Accredited official statistics

Details of Revisions to Data Series up to June 2024

Published 3 October 2024

Applies to England

In the statistical release ‘Travel time measures for the Strategic Road Network and local ‘A’ roads: July 2023 to June 2023’ we have revised the series of local ‘A’ road tables for data from 2019 through to 2023.

The data used to inform previous publications included valid GPS data points. However, it also included a small amount of additional data that was out-of-scope of the data specification. Following investigation and arrangements with our suppliers, we have taken the decision to update the historic data that we hold and publish to ensure this is in line with the agreed data specification.

In addition, improvements in processing made by the supplier have increased the available sample size for 2023 (which has previously been reduced). 

The impact of this revision is minimal, with a slightly more pronounced effect in 2023

Although these changes have a small impact on the absolute values for speed and delay, the overall trends remain consistent.  There is a small change in the magnitude rather than previously observed trends in speed and delay. The revisions have been made across tables CGN0503, CGN0504 and ‘Percentage of data imputed for travel time statistics on the local ‘A’ road network, by month’.  This revision does not impact the SRN dataset in tables CGN0404 and CGN0405.

An overview of the difference between the original and revised data is given below.

1. National level

At a national level, the following changes were observed:

• statistics for the national level figures were revised at both monthly and annual-rolling levels. This has resulted in minor changes for all years prior to 2023, and a more noticeable impact in 2023. Overall, delay is slightly lower and speed slightly higher in the revised dataset

Year	Original Delay (seconds per vehicle per mile)	Revised Delay (seconds per vehicle per mile)	Change (seconds per vehicle per mile)
2019	48.2	48.2	0.0
2020	40.2	40.3	0.1
2021	43.4	43.3	-0.1
2022	45.5	45.4	-0.1
2023	47.9	46.7	-1.2

Figure 1: Differences in the annual-rolling statistics for average delay in England across 2019 to 2023.

Year	Original Speed (mph)	Revised Speed (mph)	Change (mph)
2019	22.9	22.9	0.0
2020	24.7	24.7	0.0
2021	23.9	23.9	0.0
2022	23.5	23.5	0.0
2023	23.0	23.4	0.4

Figure 2: Differences in the annual-rolling statistics for average speed in England across 2019-2023.

• changes to the rolling delay figures were most impacted in rural data compared to urban data. The revised dataset sees an average 0.2% decrease in urban delay and 3.6% decrease in rural delay in 2023, in comparison speed increases on average 1.2% for urban speed and increases 0.6% for rural speed

2. Sub-national level

Data for all sub-national level figures were impacted, with the following impacts by breakdown:

2.1 Regional Data

Changes were largest in 2023, with delay decreasing by 3.1% on average for all regions and speeds increasing by 1.7% on average. Between 2019 and 2022, speed and delay changed less than 1.0% between years in all regions except the South East, which saw a 1.9% and 1.5% increase in delay for 2019 and 2020 respectively.

2.2 Sub-national Transport Body

Data for 2019 and 2020 broadly saw minor increases in delay between the original and revised data, with the largest increase in England’s Economic Heartland at 2.0% for delay. Data for 2021 and 2022 saw comparative decreases in delay to the increases observed in 2019 and 2020, with the largest decrease for Peninsula Transport at 1.1%. In 2023 slightly larger increases in speed and decreases in delay were seen for all Sub-national Transport Bodies, with Western Gateway impacted most with a 2.4% increase in speed and 4.7% decrease in delay.

2.3 Local Authority Level

 As is consistent with broader figures, data is most impacted in 2023 but is more variable than other aforementioned geographic breakdowns. On average, speed changed less than 2.0%  and delay changed less than 3.0% in 2023 for local authorities.  Between 2019 and 2022, average speed and average delay changed less than 0.1%

However, there were some outliers in the changes to delay in 2023. with delay decreasing by 10.6% in Sunderland and increasing by 5.2% in Slough.

3. Road-level

Road level figures were the most impacted of all the breakdowns, with the following impacts observed:

• overall decreases in delay and increases in speed.  Delay decreased most on rural links and increased most around urban areas

• speed and delay changes at road link levels are variable but average to 2.0% increases in speed and 4.7% decreases in delay for 2023, (as the year with the greatest change between the 2 datasets)

Map 1: The average percentage change in average delay across England by road-link for 2023.

A map showing the percentage change in average delay by road link for 2023 in England. Increases in delay were strongest (over 25%) around urban areas and decreases were strongest (over 25%) in rural areas.

4. Imputation figures

Imputation levels for the revised dataset are broadly consistent with the original dataset, with the exceptions of 5 months in 2021 (increasing imputation by 0.1 percentage points each), and the year 2023 (increasing imputation by 0.4 percentage points on average).