After the war, the Special Roads Act 1949 gave highway authorities powers for the first time to provide roads restricted to certain classes of traffic, with access limited to a few specially constructed interchanges. The Minister of Transport decided that these powers should be used to provide the Maidenhead By-Pass, and the adjoining by-pass of Slough, as Trunk Road Motorways.

In 1956 the Government announced a plan for modernising the trunk road network based on five major projects, one of which was a road from London westwards to London Airport, the west of England and South Wales. The Maidenhead and Slough By-Passes were to be incorporated into this road, and construction of the Maidenhead By-Pass started again in 1959.

In 1960 the target of constructing 1,000 miles of motorway by the early 1970s was set. The London-South Wales Motorway was part of this 1,000 mile programme, and with the opening of the Maidenhead By-Pass in 1961 the first six miles of the M4 Motorway came into use.

Some six years later, in 1967, some 63 miles were open to traffic - 24 at the London end between Chiswick and Maidenhead Thicket, and about 39 at the western end between Tormarton in Gloucestershire and the Newport By-Pass in Wales. This western length included the Severn and Wye Bridges which were opened by Her Majesty the Queen in September 1966.

The 78-mile gap between Tormarton and the Maidenhead By-Pass remained to be filled. In June 1971 the gap began to close when 10½ miles between Tormarton and Stanton St Quintin were opened, and by November the motorway was extended another 18 miles eastwards to Badbury in Wiltshire, leaving only 50 miles to be completed. The opening of the 50 mile length between Badbury and Maidenhead By-Pass (Holyport) meant that there were 139 miles of continuous motorway between London and South Wales.

In January 1961 Sir Alexander Gibb & Partners were appointed as Consulting Engineers by the Minister of Transport to investigate and report on possible routes for the motorway. The terminal points were to be at Tormarton in Gloucestershire and on the Maidenhead By-Pass. The plan then existing (based on work done by the Berkshire and Wiltshire County Councils prior to the outbreak of war in 1939) was for a motorway keeping to the south of the Bath Road (A4) from Maidenhead to Chippenham, apart from a short length north of Calne, and then passing to the north of the Castle Combe valley. The lengths in Wiltshire and east of Reading were being protected from development by the local planning authorities.

At an early stage in the investigation it became evident that Reading and Swindon were the two focal points and that the protected route in Wiltshire should be abandoned. The route then recommended by the Consulting Engineers for the 30 miles eastwards from Tormarton to a point near Swindon has been adhered to ever since with only minor variations, but before a satisfactory route could be found for the 48 miles between Swindon and Holyport no less than 815 miles of routes were surveyed and studied. During these investigations there was much public concern about the possible routes for the motorway east of Swindon. One of the routes passed to the north of Swindon and followed the Vale of White Horse to two crossings of the River Thames near Wallingford and Henley. Another adhered close to the Bath Road (A4) between Maidenhead and Kintbury and known as the Direct Route, crossed the Thames at Shiplake and Streatley and proceeded almost due west, keeping to the north of Lambourn. This was the route initially preferred by the Minister, but changes were made to it to avoid most of the high downland and the Oxfordshire beechwoods. The concept was again re-examined to take account of possible developments arising from the Channel Tunnel project and the South East England traffic study, then recently completed. Finally, in August 1965, the Minister of Transport announced details of the selected route and between 1966 and 1968 draft Schemes under the Highways Act 1959 were published to establish the route between Tormarton and Holyport. After the consideration of objections the route was finally fixed with some minor modifications in August 1968.

Orders under the Highways Act 1959 also had to be made to authorise alterations to existing roads necessitated by the construction of the motorway, and nearly 50 miles of new side roads have been built along the 78 miles of motorway.

The new Motorway leaves the Maidenhead By-Pass at Holyport, on a 19 mile section constructed by Costain Civil Engineering Ltd. At Winnersh a free flow junction and further work, built by W & C French (Construction) Ltd, ensured an easy access to Reading. The Motorway passes to the south with an interchange on the Reading-Basingstoke road and another on the A4 at Theale where a new by-pass will reduce the traffic in this severely overloaded village.

