Minor road traffic estimates review: frequently asked questions
Published 28 September 2022
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This publication is available at https://www.gov.uk/government/statistics/road-traffic-estimates-in-great-britain-2021/minor-road-traffic-estimates-review-frequently-asked-questions
Previously published minor road traffic estimates have been under review with the aim of identifying areas that would improve robustness and consistency of the historic time series. As a result, annual minor road traffic estimates have been revised from 2000 to 2020 in the ‘Road traffic estimates in Great Britain: 2021’ publication.
The ‘Minor road traffic estimates review: technical report’, available on the road traffic statistics information page provides details of historic revisions made and the impact on minor road traffic estimates. This ‘Frequently asked questions’ document supplements the technical report and answers further questions of interest.
1. Why are previously published minor road traffic estimates being revised?
The Department for Transport (DfT) continuously reviews methodology for calculating road traffic estimates. Part of the minor road traffic estimates review aimed to identify areas that would improve robustness and consistency of the historic time series. Minor road traffic estimates have been revised based on the findings of this investigation.
2. Which data are affected by the review?
This historic review has revised minor road traffic estimates in Great Britain from 2000 to 2020. The changes do not affect traffic estimates for major roads (motorways and ‘A’ roads) but do result in changes to overall traffic estimates on ‘all roads’ for these years. Minor road traffic estimates have been revised for both annual road traffic statistics and provisional quarterly statistics.
Traffic estimates are used in other analyses and statistics across government, for example road safety statistics. Revising the minor road traffic series may therefore have an impact on other data series. Further information on individual teams’ plans on republishing any estimates that use the traffic data can be provided from the respective teams.
3. What has changed as a result of this review?
The DfT produces annual estimates of traffic on minor roads by undertaking traffic counts at a fixed sample of points each year. Although retaining a fixed sample ensures a more precise measure of traffic change between consecutive years, it also means that any errors in the sample will accumulate over time. In order to correct for any sampling errors, a larger benchmark sample is taken every decade, this allows for the recalibration of the traffic estimates on minor roads.
The overall approach to producing minor road traffic estimates has not been changed as a result of the review. However, the methodology used to calculate minor road traffic estimates from the minor road benchmarking exercises in 2009 and 2019 has been revised following investigations into areas for improvements. One of the key areas of improvement in the benchmark methodology was the use of GPS data to stratify the benchmark sample into ‘low’ and ‘high’ traffic flow locations. There have also been improvements made to the annual methodology for calculating minor road traffic estimates from 2000 to 2020. Details of the improvements and the revisions are given in the technical report. The overall revision for minor road traffic in Great Britain has been a decrease of 0.8% in 2009 and a decrease of 13.2% in 2019. Table 1 shows the impact of the revisions compared to previously published figures after applying the revisions.
Table 1: Estimated minor road traffic (billion vehicle miles) for Great Britain before and after the minor roads review, 2000 to 2020
| Year | Before review | After review | Revision to estimate |
|---|---|---|---|
| 2000 | 103.2 | 103.1 | -0.1% |
| 2001 | 103.6 | 103.4 | -0.2% |
| 2002 | 107.2 | 106.8 | -0.3% |
| 2003 | 107.3 | 106.9 | -0.3% |
| 2004 | 107.7 | 106.4 | -1.1% |
| 2005 | 107.9 | 106.9 | -1.0% |
| 2006 | 109.1 | 108.1 | -0.9% |
| 2007 | 111.8 | 111.1 | -0.7% |
| 2008 | 110.3 | 109.8 | -0.5% |
| 2009 | 108.1 | 107.3 | -0.8% |
| 2010 | 108.4 | 105.8 | -2.4% |
| 2011 | 109.4 | 105.5 | -3.6% |
| 2012 | 110.8 | 106.4 | -4.0% |
| 2013 | 112.7 | 106.6 | -5.4% |
| 2014 | 118.9 | 111.2 | -6.5% |
| 2015 | 122.1 | 112.9 | -7.5% |
| 2016 | 125.6 | 115.4 | -8.2% |
| 2017 | 130.1 | 117.4 | -9.7% |
| 2018 | 131.9 | 116.6 | -11.6% |
| 2019 | 135.8 | 117.9 | -13.2% |
| 2020 | 112.4 | 97.8 | -13.0% |
4. Does this mean that previously published figures are incorrect?
The figures published previously were the best estimate at the time of publication. All traffic figures are published as estimates, it is not feasible to get a precise measure of traffic as this would require every road in Great Britain to be sampled every day of the year. The revised figures are still estimates of minor road traffic, now based on a more consistent and more robust back series.
As a result of this review the figures published in ‘Road traffic estimates in Great Britain: 2021’ will replace previously published estimates.
5. How do the revised minor road traffic estimates compare to those published before the most recent minor road benchmarking exercise?
Chart 1 shows the minor road traffic estimates for Great Britain, before and after the revisions applied from this review, as well as the estimates published before the 2019 minor road benchmarking exercise. This shows the impact of the high adjustment that the original 2019 benchmarking exercise had on previously published minor road traffic estimates. Comparatively, the newly revised estimates show a lesser increase in minor road traffic.
