Congestion on inter-urban roads

Monthly provisional estimates: May 2009
(Includes final data for April 2009)

The latest monthly National Statistics on congestion on inter-urban roads in England produced by the Department for Transport according to the arrangements approved by the UK Statistics Authority.

The inter-urban network consists of all motorways and trunk ‘A’ roads managed by the Highways Agency, as well as the M6 Toll.  This is also known as the Strategic Road Network (SRN).

The indicator used to monitor reliability is the average delay in minutes per 10 miles (derived from the differences between observed journey times and a reference journey time) experienced on the slowest 10 per cent of journeys for each monitored route.

• Provisional figures for the year ending May 2009 show that average vehicle delay on the slowest 10% of journeys fell to 3.39 from 3.90 minutes per 10 miles since the baseline year ending March 2008, a decrease of 13.0 per cent.
• The final figure for the year ending April 2009 is an average vehicle delay of 3.41 minutes per 10 miles, which is a decrease of 12.4 per cent from the baseline year ending March 2008.

Journey time reliability measure on the Strategic Road Network

Notes

1. The data quality for all 103 routes included in the SRN was reviewed prior to the release of data for October 2008. Improvements in data quality allowed an additional five routes to be included for the year ending March 2008 onwards, but deterioration of data on one route (M1 J6a-13) in 2008 led to it being temporarily excluded. As a result the total number of routes included increased from 91 to 95 routes for the year ending March 2008 onwards. The previously published data for years ending March 2008 to September 2008 were revised, and there is now a small discontinuity with earlier periods.
2. The effect of the increase in the number of routes was to change the baseline figure of average vehicle delay for the year ending March 2008 to 3.90 minutes per 10 miles for 95 routes, compared with 3.95 for 91 routes.
3. For the Comprehensive Spending Review 2007 (CSR07), the Department has a Public Service Agreement (PSA) to deliver reliable and efficient transport networks that support economic growth. One of the four indicators used to measure success against this PSA is reliability, measured using average vehicle delay on the SRN's slowest 10% of journeys. The baseline is the year ending March 2008, and the measure will be monitored for the period up to the year ending March 2011. Reliability performance will be assessed in the context of an expected increase in traffic of 1-2% per year. There is no specific numerical target.
4. For the Spending Review 2004 (SR04), there was a PSA target that the average vehicle delay on the SRN’s slowest 10% of journeys should be less in the year ending March 2008 than in the baseline period August 2004 – July 2005.
5. For both CSR07 and SR04, the PSA indicator is the average vehicle delay, derived from the differences between observed journey times and a reference journey time (the time that could theoretically be achieved when the traffic is free-flowing), weighted by traffic flows for each route of the network. The slowest 10% of journeys are selected for each 15-minute departure time between 6 am and 8 pm for each day of the week, on each route. The indicator therefore reflects journeys experienced on all types of route on all days at all times of the day.
6. It should be noted that where one or more lanes are closed, for example because of road works or incidents, journey times are estimated from monitored data where available or infilled using data from other periods. Flow data used in calculations is the average for that time of day and day of week rather than actual flow, as the latter would be affected by vehicles diverting off the network.
7. The data for May 2009 are provisional because there has not been full quality assurance at a route level.
8. Note that percentage changes are calculated from unrounded delay figures and may differ from those produced using the rounded figures shown in this release.
   

Publication details

Published on 2 July 2009 by Transport Statistics.

The next release in this series will be published on Thursday 6 August 2009.

Email: david.robinson@dft.gsi.gov.uk or telephone +44 (0)20 7944-6559 for further information and queries.