Its planners did not suspect that this length of motorway would, one day, be the busiest in the UK. In fact, if there's anything wrong with Ringway 4's Western Section, it's that it's a bit too useful.
In fact its usefulness is hard to overstate. Not only is it one of the most critical sections of road in the whole national motorway network, it's also virtually impossible to avoid when something goes wrong. Without the complementary Ringway 3 Western Section, and with local roads inadequate and badly congested, it is the only way to get around the edges of West London.
Back in the 1960s, when planning was still at a relatively early stage, it was already known that this road would be fundamental to London's highway network. Ringway 3 was supposed to carry long-distance traffic around the edge of London, but in the west, the direct line offered by Ringway 4 meant that many orbital journeys between the west and north would use this section of motorway, hopping over to Ringway 3's Northern Section using the Denham Spur. That is why Ringway 3 had the Denham Spur in the first place.
Ringway 4 is, of course, made up of the North and South Orbital Roads. The "Western Section" described here is really a bit of both - the South Orbital south of the Thames and the North Orbital north of it - which we are treating as a separate length of road simply to break up Ringway 4 into more manageable sections. The whole of this length would have been part of the M25, along with the South Orbital Road.
The Western Section does have its own identity, and it certainly makes sense to look at this length, between the M3 and A41, as a distinct project. Unlike the rest of Ringway 4, it was expected to carry a significant amount of orbital traffic, rather than serving east-west journeys between satellite towns. It also involved some of the most difficult planning work, threading a major motorway between villages, reservoirs, gravel workings and London's main airport.
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This description begins at the southern end of the route and travels north.
Thorpe to Hunton Bridge
At Thorpe, a major free-flowing interchange connecting to the M3 was built as planned, and exists today as M25 junction 12. Travelling north from there as a three-lane motorway, the route would connect to the A30 at Staines, crossing the Thames as it did so. The bridge across the river, originally constructed as part of the Staines Bypass, marks the transition from the South Orbital Road to the North Orbital Road.
The route north from there is similar to the M25 as eventually built, though it would have taken a smoother course past Wraysbury Reservoir, which was proposed to be smaller. A junction at Poyle would provide access to a spur road towards Heathrow Airport and, potentially, a motorway service area.
An interchange with the M4 would follow at Thorney, slightly west of the site of the present-day junction, and different in form. This section of motorway was planned by Buckinghamshire County Council in the 1960s, and their proposal was for a junction that could initially open as a large diamond-shaped roundabout before having additional sliproads built to form a free-flowing layout.
Between the M4 and M40, the eastern line for the motorway was chosen, which is similar to that eventually built with minor differences in alignment. Planners rejected an alternative western line, which would have run west of Richings Park and Iver, with a sharp turn at Iver Heath that was a reminder of its origin as an alignment originally plotted by Bressey in 1937. The eastern line was developed in the 1960s as a more suitable line for a motorway.
Ringway 4 would cross the M40 at Denham Interchange, just west of Denham Roundabout, with a four-level stack interchange interlinked with the roundabout and the Ringway 3 Denham Spur by braided sliproads. It would then continue north, shadowing the A412 North Orbital Road - an earlier section of this route built in the 1930s and fit only for replacement.
At Maple Cross, the motorway would swing a little way to the west, rejoining the modern line of the M25, with interchanges exactly as built at Maple Cross and Rickmansworth. This section, from M25 junction 17 to 19, was designed and built in the early 1970s as part of the North Orbital Road project and was a product of the Ringway era.
Following what is now the spur at M25 junction 19, the motorway would turn east to reach Hunton Bridge Roundabout on the A41, where it would terminate. The spur was built to be the mainline, with three lanes in each direction, and is today reduced to two lanes only by extensive use of paint. The roundabout was the first stage of an eventual three-level junction, where traffic could join the A41(M) north towards Hemel Hempstead, or the A41 south-east towards London and the next section of Ringway 4.
Walk the line
Discussing the Western Section in the context of the Ringways plan is difficult, because in the era the Ringways were being planned, prior to 1973, most of this road had no fixed line.
Between the Thames and the M4, the route was complicated by proposals to build a huge new reservoir at Wraysbury, and by some uncertainty over Heathrow Airport. At the time, a new third London airport was proposed at Maplin Sands (perhaps to be served by the M12 and M13), and the future role of Heathrow had yet to be nailed down.
North of the M4, the difficulties became greater. The line protected for the previous 30 years passed west of Iver and turned a sudden corner at Iver Heath, but finding a new route to replace it meant navigating countryside already strewn with railway lines, electricity pylons, gravel pits and other careless industrial scars, ideally without despoiling what little greenery remained. There were several competing options to choose from.