Chart 1: Estimated minor road traffic (billion vehicle miles) in Great Britain before and after applying historic revisions 1993 to 2020, and before the 2019 benchmarking exercise 1993 to 2018
6. How have these revisions affected total road traffic in Great Britain?
The minor road traffic estimate for Great Britain in 2019 has decreased by 13.2% as a result of this review. Minor road traffic accounts for approximately one third of all traffic in Great Britain. The traffic estimate for all roads in Great Britain for 2019 has decreased by 5.0% as a result of the review. As traffic on minor roads accounts for a smaller proportion than major roads, the overall impact is less.
Chart 2: Estimated traffic (billion vehicle miles) on minor roads and on all roads in Great Britain before and after applying historic revisions, 1993 to 2020
7. How have these revisions affected year on year trends?
The revision to the minor road traffic estimates also impacts the published year on year changes in traffic. Looking at the most recent pre-pandemic estimates, before the revision the increase in minor road traffic from 2018 to 2019 was 2.9% and after the revision was 1.1%. The impact is smaller for traffic on all roads, with a 2.0% increase from 2018 to 2019 before the revision and a 1.3% increase after.
8. Did the size of revisions vary between regions?
The level of revision to the previously published minor road traffic estimates varied by region, and over time. Within England, the level of revision to the 2019 estimates varied between 0.3% in the South West and 25.3% in the North West. The main reason for the regional differences in revisions is the introduction of the stratification by flow group and further details are provided in the technical report.
Table 2: Estimated minor road traffic (billion vehicle miles) by region before and after the minor roads review, 2009 and 2019
| Region/Country | 2009 estimate before review (bvm) | 2009 estimate after review (bvm) | Revision to 2009 estimate | 2019 estimate before review (bvm) | 2019 estimate after review (bvm) | Revision to 2019 estimate |
|---|---|---|---|---|---|---|
| East Midlands | 8.7 | 9.1 | 4.1% | 10.7 | 10.4 | -3.5% |
| East of England | 13 | 12.7 | -1.9% | 15.3 | 13.1 | -14.5% |
| London | 6.6 | 8 | 21.6% | 10.4 | 8 | -22.7% |
| North East | 4.5 | 4.2 | -6.2% | 5.8 | 4.8 | -17.4% |
| North West | 10.9 | 10.4 | -4.6% | 15.4 | 11.5 | -25.3% |
| South East | 17.1 | 15.5 | -9.4% | 19.7 | 17 | -14.1% |
| South West | 11.8 | 12.6 | 7.2% | 14.5 | 14.5 | -0.3% |
| West Midlands | 10.9 | 10.8 | -1.0% | 13.2 | 11.6 | -12.6% |
| Yorkshire and The Humber | 9.1 | 9.2 | 0.4% | 12.9 | 10.2 | -21.1% |
| England | 92.6 | 92.6 | -0.1% | 118.0 | 100.9 | -14.5% |
| Scotland | 9.3 | 8.9 | -4.3% | 10.1 | 10.1 | 0.0% |
| Wales | 6.1 | 5.8 | -5.6% | 7.8 | 6.9 | -11.2% |
| Great Britain | 108.1 | 107.3 | -0.8% | 135.8 | 117.9 | -13.2% |
Note that Scotland was not included in the 2009 benchmark exercise. The available GPS dataset, which is the basis of the most significant methodological changes introduced in this review to the 2009 and 2019 benchmark methodologies, only holds data for roads in England and Wales. Therefore, the revisions to minor road traffic estimates for Scotland are only based on the improvements detailed in the ‘Consistency of annual methodology’ section of the technical report.
9. Did the size of revisions vary between different road categories?
The level of revision to the previously published minor road traffic estimates varied by road category. Table 3 show a larger revision for ‘Classified unnumbered and unclassified roads’ in 2019 (a decrease of 16.7%) and a larger revision for ‘B’ roads in 2009 (an increase of 30.7%).
One of the reasons for the level of these revisions is the introduction of the stratification by flow group; further details about the methodology changes can be found in the technical report.
Table 3: Estimated traffic (billion vehicle miles) by road category before and after the minor roads review for the 2009 and 2019 benchmark exercises, with the level of revision applied for each category
| Road category | 2009 estimate before review (bvm) | 2009 estimate after review (bvm) | Revision to 2009 estimate | 2019 estimate before review (bvm) | 2019 estimate after review (bvm) | Revision to 2019 estimate |
|---|---|---|---|---|---|---|
| ‘B’ roads | 22.5 | 29.4 | 30.7% | 30 | 29.8 | -0.8% |
| Classified unnumbered and unclassified roads | 85.6 | 77.8 | -9.0% | 105.8 | 88.1 | -16.7% |
| All minor roads | 108.1 | 107.3 | -0.8% | 135.8 | 117.9 | -13.2% |
For the 2019 benchmark the adjustments also vary by urban or rural location, with urban roads having a greater adjustment factor than rural. The minor road traffic estimate for urban roads is 18.9% lower than the figure published before the review. This is because urban roads in the 2019 benchmark sample had a higher proportion of ‘high flow’ roads than in the 2009 sample, which has been accounted for in the revised estimates.
