The Motorway Archive
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What is the Motorway Archive?
Work on developing the UK Motorway system, which transformed British travel, started in the mid-1950s. The Motorway Archive celebrates the engineering achievement involved in the conception, planning, design and construction of this transport network by thousands of dedicated professionals. The Archive itself is a collection of as many of the documents and artefacts, which were associated with the development, as it has been possible to find. From this wealth of material has come the story of each motorway developed in Britain over the last 50 years. This is the story of the London Orbital Motorway.

Region: East/London/South east

The M25 London Orbital Motorway

Planning of the various lengths - Route selection - Economic and traffic aspects - Interchanges - Tunnels - Bridges -
Pavement construction - Signing - Communications - Lighting - Landscaping - Service areas -
Dartford Tunnel - Conclusion - Contract details
[See also:The origins of the London Orbital Motorway - Northern section (J13 to J30) - Southern section (J1 to J13)]

The text and pictures reproduced on this web page are taken from an edition of the Journal of the Institution of Highways and Transportation dated November 1986. Their permission to reproduce these items is gratefully acknowledged.


M25, although it is now looked on as an entity for a complete radial route of London, has developed in a piece-meal fashion, with each length having to be proposed statutorily and justified on its own merits. Individual lengths have been taken forward more quickly or slowly according to the problems encountered in passing through the statutory procedures. This has meant that differing lengths may seem to have been brought into service in an apparently haphazard way, but the intention has always been to make the different lengths available to traffic at the earliest opportunity - for the benefit of both the road users and the communities relieved of traffic.

In all, 39 public inquiries before independent Inspectors were held, taking over 700 sitting days - with many more working days (and nights) preparing evidence. A wide variety of alternatives was put forward by objectors including of course the argument that no route was required at all. Each of these needed to be answered with painstaking presentation of evidence, including predictions of traffic and parameters affecting growth of traffic, in the light of information available at that time, open to public scrutiny and debate. The public inquiries themselves varied in duration - some lasted one day, one lasted 97 sitting days over a period of 13 months. In addition some of the statutory proposals had to go before Parliament and a special Act was needed to authorise the crossing of the northern tip of Epping Forest by the route in tunnel.

The chronological sequence of the various lengths therefore emerged largely from local circumstances, with those lengths which had an obvious local traffic relief being constructed first. The three-mile length between South Mimms and Potters Bar (Al (M) to A111) completed in 1975 and the five-mile length between Godstone and Reigate (A22 to A217), 1976, are early examples.

Planning of the various lengths

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This sequence affected the route selection. Another major factor was the location of interchanges to connect traffic with the main radial routes.

At the eastern crossing of the Thames, the Dartford Tunnel and its A282 all-purpose approach roads, extending over a total of four miles, governed the location of the northerly merging of M25 with A282 at Junction 31 (A13) near Thurrock. It also governed the southerly merging of A282 with M25 at Junction 2 (A2) in Kent.

From Junction 2 clockwise some 28 miles of the 57-mile southern length of M25 passes through or near the Areas of Outstanding Natural Beauty and Great Landscape Value of the North Downs. By its very location in such sensitive environmental areas, M25 called for careful treatment of the chosen route. For example, the Swanley - Sevenoaks length, which runs parallel to, but some distance from, the River Darenth was seen by many as a major and unacceptable intrusion into picturesque countryside. But the skilful use of earth mounds has effectively concealed the motorway in all but a few locations. As a corollary, of course, the view for the motorist is inevitably obscured by the earth mounds denying him the distant vistas which help to make motoring a pleasure.

In other areas, for instance between Reigate and Leatherhead, the road has been depressed below the optimum engineering level to improve conditions for those living in the vicinity.

