Originally built almost wholly as a dual three-lane motorway, much of the motorway has been widened: to dual four lanes for almost half, to a dual five-lanes section between junctions 12 and 14 and a dual six-lane section between junctions 14 and 15. Further widening is in progress of minor sections with plans for managed motorways in many others.
To the east of London the two ends of the M25 are joined to complete a loop by the non-motorway A282 Dartford Crossing of the River Thames between Thurrock and Dartford. This crossing, which consists of twin two-lane tunnels and the four-lane QE2 (Queen Elizabeth II) bridge, is named Canterbury Way. Passage across the bridge or through the tunnels is subject to a toll, its level depending on the kind of vehicle. This stretch being non-motorway allows traffic, including that not permitted to use motorways, to cross the River Thames east of the Woolwich Ferry; the only crossing further to the east is a passenger ferry between Gravesend, in Kent, and Tilbury, in Essex.
At Junction 5, the clockwise carriageway of the M25 is routed off the main north–south dual carriageway onto the main east–west dual carriageway with the main north–south carriageway becoming the A21. In the opposite direction, to the east of the point where the M25 diverges from the main east–west carriageway, that carriageway become the M26 motorway.
The radial distance from London (taken as Charing Cross) varies from 12.5 miles (20.1 km) in Potters Bar to 19.5 miles (31.4 km) in Byfleet. Three Greater London boroughs (Enfield, Hillingdon and Havering) have realigned their boundaries to the M25 for minor stretches; while in others, most notably in Essex and Surrey, the radial gap between Greater London and the motorway reaches 7.8 miles (12.6 km), neither of which coincide with the Metropolitan Green Belt. Major towns listed as destinations (right), in various counties, adjoin the M25. North Ockendon is the only settlement of Greater London situated outside the M25. In 2004, following an opinion poll, the London Assembly mooted for consultation alignment of the Greater London boundary with the M25. "Inside the M25" and "outside/beyond the M25" are colloquial, looser alternatives to "Greater London" sometimes used in haulage. The Communications Act 2003 explicitly uses the M25 as the boundary in requiring a proportion of television programmes to be made outside the London area.
Two motorway service areas are on the M25, and two others are directly accessible from it. Those on the M25 are Clacket Lane between junctions 5 and 6 (in the south-east) and Cobham between junctions 9 and 10 (in the south-west). Those directly accessible from it are South Mimms off junction 23 (to the north of London) and Thurrock off junction 31 (to the east of London). Cobham services opened on 13 September 2012.
Originally, the M25 was unlit except for sections around Heathrow, major interchanges and Junctions 23–30. Originally, low pressure sodium (SOX) lighting was the most prominent technology used, but widening projects from the 1990s onwards have all used high-pressure sodium (SON) lighting and this has diminished the original installations. By 2014 only one significant stretch was still SOX-lit (Junction 25–26) and the units were removed the same year.
The motorway passes through five counties. Junctions 1A–5 are in Kent, 6–14 are in Surrey, 15–16 are in Buckinghamshire, 17–25 are in Hertfordshire, and 26–31 are in Essex. Policing of the road is carried out by an integrated policing group made up of the Metropolitan, Thames Valley, Essex, Kent, Hertfordshire and Surrey forces.
The M25 is one of Europe's busiest motorways. In 2003, a maximum of 196,000 vehicles a day were recorded on the motorway just south of London Heathrow Airport between junctions 13 and 14.
A precursor of the M25 was the North Orbital Road (see A414 road).
The idea of an orbital road around London was first proposed early in the 20th century and then re-examined in Sir Charles Bressey's and Sir Edwin Lutyens' The Highway Development Survey, 1937. Sir Patrick Abercrombie's County of London Plan, 1943 and Greater London Plan, 1944 proposed a series of five roads encircling the capital. The northern sections of the M25 follow a similar route to the World War II Outer London Defence Ring.
Little was done to progress these plans until the 1960s when the Greater London Council developed its London Ringways plan consisting of four "rings" around the capital. Sections of the two outer rings – Ringway 3 (the 'M16 motorway') and Ringway 4 – were constructed in the early 1970s and were integrated into the single M25 orbital motorway. But the Ringways plan was hugely controversial owing to the destruction required for the inner two ring roads, (Ringway 1 and Ringway 2). Parts of Ringway 1 were constructed (including West Cross Route), against stiff opposition, before the overall plan was abandoned in 1973 following pressure from residents in the threatened areas.