This was a job for the Ministry of Transport's Landscape Advisory Committee. On 6 March 1973, sixteen members of the Committee boarded a coach at Denham station to spend the day touring possible alignments for the North Orbital around Colnbrook, Iver and Richings Park. The meticulous record of their outing notes that they were served a buffet lunch in the Oxford Room at the Bull Hotel in Gerrard's Cross, to which the coach driver was also invited.
The Committee took a dim view of the original line west of Iver. Dozens of houses had been built right up to the protected corridor, and a school was also hard up against the boundary. It was not remotely suitable for a new motorway. Another line, Route IIa, was dismissed because it passed through parkland.
That left Route III, the eastern route, which skirted the water treatment works at Iver, diverted the River Colne and sliced an edge off Huntsmoor Park. The Committee concluded that it was the least poor of several poor options, and made a number of recommendations to move it a little further from Iver Village and make it less intrusive elsewhere. The line they recommended is the M25, as eventually built, from the M4 north to Iver Heath.
One of the other places that concerned the Landscape Committee was Denham, and with good reason.
An entirely artificial problem
Denham is a small and unassuming village in Buckinghamshire, just outside the Greater London boundary, not far from the old Oxford Road. It's picturesque, secluded and easily reached from the bright lights of London, which is why it's been home to a long list of the rich and famous. But some of them might have moved elsewhere if Denham Interchange had been built.
Back in the 1920s, Denham was a sleepy place where the A40 stumbled north-west out of Uxbridge town centre and met the A412. Over the following decades, for some reason, the Ministry of Transport seemed determined to bring as many new roads here as possible. First came Western Avenue (originally A403, but now A40) which travelled out of London to finish here. Then a diversion of the A332 (now A412) to meet the same junction.
By the early 1960s they planned to bring three more roads here - all of them motorways - to form a sort of super-junction with four motorway arms and six A-roads radiating away in all directions. Civil servants referred to it as the "Denham Complex".
The massive Denham Roundabout that exists today was built in preparation for it, but was only the first phase of a much larger project. That multi-lane roundabout was so vast that it proved almost impossible for vehicles to safely enter, because circulating traffic moved at such high speeds, and it has long since been reconstructed into a two-way road with multiple smaller roundabouts and signalised junctions.
From the west would come the A40(M) (now the M40) running end-on into Western Avenue. From the north-east would come Ringway 3's Denham Spur. And passing through on a north-south line, just west of the roundabout, would be Ringway 4's Western Section, part of the North Orbital.
Linking all ten arms (M40, two A40s, two Ringway 4s, A4020, two A412s, an A413 and the Denham Spur) with a single junction proved to be a considerable headache and generated a lot of irritable correspondence within the Ministry - but it was a problem entirely of their own making, a result of a concerted effort to bring as many new roads as possible piling up against each other just outside the sleepy village of Denham.
Buckinghamshire County Council had the job of designing this junction, as part of their work on the North Orbital, and in 1966 published plans for a four-level stack interchange with a tangle of braided sliproads connecting it to Denham Roundabout. The west-facing sliproads from the roundabout were built according to this plan, curved to account for other sliproads crossing over them. This plan was further complicated with the addition of the Denham Spur in 1967, but despite making it official the Ministry weren't quite convinced by it.
By 1970, the London North Orbital Feasibility Study had taken over design work, as part of a wider project to work out how Ringways 3 and 4 should be designed and laid out across the whole of north London. They reported, in 1971, with a series of four possible layouts that could charitably be described as "ambitious", and their preferred one - scheme 4 - is unorthodox to the point of being silly.
The Ministry's "General Planning Highways Division" were even less convinced with this new proposal, and sidelined it. One internal memo recommends considering the Feasibility Study's proposal as "preliminary to a preliminary report" on the junction complex. And then, of course, before anything further could happen, the Layfield Report was published and the Ringways were abandoned. There would be no need for such a monstrous junction after all.
By the 1980s, when the North Orbital was finally built as part of the M25, it was moved to run further west, away from Denham, and it now has a simpler interchange further along the M40. The residents of Denham must have been relieved.
A bucket full of holes
An orbital route around north London was an early priority in the inter-war roadbuilding programme. A section of North Orbital from A41 to A1 was opened in 1932; a second length, from the A40 to Maple Cross, was inaugurated in 1937. That left a gap between Maple Cross and the A41 that was supposed to be filled quickly afterwards, bypassing Watford - but the war got in the way.