As an example of the procedure involved in planning and route selection, the 13-mile length from Poyle at Junction 14 south-west of Heathrow to Maple Cross at Junction 17 south-west of Rickmansworth was dealt with as an entity. Public consultation into five basic optional routes with many varied combinations was carried out in 1974. Some 15,000 questionnaires were distributed and some 5,400 were completed and returned. A preferred route was announced in 1976 and the proposals advertised formally under the Highways Act procedures in 1978. These attracted some 2000 objections and a public inquiry was held between October 1979 and May 1980 when several alternative

routes were put forward by objectors, for which the Department's witnesses provided economic and environmental comparison with the published route. The Inspector recommended that the route as published should be followed and the announcement that the Secretaries of State had accepted this was made in 1982 with the construction starting a few months later.

The route follows the Colne Brook valley between the developed areas at Richings Park and Iver to the west and West Drayton to the east. It then crosses the Alder Bourne valley and follows the open higher land between Gerrards Cross and Chalfont St Peter to the west and Denham and West Hyde to the east. It crosses the London to High Wycombe railway line passing through two of the five spans of the Chalfont viaduct and then in open undulating topography continues to A412 interchange at Maple Cross (Junction 17). The route incorporates extensive noise and visual mitigation measures which include a 4m high embankment with a slope of 1 in 8 facing Richings Park.

The six mile length of M25 between Maple Cross and the Hunton Bridge roundabout with A41 was conceived originally as part of the north orbital route. Although it was constructed to motorway standards, when it was completed in 1976 it was given the number A405, this being changed to M25 when the length to the south was opened in 1985.

Care was taken to curve the route sympathetically into this environmentally sensitive area of the Chilterns. The viaduct at Berry Lane, designed by Ove Arup, received a Concrete Society Award for the manner in which the slender piers and superstructure helped to merge it into the trees though which the route passed.

The route selection of the 12 mile length between Micklefield Green and South Mimms is described in the Eastern region pages.

For the nine mile length from South Mimms at the interchange with A1(M) (Junction 23) to Waltham Cross at the interchange with A10 (Junction 25) the route selected was governed mainly by the necessity to thread between the developed areas at Potters Bar to the north and Barnet, Hadley Wood and Enfield to the south. Here again the flowing alignment helps to sit the route easily into the scenery.

At Waltham Cross a corridor through the dense housing and industrial development had been reserved for the D-ring although of a much narrower width than a dual three-lane motorway. The route would also have to cross Epping Forest and the chosen route is in tunnel in both these areas to enable traffic to pass through unobtrusively. Many variations of vertical and horizontal alignment were investigated on this length before homing in on the final selection. On this eight mile length between A10 and M11 about four miles of embankments and acoustic fences have been provided to alleviate the effect on people nearby.

From the A12 east of Brentwood (Junction 28) to A13 near Thurrock the alignment finally selected largely followed that protected in the Essex County development plan for 1957. The route avoids several residential areas and, as was the case elsewhere on M25, passes partly through old gravel workings which have been backfilled with domestic and industrial refuse.

Route selection

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Each length of M25 was evaluated on its own merits at the time. The forecasts for the earlier lengths, including the southwest sector, were prepared in the late 1960s. At the public inquiries in the first half of the 1970s there was determined opposition to motorways and it was claimed by objectors that the Department's forecasts were excessive, a claim that was apparently reinforced by the oil crisis in 1973. This background should be borne in mind when considering the decision to build those sections to dual three-lane standard.

Table 3 shows the original traffic forecasts presented at public inquiries, the forecasts updated in 1984 and the actual flows in April, 1986 on a 16 hour average daily basis. Continuous traffic counts have not been taken on all sections, but further installations will provide this data by the end of 1986.

The economic justification for most lengths, and all the later ones, was evaluated using COBA, the Department of Transport's cost benefit analysis programme. But the planning, design and construction have taken place over some 20 years, during which period many editions of COBA have been applied; and indeed some of the earliest sections, such as Reigate - Godstone and the Potters Bar bypass, predate even the first edition of COBA. However there can be no doubt that economic benefits derived by traffic are substantial. Traffic which now uses M25, whether the local movements or transferring between major radials, already experience substantial time savings.

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