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Journey Reliability
June 2008
The term “reliability PSA target” is often quoted but not everyone understands what it means. This article explains what the reliability PSA target is and how everything related to Traffic Incident Management contributes to it.
What is the PSA Target?
In July 2005, the Department for Transport announced two new Public Service Agreement (PSA) targets aimed at tackling road traffic congestion. The first of these targets (PSA 1) focuses on improving journey time reliability on the Strategic Road Network and the second (PSA 5) is concerned with congestion in England's ten largest urban areas. PSA 1 relates to the Highways Agency’s network.
Unreliable journey times can cause significant frustration for road users, making it hard to plan a journey. If a route is known to be congested at certain times drivers can plan accordingly. When additional congestion occurs at a time when it is not expected it disrupts journeys and makes road users late. Road users find this most frustrating and the performance indicator and target aim to combat this issue.
The indicator was set using a base year of data from August 2004 to July 2005, and the 2007-08 target is simply to have less delay than the base year. The target for the next 3 years is as follows:
"Minimise increases in journey time unreliability through the delivery of the CSR07 programme of interventions."
Reliability
The performance indicator is measured using the worst 10% of journeys starting between 6am and 8pm along a selection of routes covering most of the Network. The precise method of calculating it is complex but essentially traffic speed samples for every 15 minutes for all journeys are compared to reference speeds. From this it is possible to see what the delays were on the worst 10% of journeys at each time. These are combined to produce an indicator measured in minutes delay per 10 miles travelled per vehicle. Please follow the link to view a map (opens in a new window) displaying this data.
The mathematics and data processing behind the delay figures are not the most important things. The key is that all big or small delays in the worst journeys contribute to the target and therefore it is important to prevent, reduce and mitigate against them. If you want more detail of the way the indicator is calculated a full description can be found on the DfT website (opens in new window).
The average vehicle delay baseline indicator is 3.8 minutes per 10 vehicle miles travelled. This is the national average so some routes are better and some are far worse than this. Of the route sections considered the spread of delays ranges from 0.3 to 19.5 minutes per 10 vehicle miles travelled. This helps prioritise the mitigation measures being implemented. The plan shows how these delays are distributed across the network.
The 07/08 financial year was the first time the performance indicator was used to measure the success of mitigation schemes against a defined national target. A full report can be found on the DfT website (opens in new window) showing that the target was unfortunately not achieved. Although not the desired outcome, all those who contributed to reducing delays on the strategic road network are congratulated for their efforts. Congestion will remain a key issue in the future, so it is important that these efforts are sustained to deliver reliable journey times.
The benefits already realised
Some of the key measures that have made contributions to the PSA 1 target are detailed below. Many of these have been covered in previous issues of the TIM Bulletin.
Off-network diversion routes have been installed on over 50% of the Strategic Road Network. Studies show that they are being used 23% more than forecast and having a signed diversion in place results in an estimated 1000 more vehicles diverting off the network in the case of an incident.
90,000 Fresnel lenses were distributed to drivers of left-hand drive lorries at ports in Dover, Hull, Liverpool, Immingham and Harwich in November, and Portsmouth in December 2007. The number of side swipe incidents involving HGVs reduced by over 50% in the three months following the roll out of 40,000 Fresnel lenses in the SE region.
More than 5000 driver location signs have now been rolled out on over 1000km of priority routes including the whole length of the M25 & M6. The Police, Traffic Officer Service and Service Providers have all reported faster response to incidents due to the more accurate location information being provided from the scene.
Since ATM has been implemented, the section of the M42 in question has seen average vehicle delay on the 10% worst journeys fall by 18%. The average journey time on the M42 J3a – J7 during the Friday afternoon peak reduced from 18 minutes to 6 minutes.
The Police Collision Investigation Equipment has now been provided to all 37 Police Forces operating on the Agency’s network. Feedback has been encouraging, with use of the survey equipment so far saving an average of 37 minutes per incident.
The immediate despatch of ISUs in conjunction with the enhanced ISU service has seen ISU response times improve by 20% and the time ISUs typically spend at the scene reduce by 13% leading to more efficient clearing of incidents from the network.
Has this article been worthwhile reading? Why not take a moment to send us your comments, thoughts or questions. Please e-mail TIMbulletin@highways.gsi.gov.uk.