Construction of the next length fell to Sir Lindsay Parkinson & Co Ltd After crossing the Theale - Pangbourne road the climb on to the Berkshire Downs begins, and at this point some Roman villas were unearthed and were fully documented by local Archeological Societies before the work proceeded. The section across the Downs with its flowing curves and views of valley and woodland and more distant glimpses of the Hampshire Downs to the south is possibly the most delightful section of the Motorway; though its complex geology made construction difficult. After a junction to Newbury 4 miles to the south, the Motorway crosses the Lambourn Valley on a short viaduct and climbs over higher up the spine of the Downs to the highest point at Baydon, approximately 800 ft above sea-level. After the crossing of the Lambourn valley, A Monk & Co Ltd took over construction and of all the Contractors engaged on the works, probably faced the bitterest weather and the most unpleasant conditions.

After the high point at Baydon, a new section begins built by W & C French (Construction) Ltd; the Motorway drops abruptly to the lower ground south of Swindon. A connection to Swindon is provided on the A345, and immediately afterwards a length of grade separated carriageways marks a difficult area where the Contractors contended valiantly with slipping ground which was finally stabilised by the bank on which the southern carriageway is laid. The Motorway passes close to Swindon on its southern side and then to the north of Wootton Bassett, at which point Sir Alfred McAlpine & Son Ltd took over. The Motorway runs through Dauntsey Vale and the plain of the Bristol Avon. Here the Contractor had to contend with some very weak clays and emerged triumphant from a bitter struggle to complete the embankment close to the Western Region main line.

The Motorway crosses the Bristol Avon and the climb up to the ridge of the Cotswolds begins. There is a connection to Chippenham, 4 miles to the south, on the A429 and thence with sweeping curves of up to 23 miles radius, reminiscent of Brunel's railway, the Motorway achieves the ridge of the Cotswolds and the junction with the completed section leading to the Severn Bridge and South Wales.

The line of M4 starts at Holyport in the London Clay, which is overlain in places by Thames gravel, and passes successively through the Reading Beds, the Upper, Middle and Lower Chalk, the Upper Greensand, the Gault and Kimmeridge Clays, the Corallian strata, the Oxford Clay, Cornbrash and the Forest Marble series finally meeting the Great Oolite Limestone of the Middle Jurassic series at Tormarton.

Nearly 15 million cu yds of material were excavated from the line of the motorway, including 1 million cu yds of rock. About 24% of this material proved to be unsuitable for use in the embankments and was carted to tip. Replacement of the unsuitable material, together with requirements for embankments in low lying areas which could not be met from cuttings made necessary a total import of some 8.5 million cu yds, 55% of which was used on Contract 5 alone.

On the whole, the clay strata gave rise to fewest problems, and these arose chiefly where the clay was associated with other materials. For example in the Forest Marble series, bands of clay interbedded with limestone seams became very wet when water lying in the fissured rock was released by excavation. In the same way, clay overlying chalk became unusable if allowed to mix with the water given up by excavated chalk.

The Reading Beds of Berkshire were a particular problem owing to the variety of materials and their thinly bedded nature. They range from clean single sized sands to fatty clays, and it was impossible to separate suitable from unsuitable material.

Excavation was carried out almost exclusively by scrapers, including some of the biggest machines available, such as the Caterpillar 657 of 44 cu yds pay load. The scraper fleet in June 1970 totalled 166 machines.

Side slopes in cuttings are normally 1 in 3 in clay, although in certain deep cuttings this was increased to 1 in 4. Cutting slopes in rock are 1 in 14. Side slopes to embankments are 1 in 2.

The pavement consists of dual 36 ft carriageways throughout with 9 ft 6 in hard shoulders, 5 ft 0 in verges and a 13 ft 0 in central reservation. Alternative tenders for rigid and flexible construction respectively were invited for all contracts. As result, flexible construction was adopted for the main M4 contracts while in Contract 7 for the Winnersh Link a rigid pavement was provided.

Sub-base thicknesses were determined in accordance with Road Note No 29 and ranged from 6 in to 20 in Isolated pockets required even greater thicknesses. Lower sub-base materials, that is all except the top 6 in, are all local gravels except on Contract 1 where a low grade crushed limestone was taken from a quarry adjacent to the motorway. The upper sub-base consists of either crushed limestone, brought in by both road and rail, or cement bound granular material from local sources.

For the flexible construction all contractors elected to use a composite base throughout, comprising 7 in of lean concrete under 3 in of dense tarmacadam or dense bitumen macadam. The surfacing is 4 in of hot rolled asphalt.

Contract 1 is close enough to the source of supply in the Mendips for coated roadstone to have been delivered direct from individual quarries, but in all other cases the contractors chose to erect special asphalt plants to produce material exclusively for their own contracts.

Supplies of stone were drawn from the Mendips and arrived by rail at Swindon, Newbury and Theale, together with further quantities direct by road.

