A new road from London to South Wales was first proposed in the 1930s, and the Ministry of Transport announced plans for the M4 as one of the first major post-war trunk road improvement projects in 1956.
The Chiswick flyover, a short section of elevated dual-carriageway built to reduce the impact of traffic travelling between central London and the west, opened in 1959; it was not originally classed as a motorway. The Maidenhead bypass opened in 1961 whilst J1-J5 opened in 1965. The stretch from J18 to the west of Newport was opened in 1966, including the Severn Bridge (now part of the M48). The Port Talbot by-pass, also built in the 1960s and now part of the M4, was originally the A48(M) motorway, a number now allocated to a short section of motorway near Cardiff. The Ministry of Transport originally intended that the M4 would terminate at Tredegar Park west of Newport, and it was only following the creation of the Welsh Office that the Government became committed to a high-standard dual carriageway to Pont Abraham in Carmarthenshire.
The English section of the motorway was completed on 22 December 1971 when the 50-mile (80 km) stretch between Junctions 9 and 15 (Maidenhead and Swindon) was opened to traffic. The Welsh section was completed in 1993, when the Briton Ferry motorway bridge opened. The Second Severn Crossing opened in 1996, together with new link motorways on either side of the estuary to divert the M4 over the new crossing. The existing route over the Severn Bridge was redesignated the M48, and the new M49 was opened to connect the new crossing to the M5.
In June 1999 the section of the third lane (the lane nearest the central reservation) between Junctions 2 and 3 was converted to a bus lane, first as a pilot scheme and then permanently in 2001. A lower speed limit was introduced along the bus lane section at the same time (the lane was however reverted to general traffic use by the incoming coalition government in late 2010).
In April 2005 speed checks carried out by police camera vans between Junction 14 and Junction 18 led to a public protest, involving a "go-slow" of several hundred vehicles along the affected sections of the motorway.
Between 2007 and January 2010 the section from Castleton (Junction 29) to Coryton (Junction 32) was widened to six lanes. The scheme was formally opened on 25 January 2010 by Ieuan Wyn Jones the Deputy First Minister for Wales.
During 2009 the Newport section of the motorway between Junctions 23a and 29 was upgraded with a new concrete central barrier. In February 2010 it was proposed that the M4 in South Wales would become the first hydrogen highway with hydrogen stations provided along the route, with an aspiration for further stations to be provided along the M4 into South West England over time. A similar claim was made for a 30-mile (48 km) section of road in Scotland close to Aberdeen in September 2009 with refuelling points at Bridge of Don, Ellon and Peterhead.
The original (A48(M)) bridge over the River Neath
is to the right, the new M4 bridge is to the left
In October 2010 the new Secretary of State for Transport, Philip Hammond, announced that the bus lane in the London section would be suspended for 18 months from 24 December 2010, be brought back into use temporarily for the 2012 Summer Olympics, and then be removed.
Between 2008 and 2010, Junction 11, near Reading, was extensively remodelled with a new four-lane motorway junction, two new road bridges and other works. The £65m scheme included work on the Mereoak roundabout and part of the A33 Swallowfield Bypass near Shinfield, and also the conversion of the two existing bridges, one of which is available only to pedestrians and cyclists and the other to buses. It also involved the movement of the local Highways Agency and Fire Service offices, and the construction of a long footbridge network, a new bus-lane and a new gyratory. Sound barriers for nearby residential areas were also installed. In April 2008, the decision to preserve a rare Vickers machine gun pillbox and turn it into a bat roost was announced by the developers.
The table below shows the timeline for the construction of the motorway on a section by section basis.
An animated map showing build progress at five-year (or greater) intervals.
: When the Second Severn Crossing
was opened in 1996 the M4 was re-routed to cross the new bridge; the previous section of motorway between Junctions 21 to 23 was renamed the M48