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M25 Construction, Signing, Communications and Lighting


Paving TrainIn accordance with Departmental policy, alternative tenders were invited for rigid and flexible construction, with the exception of one contract in the north-east quadrant which, for monitoring of performance purposes, was specified as continuously reinforced concrete pavement only. As a result about 54 per cent of the motorway is of flexible and 46 per cent rigid construction.

As the motorway has been built over a long period, there have been improvements in the standards of design and construction. The early sections were all designed in accordance with Road Note 29 (1970) and the Notes for Guidance to the 1976 Specification. The calculation of the million standard axle loading (msas) was changed in 1978 and capping layers for the formation were introduced at the same time. By 1979 the results of TRRL research on flexible roads became available and, although new design standards were not issued, advice was given from the Department's headquarters on an ad-hoc basis on the design of heavily-trafficked roads (80 -100 mas).

Apart from the introduction of capping layers and the change to the calculation of msa's (million standard axles), there was no significant change to rigid pavement standards until 1984 when the specification was changed. Two lengths have been constructed in accordance with this.


Although M25 is an orbital motorway it was felt that the main principles of signing linear motorways could be applied to it as well. The objective was to get traffic joining M25 to 'orbit' in the desired direction, to confirm to drivers that they were travelling in the correct direction, and finally to sign their exits clearly.

The system adopted, following extensive consultations, uses five "key destinations" - Heathrow, Gatwick, Dartford Tunnel, Harlow and Watford. The signs at the approaches to M25 show the next key destination together with route numbers of other main radials, e.g. M1, M4, etc., and local destinations on each side to enable the driver joining the motorway to orbit in the desired direction. Once on M25, the route confirmatory sign (which is provided after each junction) repeats the same key and local destination and also the next key destination, together with mileage to each place and long distance destinations such as The West and The North. At the approach to each junction, the driver leaving the motorway is given the route number, the key destination (if one exists), and the next primary destination coupled with local destinations, as appropriate. The driver who is staying on the motorway will be given the next two key destinations together with the primary destination that can be reached via the next junction. These will of course be repeated on the route confirmatory sign immediately past the junction. (Figures 18 and 19). Signs are generally post-mounted except at junctions with radial motorways where they are on gantries. Additional free-standing signs to long distance destinations, e.g. The North and The West, are also provided at critical locations.

This system has enabled complete plans to be drawn up long before the construction of many sections of the motorway were even started and allowed for sections of the motorway to be signed and brought into use on completion.

As well as the routine signing, the system of signing during emergencies is also under continual review. Variable legend signs operated by remote control are under consideration for symbolic signing of diversion routes in the north east sector. Here also sensor loop systems have been installed at intervals on the carriageway surfacing of the motorway and slip roads to detect vehicle movements, and to enable action to be taken by signing or patrolling for potential problems.


THE communications system developed for M25, and for which contracts have been placed for completion by end of 1987, is quite new. It makes use of the enhanced capabilities of modern technology by means of microprocessor based control systems for signals to telephones and incident detection and is a considerable improvement on that of the existing motorway system.

When the new system is brought into use by the end of 1987, the road equipment, which conveys information to travellers, will not appear markedly different from that on other motorways. Post-mounted matrix signals are provided in the central reservation at 2km spacing (3km on the normal inter-urban network) and telephones at 1km intervals (1.5km on the rest of the network). A series of three gantry-mounted signals is provided at the approach to each major junction in order to slow down traffic on the three lanes of a carriageway and then divert it when the occasion demands. Closed circuit television is also provided at strategic points on each major interchange.

The new National Motorway Communications System (NMCS II) is technologically superior to the old one. The result is a better service to the motorist through a more reliable telephone system; signals can be set without the need to communicate with a remote central computer.

When the whole system has been commissioned, it will be operated from the four control centres at Godstone, Surrey; Heston, West London; Welwyn, Hertfordshire and Chigwell, Essex. Dartford Tunnel of course will continue to have its own control centre. Each centre will control roughly one quarter of M25 with some also controlling communications on a part of the radial motorway in their ambit. The patrolling of M25 is by the county forces of Kent, Surrey, Essex and Hertfordshire, and by the Thames Valley and the Metropolitan forces.

Automatic incident detection is being provided in the area of Holmesdale and Bell Common Tunnels. The new system will also have the capability to receive and convey messages from ice and fog detection devices. It will be possible to automate the setting of signals on the receipt of such messages. Automatic television camera control and wide-area diversion control can also be added.


The provision of lighting has been based on the Department's policy prior to July 1986 when new policy, which aims to obtain better value for money and give greater consideration to special road safety factors, was introduced. Before July the policy was to light motorways and trunk roads only if a night-time accident problem developed in service or if there were other special circumstances. During consultation, and the statutory procedures, concern was expressed about the environmental effects of lighting and in some areas undertakings were given that lighting would not be provided on opening. At the time of writing, lighting has been, or is being, provided, on 50 miles of the motorway with 22 of the 31 junctions fully lit and six partially lit. Thus about 43 per cent of the motorway will be lit compared to 22 per cent for all trunk motorways in England.