During 1971 it was estimated that about 950,000 tons of bituminous and tarbound materials were laid, involving some 31/4 million sq yds of road surface, and a further 273,000 tons were laid the previous year.

The key to the timely completion of motorway contracts with short contract periods is an early start on bridges. It was clear that on a length of 78 miles there would be considerable scope for standardisation of bridge parts, and for both these reasons it was decided to let separate contracts for the manufacture of precast units for bridges. The first contract was let in November 1968 with the object of having units ready for the first two motorway contracts, and two others were let subsequently.

Underbridges carrying the motorway over side roads or railway lines utilise standard bridge beams seated on rubber pads on vertical abutment walls. The latter are normally precast panels supported on in-situ concrete foundation blocks, again on rubber pads, and post-tensioned horizontally but in situ abutment wall construction has been used in certain cases when loading conditions required it.

All beams are set at right-angles to the abutments, and in the case of skew bridges a number of beams at each side of the bridge have one end supported by a heavy edge beam, normally of the hollow box type.

Underbridges are founded on spread footings where the ground permits, but in other cases closely spaced cast in situ bored piles have been used on all contracts except No 4 where driven, cast in situ Holmpress piles were used.

Overbridges are all basically single box beams with propped cantilevers on both sides. The box consists of precast soffit and deck slabs in conjunction with cast in situ webs. The outer faces of the webs are cast against precast walls forming part of the cantilevers. These cantilever units are up to 30 ft long and consist of the vertical web shutter just referred to, and a horizontal deck member, together with supporting diagonal struts. The span is stiffened by torsion frames at the supports.

After constructing the webs, the spans were prestressed longitudinally, and in the case of wider bridges, transversely as well.

Although varying conditions of loading, span and skew required a large number of different units, the differences were confined mainly to reinforcement, duct positions etc., and comparatively few moulds were required.

Supports for overbridges are normally three in number together with two bankseats. The fixed bearing is normally a rubber pad while the others are rubber blocks or PTFE sliding bearings.

Where considerations of visibility demand it, the central support has to be omitted, and in this case the bridge span forms a two pin portal with a triangular frame support at each end. Bridges of this type incorporate prestressed ties below ground level between the two hinges.

Overbridges are supported on spread footings where the ground conditions permit, but in the majority of cases it was found necessary to use piles. Normally 2 or 4 bored piles of diameters ranging from 24 to 36 in were used, together with enlarged base if necessary.

Farm access bridges, although designed for HA loading have a road width of only 12 ft and a simplified form of span was possible. These bridges are of trough form, the vertical sides being precast and the deck formed in situ. The supports are similar to those used for the single span highway bridges, that is, triangular frames forming a two pin portal with ties between the hinges.

Footbridges have a fully precast deck comprising trough-section cantilevered units at each side supporting a central span on rubber bearing pads.

The design of the supports follows the triangular frame scheme but in this case without the underground ties between foundations. In order to provide counterweight for supporting the central spans, the rear ends of the cantilevered units are tied to concrete blocks buried in the approach ramps.

Culverts, whether for access, services, services or waterways are all constructed from precast units of two basic types. The walls consist of L-shaped units of varying height and base and 3 ft 5 in wide, while the roof is formed from beams of varying length, but again 3 ft 5 in wide.

The culverts were constructed by setting the wall units on a suitable bedding and filling the remainder of the floor with in situ concrete. The gaps between units were then filled and the roof units placed.

The advantages of this form of construction are speed of erection and the ability to cater for different combinations of height and width with a limited number of unit types.

All bridge decks are fully waterproofed. The original specification called for a butyl rubber membrane but this type was subsequently deleted from the list (BE27) approved by the Department of the Environment and contractors were asked to choose an alternative from the approved types. They were given a limited choice in order to take advantage of any specialised skills in laying that might be available.

The choice in most cases was a self adhesive preformed rubberised bitumen membrane carried on a nylon fabric, although in a few instances, a brushed on epoxy membrane was used. Where a preformed membrane was used, it was stopped short at kerbs and the area under the latter was waterproofed with an epoxy compound. This course was adopted because it is difficult to set kerb stones firmly on a preformed membrane and they are liable to be displaced by traffic.

In all cases the waterproof layer was protected by 3/4 in of sand asphalt before the 2 in wearing course of rolled asphalt to BS 594 was placed.

Bridge parapets are in aluminium, designed to Department of the Environment rules and incorporate wire mesh panels protected with a nylon coating.