Congestion on inter-urban roads

Monthly provisional estimates: August 2008
(includes final data for July 2008)

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 August 2008 show that average vehicle delay on the slowest 10 per cent of journeys fell to 3.66 from 3.95 minutes per 10 miles since the CSR07 baseline year ending March 2008, a decrease of 7.2 per cent.
• The confirmed figure for the year ending July 2008 is an average vehicle delay of 3.74 minutes per 10 miles, which is a decrease of 5.3 per cent from the CSR07 baseline year. 
• Compared with the previous SR04 baseline year ending July 2005, average vehicle delay fell by 3.1 per cent to August 2008.

The chart below shows the monthly trend, on a rolling 12 month basis.

Journey time reliability measure on the Strategic Road Network

 

Notes

1. 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.

2. For the Spending Review 2004 (SR04), there was a PSA target that the average vehicle delay on the SRN’s slowest 10 per cent of journeys should be less in the year ending March 2008 than in the baseline period August 2004 – July 2005.

3. 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 per cent of journeys are selected for each 15-minute departure time between 6 am and 8 pm for each day of the week, on each of the 91 routes. The indicator therefore reflects journeys experienced on all types of route on all days at all times of the day. Note that percentage changes are calculated from unrounded delay figures and may differ from those produced using the rounded figures shown in this release.

4. The inter-urban congestion data are now National Statistics. The data for August 2008 are provisional because there has not been full quality assurance at a route level, but it is unlikely that they will be revised.

Publication details

Published on 2 October 2008 by Transport Statistics.

The next release in this series will be published on Thursday 6 November 2008.

Email: sophie.chase@dft.gsi.gov.uk  or telephone +44 (0)20 7944-6392 for further information and queries.