With an orbital road of this magnitude, serving a conurbation of some seven million people, the connections with strategic radial motorways and trunk roads were a vital part of the design. Each interchange had to take account of both traffic and the environment and the provision for complex turning movements had to be tempered by regard for the environment. The ring includes a total of 31 interchanges, numbered consecutively from Junction 1 south of Dartford Tunnel connecting with the A225, clockwise round to Junction 31 north of Dartford Tunnel connecting with A13. (The total is 32, if Junction 21A is counted separately). These interchanges include nine major motorway-to-motorway interchanges described below. For each interchange the number of levels, number of legs and number of traffic movements are indicated; for example, a two-level interchange between two motorways catering for all traffic movements would be designated (2, 4, 12).

The first motorway interchange encountered when travelling clockwise from Dartford is at Swanley. This was originally designed as a conventional three-level motorway-tomotorway interchange with turns catered for in a large roundabout. Connecting links were subsequently added to allow the major flows of motorway traffic between M25 north of the junction and M20 to the east to do so without entering the main junction. Because of the high incidence of local movements, traffic signals incorporating a pedestrian phase have been added to the roundabout: through traffic on both motorways is unaffected.

At Sevenoaks M25 changes direction from south to west and is joined by M26 from Wrotham and A21 from Hastings in two-level free-flow design. A tong viaduct carries a local road from Chipstead to Chevening and the whole interchange lies in a sensitive setting on the outskirts of Sevenoaks. Because of the low forecast demand, the movements from north to east and from the east to north and south are omitted at this stage: this has provoked some criticism and the need to provide for these movements will be monitored now that the ring is completed.

M23 interchange (Junction 7) (4, 4, 12)

The junction between M25 and M23 at Merstham is a four-level design with direct free-flow connection between the two motorways. It was built as part of the first length of M25 constructed south of the Thames. The M23 is carried on a five-span steel box girder bridge at a height of 23m above M25.

The M25 and M3 intersect near Chertsey. The site is on the edge of the Thames Valley near St Anne's Hill and a two-level cyclic design was chosen to reduce the overall height so as to fit it more easily into the topography. This type of layout features a large number of bridge structures and in this case the construction depth was minimised by using continuous decks of voided reinforced concrete.

The interchange between M25 and M4 near Heathrow offered one of the major challenges in the whole orbital route, in terms both of fitting the layout into the flat terrain and of the operational need to keep traffic flowing along the busy M4 into and out of London at all times. Several types of free-flow junction were considered, the eventual choice being a four-level design with an overall height of about 21m. There are 11 bridge structures, including three major viaducts between 182m and 264m, within the interchange. The original design of the decks was in prestressed concrete but, following the submission of a contractor's alternative the final construction medium was composite steel beams and concrete slab decks designed to BS 5400. Some 3½m cubic metres of filling material were imported for the engineering construction and for the substantial contoured landscaping, which helps to marry the links and the terrain.

In the Alder Bourne Valley selected for the interchange, there were two main design constraints. The interchange had to be unobtrusive and provide for freeflow traffic between M40 west of the interchange and M25 south. The result is a two-level junction with links and loops. Loose curves are provided for the south and westbound traffic and tighter radii for east and north moving traffic. The junction is well below the skyline and extensively landscaped.

M1/A405 interchange (Junctions 21 and 21A) (4,4,8 and 2,4,12)

THE interchange with M1 and A405 is in effect two interchanges with connecting links. The M1 part has been designed as a four-level structure with the top link road from M25 east to M1 north passing over M1, while the M25 and the M1 north to M25 west link road pass under M1. This has been done to reduce the visual impact of the interchange on the area. Extensive contouring and planting has also been incorporated into the design. The A405 part of the interchange is a conventional roundabout above the motorway. The M1 part omits the minor traffic movements from M25 east and west to M1 south and M1 south to M25 east and west. These movements are catered for by the A405 part of the interchange and M1 Junction 6.

THE A1(M) interchange at South Mimms required the enlargement and rearrangement of the existing roundabout between AM), M25 and A6. The completed M25, built largely along the route of the present A6, is taken over the roundabout.

THE M11 Theydon Interchange incorporates a two-level interchange where traffic movements in all directions are catered for by two-lane free-flow links with hard-shoulders. Limiting the interchange to two levels reduced the effect on surroundings and necessitated the construction of seven bridges, five over the live M11.