Speed limits For the majority of its length, the
national speed limit applies. Exceptions include the following: • on the
Chiswick Flyover within London in both directions. • between junction 4 and the Chiswick Flyover, eastbound only. • on the
Port Talbot elevated section between junction 40 and junction 41. The fixed speed camera was removed in 2006 as it was believed to be causing tailbacks. In July 2014, an
average speed camera system was installed; it generated around £500,000 in fines in the first six months. • between junctions 24 and 28 at
Newport.
Smart motorway scheme at junction 27 and a new concrete reservation (2010) The first section of
smart motorway on the M4, between junctions 19 (M32) and 20 (M5) north of Bristol, has variable speed limits and includes a part-time hard-shoulder. Completion was in summer 2014. Another section between junctions 24 and 29 in Newport had variable speed limits until 2021, when it was changed to a permanent limit with average speed cameras. In 2010, it was announced that the motorway would be changed to a
smart motorway between junctions 3 and 12. With a length of , on completion it became the longest smart motorway scheme in the United Kingdom. Work started in autumn 2018 and was completed ahead of schedule in December 2021 at a cost of £848 million. Further conversions to smart motorways have been halted indefinitely.
Brynglas tunnels – western portals The
Brynglas Tunnels carry the M4 under
Brynglas Hill in Newport. The tunnels are the first and only twin–bored tunnels in the UK motorway network (the
Dartford Tunnel is not classified as part of the
M25 motorway). In July 2011, a lorry fire in one tunnel closed the motorway. Although there were no injuries and no deaths, the tunnel remained closed and a contraflow system was in place in the remaining tunnel for about one month, causing major travel delays.
Bus lane In June 1999, the
M4 bus lane was created on the third lane between junctions 2 and 3, initially as a pilot scheme and then a permanent arrangement from 2001. A lower speed limit was introduced along this section at the same time. The bus lane was on the eastbound carriageway; from the western end of the Chiswick Flyover near
Brentford to junction 3 (A312), covering part of the journey between
Heathrow Airport and central London. The lane which had no intermediate exits was for use by buses, coaches, motorcycles, emergency vehicles and licensed taxis but not mini-cabs. In December 2010, the bus lane was suspended for 18 months. It was reinstated temporarily for the
2012 Summer Olympics and then permanently decommissioned.
Porous road surface Near junction 35 of the M4, there is a stretch of the motorway that has a surfacing of porous
asphalt that improves drainage and reduces noise. When driving in heavy rain drivers notice a reduction in
road spray from other vehicles and improved visibility. This special surface was publicised in an episode of the BBC's ''
Tomorrow's World'' programme. This was the site of the first trial of the new road surface when it was laid down in 1993.
Elevated and heated section The elevated section of the M4 in West London, built in the 1960s, is mostly directly above the A4 and extends over parts of
Brentford's
Golden Mile. This section was designed to have a heated road surface to reduce icing in winter.
Four-level stack interchanges near
Heathrow Airport The M4 has two of the three four-level
stack interchanges in the UK, including the first UK example at junction 20, the "
Almondsbury Interchange" with the M5. The other is at junction 4b, the "
Thorney Interchange" with the M25; this interchange has to make provision for a railway line passing beneath the M4. Due to the nature of these junctions, one cannot make a U-turn at either of them.
Notable junctions Junction 8/9 near
Maidenhead,
Berkshire, and
High Wycombe,
Buckinghamshire is the only one in the UK with dual numbers. This dates from the time when the M4 turned north near junction 8, where it met the A308, and headed for the original junction 9, where the motorway ended at a
roundabout interchange with the
A4. When the westward extension was opened, junction 8 was closed and a new junction built a little to the west, taking both numbers. The road to the A4 became A423(M) and later
A404(M), and the junction with the A4 became 9B. Junction 9A is the exit for
Cox Green and
White Waltham. To the west of junction 13 on the eastbound carriageway there are a set of sliproads signposted "Works Unit Only". The signs have red borders, implying a military exit. It is a back entrance to
RAF Welford, a Second World War airfield and now an RAF/
USAF military installation mainly used for storing munitions. The M4 entrance allows easier access for the large vehicles used to carry the munitions. Plans for the "missing" Junction 31, also known as the
Thornhill interchange, for which
planning permission was originally granted in September 1991 (but subsequently expired), were rekindled after proposals for a new business park on a site north of the M4 were submitted in 2007 to
Cardiff Council. The developers of the business park,
St Modwen Developments, would likely fund the new junction, which would be on the
A469. A freedom of information request in 2010 to Cardiff Council shows that whilst the land that would enable this junction should continue to be strategically protected, the decision to formally abandon the proposed Junction 31 Thornhill was made in October 2007 and there had been no subsequent mention of it in Cardiff Council Strategic or Planning meetings since. In South Wales, the M4 has to thread its way through mountainous terrain and built-up areas, so there are some unusual junction layouts. Junction 27 (
High Cross) is a normal grade-separated roundabout junction, but has severe space constraints: traffic joining the motorway must initially travel in the opposite direction to the intended direction of travel, before making a sharp left-hand turn from the slip road onto the motorway. At the time of construction, junction numbers 30 and 31 were reserved for future intermediate interchanges. Junction 30 (
Cardiff Gate) has since been added, but there are no current plans to construct Junction 31 (
A469 road). Junction 39 can only be used to access the motorway from a single slip road onto the westbound carriageway from the A48 at junction 38. There is no exit from the motorway at this junction. Junction 41 comprises two different junctions; one for local traffic to and from the west and one from the east. The former leads to and from a spur leading to the roundabout in
Briton Ferry, formerly known as junction 41a, and the original bridge over the
River Neath, which would allow access onto the stretch of the M4 from junction 43 westward. The second, eastern junction leads to and from the A48 towards
Port Talbot. As a result, one can travel for almost on the motorway in either direction, both joining and then leaving the motorway at junction 41. Junction 44 is unusual in that the eastbound entrance dives under the inside of the junction, effectively creating a "right-turn" on a roundabout. Similarly, slip roads pass under or over the main motorway at junctions 41 and 42. There have been calls to close the
slip roads at junctions 40 and 41 to improve traffic flow. The motorway has only two lanes on this stretch and is a major traffic congestion blackspot. The short slip roads have not been modernised. A small-scale trial of closing the westbound on-slip of junction 41 between 7 am and 9 am and from 4 pm to 6 pm on weekdays started on 4 August 2014 but following heavy criticism from local businesses and residents, was stopped on 29 May 2015. ==Relief road==