Construction of parts of the two outer ring roads, Ringways 3 and 4, began in 1973. The first section, between South Mimms and Potters Bar in Hertfordshire (junction 23 to junction 24) opened in September 1975 and was given the temporary general purpose road designation A1178 (a section of motorway-standard-road, originally the M16, which eventually was incorporated into the M25) was completed and operational before this. A Watford-avoiding route between the M1 and the A40 between north Watford and Denham was locally known as the Croxley Green/Rickmansworth bypass, and was operational about 1973/4; a section south of London (junction 6 to junction 8) opened in 1976. A section of Ringway 3 south of the river between Dartford and Swanley (junction 1 to junction 3) was constructed between 1974 and 1977. In 1975 the plans for Ringway 3 were modified to combine it with Ringway 4, the outermost Ringway. The M25 as a component of ringway 4, was first conceived to be an east-west road south of London to relieve the A25, and running parallel to it, with its eastern end following the route of what is now the M26. However, it was subsequently routed northwards towards the Dartford Tunnel to form, in conjunction with similar roads, including the M16 planned to the north of London, part of the London Orbital. The combined motorway was given the designation M25 which had originally been intended for the southern and western part of Ringway 4 and the M16 designation was dropped. The section of Ringway 3 west of South Mimms anti-clockwise around London to Swanley in Kent was cancelled. The stages were not constructed contiguously but in small sections. As the orbital road developed the sections were linked. Each section was presented to planning authorities in its own right and was individually justified, with almost 40 public inquiries relating to sections of the route. Maps at this time depicting these short sections named the route as the M16 but this changed before completion.
The section from Potters Bar to the Dartford Tunnel was constructed between 1979 and 1982. Construction of the M25 continued in stages until its completion in 1986. Prime Minister Margaret Thatcher officially opened the M25 on 29 October 1986, with a ceremony in the section between J22 and J23 (London Colney and South Mimms). The initial tenders for the construction of the M25 totalled £631.9 million. This did not include compulsory purchase of land and subsequent upgrades and repairs.
Soon after the motorway opened in 1986 traffic levels exceeded the maximum design capacity and in 1990 the Secretary of State for Transport announced plans to widen the whole of the M25 to four lanes. By 1993 the motorway, which was designed for a maximum of 88,000 vehicles per day, was carrying 200,000 vehicles per day. 15% of UK motorway traffic volume was on the M25 and there were plans to add six lanes to the section from Junctions 12 to 15 as well as widening the rest of the motorway to four lanes.
In parts, particularly the western third this plan went ahead, due to consistent congestion. Again, however, plans to widen further sections to eight lanes (four each way) were scaled back in 2009 in response to rising costs. The plans were reinstated in the agreed Highways Agency 2013-14 business plan.
In 1995 a contract was awarded to widen the section between Junctions 8 and 10 from six to eight lanes for a cost of £93.4 million and a Motorway Incident Detection and Automatic Signalling (MIDAS) system was introduced to the M25 from Junction 10 to Junction 15 at a cost of £13.5m in 1995. This was then extended to Junction 16 at a cost of £11.7m in 2002. This consists of a distributed network of traffic and weather sensors, speed cameras and variable-speed signs that control traffic speeds with little human supervision, and has improved traffic flow slightly, reducing the amount of start-stop driving.
In 1995 there was a proposal to widen the section close to Heathrow Airport to fourteen lanes. This attracted fierce opposition from road protesters opposing the Newbury Bypass and other schemes and it was cancelled shortly afterwards. In 1997, however, the Department of Transport announced new proposals to widen the section between Junction 12 (M3) and Junction 15 (M4) to twelve lanes. At the Terminal Five public inquiry a Highways Agency official said that the widening was needed to accommodate traffic to the proposed new terminal, however the transport minister said that no such evidence had been given. Environmental groups objected to the decision to go ahead with a scheme that would create the widest motorways in the UK without holding a public inquiry. The decision was again deferred. A decision to go-ahead was given for a ten-lane scheme in 1998 and the £148 million 'M25 Jct 12 to 15 Widening' contract was awarded to Balfour Beatty in 2003. The scheme was completed in 2005 as dual-five lanes between Junctions 12 and 14 and dual-six lanes from Junctions 14 to 15.
In 2007 capacity at Junction 25 (A10/Waltham Cross) was increased and the Holmesdale Tunnel was widened to three lanes in an easterly direction at a cost of £75 million.
Work to widen the exit slip-roads in both directions at Junction 28 (A12 road/A1023) was completed in 2008. It was designed to reduce the amount of traffic queueing on the slip roads at busy periods, particularly traffic from the clockwise M25 joining the northbound A12 where the queue extended onto the inside lane of the Motorway.
In 2006 the Highways Agency proposed to widen 63 miles (101 km) of M25 from six to eight lanes, between junctions 5–6 and 16–30 as part of a Design, Build, Finance and Operate (DBFO) project. A shortlist of contractors was announced in October 2006 for the project which was expected to cost £4.5 billion. Contractors were asked to resubmit their bids in January 2008 and in June 2009 the new transport minister indicated that the cost had risen to £5.5 billion and the benefit to cost ratio had dropped considerably. In January 2009 the government announced that plans to widen the sections from Junction 5–7 and from 23–27 had been 'scrapped' and that hard shoulder running would be introduced instead. However widening was reinstated to four lanes in the 2013–14 Highways Agency Business Plan.