Table 4: Estimated minor road traffic (billion vehicle miles) by urban or rural location before and after the minor roads review for the 2009 and 2019 benchmark exercises, with adjustment factors applied for each location type
| Urban/Rural | 2009 estimate before review (bvm) | 2009 estimate after review (bvm) | Revision to 2009 estimate | 2019 estimate before review (bvm) | 2019 estimate after review (bvm) | Review to 2019 estimate |
|---|---|---|---|---|---|---|
| Rural | 42.4 | 43.9 | 3.4% | 52.6 | 50.4 | -4.2% |
| Urban | 65.7 | 63.4 | -3.4% | 83.2 | 67.5 | -18.9% |
| All minor roads | 108.1 | 107.3 | -0.8% | 135.8 | 117.9 | -13.2% |
10. How did the review affect minor road traffic estimates in London?
London had one of the highest benchmark adjustments in the original 2019 benchmarking exercise. Prior to the benchmarking exercise London had seen a fall in its minor road traffic, dissimilar to the pattern seen in most regions.
London has had some of the largest revisions resulting from this review, a 21.6% increase in 2009 and a 22.7% decrease in 2019. The revisions in 2019 and 2009 are due to a combination of the introduction of the stratification by flow group and by separately calculating traffic estimates for Inner and Outer London. The methodology used for the original 2009 benchmark did not incorporate Inner/Outer London stratification. The introduction for this stratification has identified that the 2009 benchmark underestimated minor road traffic in Outer London. Chart 3 shows that the revision to 2009 estimates was much larger for Outer London than for Inner London.
Chart 3: All roads traffic estimates for Inner and Outer London, before and after minor roads revisions, 1999 to 2020 (with benchmark years highlighted)
11. How do minor road traffic trends compare to that of major roads?
Minor road traffic accounts for about one third traffic in Great Britain. Chart 4 shows the impact of the historic revisions to minor road traffic, by comparing them against the major road traffic estimates.
Chart 4: Estimated minor road traffic (billion vehicle miles) in Great Britain before and after applying historic revisions and major road traffic, 1999 to 2020
Chart 5 shows the same data indexed to 1999. This shows that trends over time in minor road traffic estimates are now more consistent with that of major roads after applying revisions.
Chart 5: Estimated minor road traffic (vehicle miles indexed to 1999) in Great Britain before and after applying historic revisions and major road traffic 1999 to 2020
12. What is the minor road benchmarking exercise?
The minor road benchmarking exercise is carried out every 10 years to rebaseline the traffic estimates for minor roads in Great Britain. Annual traffic estimates on minor roads are based on traffic counts at a fixed sample of locations. Although retaining a fixed sample ensures a more precise measure of traffic growth between consecutive years, it also means that any errors in the sample will accumulate over time. In order to correct for any sampling errors, a larger benchmark sample is taken every decade, this allows for the recalibration of the traffic estimates on minor roads. The aim of the benchmarking exercise is to reduce the error incurred by having a fixed sample and to give a more accurate estimate of traffic on minor roads.
13. How are annual minor road traffic estimates usually produced?
The scale of the minor road network in Great Britain means it is not possible to count traffic on every stretch of road. Instead, a representative sample of minor road sites are counted each year. The change in the traffic flows between two consecutive years is applied to the overall minor road traffic estimates for the previous year, to calculate regional and national estimates for the latest year.
14. Will there be another benchmarking exercise, and will it affect figures published in the future?
The current methodology for estimating traffic on minor roads uses a fixed sample to estimate the level of traffic growth between consecutive years. The use of a fixed sample has the risk of accumulating error over time. To reduce the potential error incurred, the traffic estimates require recalibration and for the sample to be refreshed periodically. Therefore, it is likely that traffic estimates for 2020 onwards will be subject to revision following the next benchmarking exercise. This is expected to take place towards the end of the decade.
15. What impact will the findings of this review have on future minor road traffic estimates?
The methodological developments to the minor road benchmarking exercise resulting from this review will inform the methodology used for the benchmarking exercise. This is to ensure a continued consistent back series. The minor road traffic estimates review will continue to investigate options for improving the robustness of future annual minor road traffic estimates. Details of further areas of investigation are given in the technical report.
16. Where is the raw data used to calculate the revised minor road traffic estimates?
The raw count and Average Annual Daily Flow (AADF) data by count point is publicly available on the road traffic statistics website.
To enable identification of the count points used in the revised minor road traffic benchmarking exercises and for fixed annual samples, the lists of the relevant count points are provided on the road traffic statistics information page.
The road lengths used in the benchmarking calculations were based on the Ordnance Survey (OS) Highways Network. This is an OS Highways Network product and is considered a premium service, it is therefore exempt from disclosure and is protected under the FOI Exemption Section 43 (1), so we are unable to share this dataset. The department recognises the need for transparency; however, this must be balanced in allowing Ordnance Survey to protect their commercial information, and not be placed at a disadvantage in the competitive market-place in which they operate. In considering all of this, we are satisfied that the public interest in maintaining the exemption outweighs the public interest in disclosure.
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