	2.376	2.703	0.2%	-5.3%	0.4%
2033	2.110	2.509	2.694	2.114	2.377	2.703	0.2%	-5.3%	0.4%
2034	2.110	2.509	2.694	2.114	2.378	2.703	0.2%	-5.2%	0.4%
2035	2.110	2.509	2.694	2.114	2.383	2.703	0.2%	-5.0%	0.4%
2036	2.110	2.509	2.694	2.114	2.382	2.703	0.2%	-5.0%	0.4%
2037	2.110	2.509	2.694	2.114	2.382	2.703	0.2%	-5.1%	0.4%
2038	2.110	2.509	2.694	2.114	2.381	2.703	0.2%	-5.1%	0.4%
2039	2.110	2.509	2.694	2.114	2.382	2.703	0.2%	-5.1%	0.4%
2040	2.110	2.509	2.694	2.114	2.382	2.703	0.2%	-5.0%	0.4%
2041	2.110	2.509	2.694	2.114	2.382	2.703	0.2%	-5.0%	0.4%
2042	2.110	2.509	2.694	2.114	2.383	2.703	0.2%	-5.0%	0.4%
2043	2.110	2.509	2.694	2.114	2.383	2.703	0.2%	-5.0%	0.4%
2044	2.110	2.509	2.694	2.114	2.385	2.703	0.2%	-4.9%	0.4%
2045	2.110	2.509	2.694	2.114	2.386	2.703	0.2%	-4.9%	0.4%
2046	2.110	2.509	2.694	2.114	2.387	2.703	0.2%	-4.8%	0.4%
2047	2.110	2.509	2.694	2.114	2.388	2.703	0.2%	-4.8%	0.4%
2048	2.110	2.509	2.694	2.114	2.389	2.703	0.2%	-4.8%	0.4%
2049	2.110	2.509	2.694	2.114	2.390	2.703	0.2%	-4.7%	0.4%
2050	2.110	2.509	2.694	2.114	2.392	2.703	0.2%	-4.7%	0.4%
2051-2100	2.110	2.509	2.694	2.113	2.374	2.700	0.2%	-5.4%	0.2%

Clarified guidance on application of optimism bias for rail projects

TAG unit A5.3 (rail appraisal) provides recommended uplifts for optimism bias, for both capital and operational expenditure. An updated version of unit A5.3, to be published in November 2024, will include clarification on the application of optimism bias to gross or net operational expenditure. The updated guidance will advise that the analyst should consider the importance of network-wide cost savings to the intervention in question in choosing whether to apply optimism bias to gross or net operating costs.

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