Guidance

Impact of Changing Local 'A' Roads Data Source

Updated 5 December 2024

1. Background

Prior to the end of the previous travel times data contract, the Department for Transport (DfT) went through an open procurement exercise for data used to calculate the National Statistics for Travel Times on roads in England. Ctrack & Inrix were chosen as the new provider of travel times data following the procurement exercise, with the data having previously been supplied by Teletrac Navman. The data is provided on the same monthly basis to the same specification as the historic data to provide consistency through the transition.

This paper is to show the investigation DfT undertook to review the impact of the data source change and confirm the quality assurance of the data.

2. Understanding the impact of change in data source

A 3-month overlap period of data was evaluated to understand the level of change between the two providers’ estimates for travels times on local ‘A’ roads in England. This has shown that consistently throughout the period that average speed is slower and average delay is higher for the Ctrack and Inrix sample, as shown below. Speed has decreased between 8.6 and 9.9% (or around 3 mph) from Teletrac Navman to Ctrack and Inrix, whereas delay has increased between 19.9 and 29.1% (around 8 seconds per vehicle per mile) for the corresponding months.

Chart 1: Average Speed on local ‘A’ roads, England, January to March 2021

Chart 2: Average Delay on local ‘A’ roads, England, January to March 2021

This short time series shows that the two samples follow the same trends but that there is a clear magnitude step change on both urban and rural ‘A’ roads. The geographical coverage for both samples is the same with both requiring at least one observation on at least 90% of local ‘A’ road and Strategic Road Network road links every 24 hours.

The Ctrack and Inrix sample has shown greater coverage over the off-peak time periods, compared to Teletrac Navman. This gives DfT a truer understanding of travel times throughout the whole day. The data shows that there is a larger step change in off-peak time periods (7pm to 7am), with less difference between peak and off-peak travel times on the road network.

Chart 3: Average Speed on local ‘A’ roads, England, 2019 and 2021

Chart 4: Average Delay on local ‘A’ roads, England, 2019 and 2021

The level of change is similar in levels for speed and delay (around 3 mph or 8 spvpm) for the different time periods. However, this translates to very different relative changes in the respective periods. For delay this is most pronounced, with off-peak delay increasing between 41.0 and 48.9%, between Ctrack and Inrix data and Teletrac Navman, compared to increases of 15.0 to 25.9% in the peak time periods. Average speeds decreased 7.7 and 9.6% between the Ctrack and Inrix data and Teletrac Navman during morning and evening peak time periods, while off-peak speeds decreased by 10.0 to 11.0%, between the data sources.

When the Ctrack and Inrix data for 2021 is compared to historic Teletrac Navman data, it follows similar trends to previous years. Due to the unusual nature of 2020, we have compared data to 2019. The Ctrack and Inrix data available does show a step change but similar trends to 2019, especially after the end of lockdowns in March 2021.

Chart 5: Average Speed on local ‘A’ roads, England, 2019 and 2021

Chart 6: Average Delay on local ‘A’ roads, England, 2019 and 2021

3. Statistical evaluation

Evaluation through statistical testing was conducted against February 2021 and March 2021. These months were chosen to minimise the impact of national holidays on the data.

Comparisons were made at the 15-minute time-period level using a Wilcoxon ranked-pair test, which showed that for both average journey times during the full-day, night and peak hours there was significant differences between the datasets (P<0.001) in both February and March. This means that the data is significantly different enough to introduce a step-change between the datasets, as is observed.

Follow-up testing was conducted using Spearman Rank Correlations, which confirmed that there was a strong, significant, positive correlation between the Teletrac Navman and Ctrack and Inrix dataset for the full-day average journey time in February (r=0.847, p<0.001) and March (r=0.864, p<0.001). The correlation between average journey time in the two datasets is noticeably stronger during peak hours for February (r=0.880, p<0.001) and March (r=0.912, p<0.001), while the inverse is true for night hours in February (r=0.447, p<0.001) and March (r=0.429,p<0.001). Although the correlation during night hours is weaker, it is still significant and shows reasonable correlation. This effect is likely due to the reduced sample size in the Teletrac Navman dataset having a greater impact on the results.

4. Conclusion

Although there is a clear magnitude step change, the new data source shows consistency in trends with the historic Teletrac Navman data, as seen in the correlation test results. The step change in data does not appear to be particular skewed between urban, rural or specific locations.

To avoid confusion for users, we have archived the previous Teletrac Navman based data and created new table numbers for the new data suppliers.

5. Next steps

The department is working with Ctrack and Inrix to understand whether there are improvements that can be made to better understand the step change and help users.

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8. Contact details

Road congestion and travel times

Email [email protected]

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