The South Wales motorway, as planned for years before the Ringways came along. By the time the Ringways were being discussed, it was open right to its current London terminus at Chiswick, though further extensions were proposed that have not seen the light of day. There is very little uncertainty surrounding this proposed route now and it's one of the easier parts of the Ringway plan to trace.
Continues to Slough & Reading
R4 Western Section
Spur to Heathrow Airport & West Drayton
R3 Western Section
A4 Great West Road
R2 Western Section (Chiswick Flyover)
Possible local connection to Chiswick
M3 or A316
Local connections to Hammersmith & Fulham
R1 West Cross Route
Continues as A4 Cromwell Road
The M4 was intended to go exactly where it does today, in from the Slough direction, with a spur to Heathrow, then crossing Ringway 3 at present-day junction 3. Its connection with Ringway 4's Western Section might have been at the present M4/M25 junction or slightly to the west at Richings Park, and the junction with Ringway 3 would probably have been somewhat larger than the present-day junction 3. The present roundabout seems to give some limited scope for a three-level stacked roundabout junction, if nothing else.
East from there, it would continue into London as now. There is a very vague hint in some documentation submitted to the Greater London Development Plan that the present elevated section (shown left), which is only two lanes in each direction, might have been upgraded or replaced with something much larger to tie it in with Ringway 2 at Chiswick - but this remains a very insubstantial claim and it's not clear at all whether any real proposal was made to this effect.
Two different junction designs were proposed to replace the existing Chiswick Flyover; one is a fairly sensible limited-access junction and the other a more complex all-movements interchange, in which several of the turning movements appear to be made redundant by the M3 or A316 a short distance to the south-west. The latter has no connections to the existing Chiswick Roundabout. Both junctions would involve five levels of flyovers, and the resulting stack would rise 25m (85 ft) above the Chiswick skyline. There does not appear to have been any suggestion to put any roads below surface level.
Beyond Chiswick, most plans of the era show the A4 being upgraded to M4 at least as far as Hammersmith. Possibly this meant only a motorway-standard upgrade and not an actual motorway, but it's impossible to tell. Various proposals for Ringway 1's West Cross Route show either a three-level stacked roundabout junction for the A4 Cromwell Road, which indicates that the M4 was not expected to get this far, or a free-flowing west-facing junction, which indicates that it was. Probably no decision was made before the Ringway plan was halted in the early 1970s.
Part of this confusion can be explained by the fact that west of the M3 or A316 junction the A4 was a trunk road at the time, while east of there it was the responsibility of the GLC, and the two bodies are known to have had some trouble in co-ordinating their plans for major roads.
Previous versions of this page suggested that, because of the alignment of the various roads connecting at Chiswick and the uncertainty surrounding the A4 upgrade to motorway towards Hammersmith, the M4 might actually have been proposed to run via Barnes and Wandsworth to terminate at Clapham Junction. It now seems quite clear that this is not the case.
A motorway was indeed proposed connecting Chiswick, Barnes and Wandsworth, but it was part of the Western Section of Ringway 2 and would have turned south along the Wandle Valley. The motorway link onward from Wandsworth to Clapham Junction was a connecting link between Ringways 1 and 2. The M4 itself would only have been extended as described above.