In 2009 a £6.2 billion M25 DBFO private finance initiative contract was awarded to Connect Plus to widen the sections between junctions 16 and 23 and between junctions 27 and 30 and maintain the M25 and the Dartford Crossing for a 30-year period.
Works to widen the section between Junctions 16 (M40) and 23 (A1(M)) to dual four lanes started in July 2009 at an estimated cost of £580 million. The Junction 16 to 21 (M1) section was completed by July 2011 and the Junction 21 to 23 by June 2012. Works to widen the Junctions 27 (M11) to 30 (A13) section to dual four lanes also started in July 2009. The Junction 27 to 28 (A12) section was completed in July 2010, the Junction 28 to 29 (A127) in June 2011 and finally the Junction 29 to 30 (A13) section opened in May 2012.
Works to introduce managed motorway technology and permanent hard shoulder running on two sections of the M25 began in 2013. The first section between Junctions 5 (A21/M26) and 7 (M23) started construction in May 2013 with the scheme being completed and opened in April 2014. The second section, between Junctions 23 (A1/A1(M)) and 27 (M11), began construction in February 2013 and was completed and opened in November 2014.
In December 2016 Highways England completed the capacity project at Junction 30 (Thurrock) as part of the Thames Gateway Delivery Plan.
The improved junction is said to facilitate billions of pounds of investment in the region, making journeys more reliable and improving safety. In addition, the A13 through the junction has been widened to four lanes in each direction with speed limits capped to 50 mph. New dedicated link roads created and existing slip roads improved to facilitate east bound migration to the Regional Shopping Centre (Lakeside). Drainage, safety barriers and lighting on the M25 have also been upgraded as part of the improvements around Junction 30 and 31 including new electronic gantry signage.
In 2009 the Department for Transport published options for a new Lower Thames Crossing to add capacity to the Dartford Crossing or create a new road and crossing linking to the M2 and M20 motorways.
The M25 is the second-longest ring road in Europe, after the Berlin Ring (A 10), which is 5 miles (8.0 km) longer.
Other cities in the UK encircled by motorways include: Birmingham, using parts of the M5, M6 and M42, and Manchester, using the M60. Additionally, from 2011 Glasgow has an orbital motorway made of the M8, M73 and M74, although one section of the route passes through the centre of the city.
The M25 is one of the busiest motorways in Europe. Here are some comparisons:Saint Petersburg Ring Road: more than 150,000 vehicles on an average day.
Grande Raccordo Anulare (Rome): more than 160,000 vehicles on an average day
M25 around London: Average daily traffic of 263,000 vehicles a day recorded in 2014 between junctions 14 and 15 near London Heathrow Airport.
A23 (Vienna): more than 200,000 vehicles on an average day.
A 100 (Berlin): 216,000 vehicles in a day was recorded in 1998
Rotterdam Ring Road: 227,000 vehicles a day, in 8 years 280,000 a day by the Van Brienenoordbrug
Moscow Ring Road: more than 250,000 vehicles on an average day.
A4 motorway (near Paris): 257,000 vehicles a day recorded in 2002.
Iain Sinclair's 2002 book and film London Orbital is based on a year-long journey around the M25 on foot.
The M25 (including the A282 Dartford Crossing) is known for its frequent traffic jams. These have been the subject of so much comment from such an early stage that even at the official opening ceremony Margaret Thatcher complained about "those who carp and criticise". The jams have inspired jokes (e.g., "the world's first circular car park", "the London Orbital Car Park") and songs (e.g., Chris Rea's "The Road to Hell").
The M25 plays a role in the comedy-fantasy novel Good Omens, as "evidence for the hidden hand of Satan in the affairs of Man". The demon character, Crowley, had manipulated the design of the M25 to resemble a Satanic sigil.
The M25 enjoyed a more positive reputation among ravers in the late 1980s, when this new orbital motorway became a popular route to the parties that took place around the outskirts of London. This use of the M25 for these raves inspired the name of electronic duo Orbital.
The orbital nature of the motorway, in common with racetracks, lent itself to unofficial, and illegal, motor racing. At the end of the 1980s, before the advent of speed enforcement devices, owners of supercars, many employed in the financial service industry in the City and in Docklands, would meet at night at service stations such as South Mimms and conduct time trials. Times below 1 hour were achieved - an average speed of over 117 mph (188 km/h), which included coming to a halt at the Dartford Tunnel road user charge payment booths.
Data from driver location signs provide carriageway identifier information. The numbers on the signs are kilometres from a point near the River Thames, east of London, when travelling clockwise on the motorway. The table below gives details of each junction, including the roads interchanged and the destinations that are signed from the motorway on the blue advance direction signs. Figures in kilometres are from the driver location signs; figures in miles are derived from them.