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Releasing Trapped Traffic
September 2007
Introduction
A dual carriageway or motorway can become completely untrafficable following an incident in one or both directions. This could be due to a physical obstruction of the carriageway, severe damage to infrastructure or purely due to the ongoing work at the scene. In cases such as these a decision has to be taken early with regards to trapped traffic. This article examines the alternatives available for managing those unfortunate enough to become trapped between the incident and the previous network exit point.
Methods
The most common method is ‘rearward relief’. This is simply turning traffic in a controlled manner and allowing them to travel along the same carriageway in the opposite direction. Local junction layouts will determine whether traffic should then be removed from the carriageway at the next access on or via the off-slip. This is a slow process as each vehicle has to manoeuvre to turn around. On dual-two carriageways (2 lanes in each direction) there is a risk that LGVs will not be able to make the turn easily and could end up blocking the carriageway themselves. This method requires significant Traffic Officer and/or Police resource to direct traffic not only when turning but also at the exit point.
The second core option involves directing traffic through the central reserve onto the opposite carriageway. There are several mechanisms for this. Some routes have designated points for carrying out this activity either installed for this purpose or have contraflow cross-over points from a past scheme.
Where concrete central reserve barrier has been installed in accordance with TD19/06 (superseded IAN60/05) designated cross over points are the only available location to release traffic using this mechanism. There is no quick way to remove and reinstate this type of rigid central reserve barrier.
If there is no obvious crossing point some Service Providers can remove a section of the safety fence and place matting down (follow the link for further information on Tank Matting) to allow vehicles to traverse the central reserve. Some types of safety fence (such as wire rope) pose problems as the cables have to be de-tensioned by specially trained crews and may not be easily removable. Take advice from your Service Provider representatives when considering removal of the central reserve barrier.
Turning traffic through the central reserve places traffic on the opposite carriageway to return to the previous junction resulting in additional delays to this carriageway. The operation can proceed with either a total closure of the opposite carriageway whilst the traffic turns or it can be done with lane closures and careful marshalling.
The third option is only feasible if there is a junction immediately downstream of the closed section of carriageway. The traffic can be directed through the central reserve and use the opposite carriageway in the ‘wrong’ direction to the next junction. This allows traffic to continue their journey but does require a total closure of the opposite carriageway.
A fourth option has been used in Area 10 (Manchester, Cheshire, Merseyside and Lancashire) and is under further development to formalise it. This uses two central reserve crossing points to establish a single lane contraflow. This can only be employed on a carriageway with at least 3 lanes as a buffer lane is required to separate the opposing traffic. The first use of this system was documented by the TIM Bulletin in November 2006, Crossing the Central Reserve.
This is time consuming to establish and does limit capacity on the opposing carriageway but does allow drivers to continue on their journey thus reducing the congestion at the previous junction. To date this has only been used to release trapped traffic but there is ongoing development to establish if an emergency contraflow could be left in place to keep both carriageways partially open during a protracted incident. To extend this to more than one lane of traffic traversing the central reserve is fraught with issues and to date no adequate solution has been found.
If you have anything you wish to contribute to this development work please email the TIM Team.
Considerations
One of the most important considerations is how long the carriageway will be unavailable to traffic. The time to release trapped traffic is normally measured in hours. Equally once released the traffic will encounter delay on the diversion route and therefore if it is only a short term closure it may be a lesser delay to leave the traffic in situ.
Once the time prior to opening has been estimated then the possibility of releasing the traffic can be examined. Key considerations include:
- The type of central reserve safety fence
- The surface material of the central reserve (is it hardened or just grass)
- Any drainage channels or other obstacles adjacent to the carriageway
- The available resources to remove barriers and supply traffic management
- The amount of traffic to be released.
- Turning circle of vehicles (LGVs can need up to 25m or nearly 7 lane widths)
- Weather conditions (Lighting available, please see our article on article on Incident Lighting)
- Layout of the previous junction (or other exit point) can LGVs negotiate it if travelling the wrong way?
- Location of any central reserve crossing points / gates
Alternatives
If removal of the trapped traffic is not feasible there are other issues to consider. The first is the welfare of those stranded on the network. Aside of this the main access to scene is likely to be past the traffic and the resultant people out of their cars. The occupants of the trapped vehicles are also likely to become frustrated and possibly walk up to scene to find out what is happening. This could lead to problems with scene contamination, public order and most importantly health and safety. If possible get the trapped traffic cleared promptly to allow easier access to scene and one less issue for the various commanders to worry about.
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.