By the 1960s, the 30-year delay was growing tiresome, and the members of Hertfordshire County Council and Watford Borough Council were pushing the Ministry for some progress. The need to plug a gap between existing lengths of North Orbital Road is the reason why proposals entered the roads programme in the early 1970s for the section between Maple Cross and Hunton Bridge.
The trouble with waiting 30 years to continue a road project is that things move on a bit. Engineers in the 1930s designed a state-of-the-art road, with a 33 foot (13m) wide carriageway and parallel service roads through towns and villages to keep parked cars out of the way. By the 1960s, though, predicted traffic levels were a bit higher and expectations had changed.
The Maple Cross to Hunton Bridge length, therefore, occupied the route set aside in the 1930s but was designed to the latest standards - meaning it would have three lanes each way, grade-separated junctions, hard shoulders and motorway restrictions. It was one of the first sections of M25, opening in 1976, and now forms the length between junctions 17 and 19.
Once Maple Cross to Hunton Bridge was fixed, though, the trouble wasn't over. One hole in the orbital project had been plugged, but it immediately sprung another leak. The five miles of new motorway would be out of place in the midst of 1930s single carriageway roads, so now the existing sections required improvement. An on-line upgrade of the A412 between the A40 and Maple Cross was the working assumption for much of the 1960s.
By the time it reached detailed planning, though, it was clear the old road could not be widened enough for predicted traffic levels. A full motorway was needed, and so the road built by far-sighted planners in the 1930s was sidelined, destined never to be part of a finished orbital. Ringway 4's western section would bypass it entirely.
Further north, at Hunton Bridge, the route met the A41 and A41(M) with space for an eventual three-level interchange. Orbital traffic would then hop along the A41 to pick up the next bit of North Orbital Road.
By 1970 that too was looking less than watertight. Demand for travel from the M25 to the M1 couldn't be served by the A41. The three-level interchange at Hunton Bridge was scrapped in favour of a plan to bypass the whole junction with a new length of Ringway 4 to the north, to be built when the roundabout became overloaded. It would pass close to the Ovaltine works at King's Langley, and became known as the "egg farm route". Its construction would mean the abandonment of another length of orbital - this time part of the brand new M25 and the old 1930s road through Leavesden.
By the time the Ringways had been scrapped, and lengths of Ringways 3 and 4 were being linked to cobble together the modern M25, the egg farm route was just the thing. In 1986, it was used to bypass Watford to the north, forming the final length of the M25.
- Route map contains OS data © Crown copyright and database rights (2017) used under the terms of the Open Government Licence.
- Diagram of M4 interchange extracted from MT 120/234.
- Photograph of M25 near Chorleywood taken from an original by Marathon and used under this Creative Commons licence.
- Plans of corner at Iver Heath taken from Greater London Plan (1944), Patrick Abercrombie, published by University of London Press and now out of copyright; MT 106/291; MT 120/252.
- Photograph of Huntsmoor Park taken from an original by Des Blenkinsopp and used under this Creative Commons licence.
- Denham interchange sketches extracted from MT 106/290 and MT 120/233; traffic flow diagram extracted from MT 120/248.
- 1966 plan for Denham interchange extracted from MT 120/249.
- "Scheme 4" plan for Denham interchange extracted from MT 120/252.
- Photograph of NOR near Denham in the 1930s extracted from National Archives catalogue ref. INF 9/856/4.
- Photograph of M25 at King's Langley taken from an original by Mr Biz and used under this Creative Commons licence.
- Route, Thorpe-M4: MT 120/215.
- Route, M4-M40: MT 120/234.
- Route, Maple Cross-Hunton Bridge: MT 106/291; MT 138/44.
- Route, M4-M40, via Iver Heath: MT 127/80.
- Use of R4 for orbital traffic via Denham Spur: MT 120/233; MT 120/248.
- Visit of Landscape Advisory Committee: MT 127/80.
- Draft plans for Denham interchange: MT 106/290; MT 120/233.
- Bucks CC layout for Denham interchange: MT 120/249.
- Amendment to incorporate Denham Spur; LNOFS scheme 4 layout for Denham interchange; dismissal by MOT: MT 120/252.
- North Orbital 1930s opening dates: MT 39/511.
- Lobbying from Herts CC and Watford BC; on-line improvement of A412 initially proposed: MT 106/291.
- Hunton Bridge roundabout laid out for future three-level junction: CP111.6/8.
- Northern bypass for Hunton Bridge, "egg farm route"; use of route for M25: MT 120/314.