Guidance

Impact of the Coronavirus (COVID-19) pandemic on flow weighting for congestion data

Updated 6 March 2025

1. Introduction

The data used to calculate Congestion statistics is only a sample of observations for all journeys on the road network.

This data is therefore weighted using annual average traffic flow data to make them representative of all journeys.

Due to availability of data, the traffic flow data used to inform the weightings is from two years previous. Variation in traffic flow is such that this does not usually impact the applicability of the weightings.

2. Traffic flow weighting for 2022 Congestion statistics

Under normal arrangements, the annual average daily flow used to inform the weightings for 2022 congestion statistics would be based on traffic flow in 2020.

However, the fall in traffic seen on the roads in 2020 due to the coronavirus (COVID-19) pandemic means that free flow figures were significantly different to those observed in previous years.

In addition, road traffic data suggests that daily road traffic levels in January to March 2022 are similar levels to pre-pandemic traffic (Figure 1).

Figure 1 . Percentage of daily road traffic compared to levels in February 2020, England, January to March 2022

Observed decreases in percentages are from early January bank holidays and interruption due to Storm Eunice in February 2022.

If we were to use the 2020 traffic flow to inform the congestion statistic weightings, then the estimated total vehicle miles would be significantly lower (Figure 2). This could make the 2022 congestion statistics calculated from the 2020 flow weightings less representative of the road network.

Figure 2. Comparison of total vehicle miles (billions) when using 2019 flow weightings compared to 2020 flow weightings

3. Conclusion

To address the significant difference in total vehicle miles in 2020 and those expected in 2022, we have used the 2019 annual average traffic flow data to inform the weightings used to calculate the 2022 congestion statistics.

To support this decision we have investigated the impact on speed and delay averages for 2022, using traffic flows from both 2019 and 2020. Whilst the percentage differences between the two flow weighting options were relatively small for average speed and delay, the total vehicle hours and total vehicle miles were significantly different. The congestion levels estimated using the 2020 traffic flow weightings would not be representative of the current activity on the road network.

4. Further Information

For more detailed information on the methodology to calculate the congestion statistics can be found in our Background Quality Report.

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7. Contact details

Road congestion and travel times

Email congestion.stats@dft.gov.uk

Media enquiries 0300 7777 878