The M25 motorway, or London Orbital, is a  orbital motorway around Greater London.

The motorway was first suggested early in the 20th century, and a few sections were built in the early 1970s. The M25 was completed in 1986: is one of the world's longest orbital roads.

It is one of the busiest and most congested parts of the British motorway network. 196,000 vehicles were recorded in a single day near London Heathrow Airport. The M25 is a crucial link between the four main London airports: Heathrow, Gatwick, Stansted and Luton.

Bearing in mind that no motorways go through the centre of London, the M25 is a main route connecting north with south, and east with the west of England.

The motorway was originally built with only six lanes (three each way), despite research which suggested eight lanes were necessary. Now a fourth lane exists over most of the motorway, and more in special sections (see diagram). Plans were scaled back in 2009 in response to rising costs, but work is still going on.

Planning 
Public enquiries continued through the 1970s and 1980s. Each section had its own enquiry, where local citizens and interest groups could question the plans. In all, "39 public inquiries before independent Inspectors were held, taking over 700 sitting days – with many more working days (and nights) preparing evidence". Objectors could put forward alternatives, and each of these had to be answered with presentation of evidence. Evidence included predictions of traffic and events which would affect growth of traffic. Enquiries and evidence were open to public scrutiny and debate.

Some enquiries lasted a day, and one one lasted 97 sitting days over 13 months. Proposals had to go before Parliament, and a special Act was needed to authorise the crossing of the northern tip of Epping Forest by the route in tunnel.

Operational history
As predicted by research, soon after the motorway opened in 1986 traffic levels exceeded maximum designed capacity. In 1990 the Secretary of State for Transport announced plans to widen the whole of the M25 to four lanes. By 1993 the motorway that was designed for a maximum of 88,000 vehicles per day was carrying 200,000. 15% of UK motorway traffic volume was on the M25 and there were plans to add 6 lanes to the section from Junction 12 to 15 as well widening the rest of the motorway to 4 lanes.

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 and 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. The system has improved traffic flow slightly, and reduced the amount of start-stop driving. The use of variable speed limits makes this a "controlled motorway", and if hard shoulders are used at peak times it becomes a "managed motorway".

In 1995 there was a proposal to widen the section close to Heathrow Airport to 14 lanes. This attracted fierce opposition from road protesters opposing the Newbury Bypass and other schemes, and it was cancelled shortly afterwards. However, in 1997 the Department of Transport announced new proposals to widen the section from junction 12 (M3) and junction 15 (M4) to 12 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 without holding a public inquiry. A decision to go-ahead was given for a 10-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 lane between junctions 12 to 14 and dual six lanes from 14 to 15.

Costs 
The road cost £1 billion (in 1980s money) and took 11 years to build. It used two million tons of concrete and 3½ million tons of asphalt.

Dartford 
Traffic bottleneck at the Dartford Crossing in the east is the greatest problem on the motorway at present, even though the crossing is technically not part of the M25. At present the crossing is by bridge from the north, and by tunnel from the south. There are usually long tailbacks in both directions.

Consequences 
On its 25th birthday the BBC listed some of its effect on life near the capital:
 Built business empires and spread business from the capital. A new commercial property market has developed. In the M25 area there is 130 million square feet of extra commercial space. Compare this with the City of London, where the total is about 170 million square feet. The M25 is a  big catchment area in terms of employment.
 Increased house prices. Before the M25, areas just outside London were a mixture of small rural farms and commuter belt. Its arrival helped those commuters get on their way faster, pushing house prices in such areas higher. "We know from our research that house prices generally have gone up about 300% since the M25 opened in 1986," says Anthony Wardell from Knight Franks estate agents. On the western side of London, house prices have gone up nearer to 400%, he adds.
 Changed the way people plan their lives. M25 expert Chris Marshall says "You can get around London in ways that you couldn't do before and it doesn't just change the lives of people who lived around it before, or who worked around it before, you find that actually it changes the way that people plan their lives. So now not only does someone who lived in Harlow when it was opened have the opportunity to work somewhere else, but someone say who works say in Heathrow can buy a house on the other side of London."

Junctions
{| class="wikitable"
|-  style="background:#00703c; text-align:center; color:#ffd200; font-size:120%;"
| colspan="6" | A282 (Dartford Crossing)
|-
!scope=col| miles
!scope=col| km
!scope=col| Clockwise exits (A carriageway)
!scope=col| Junction
!scope=col| Anti-clockwise exits (B carriageway)
!scope=col| Opening date
|- style="text-align:center;"
|0.0
|0.0
|style="background:#dcdcfe;"| Dartford Crossing South(Queen Elizabeth II Bridge)
! scope=row| RiverThames
|style="background:#dcdcfe;"| Dartford Crossing North(Dartford Tunnels)
| November 1963 (west tunnel)May 1980 (east tunnel)October 1991 (bridge)
|- style="text-align:center;"
|3.5
|5.7
| Erith A206
! scope=row| J1A
| Erith A206, Swanscombe (A226)
| September 1986
|- style="text-align:center;"
|4.7
|7.5
| Dartford A225
! scope=row| J1B
| Exit via J2 – Dartford (A225)
| September 1986
|-  style="background:#0080d0; text-align:center; color:white; font-size:120%;"
| colspan="6" | M25
|- style="text-align:center;"
|5.5
|8.8
|  London (SE & C), Bexleyheath A2 (W), Canterbury (M2) A2 (E) Ebbsfleet International, Gillingham
! scope=row| J2
|  London (SE & C), Bexleyheath A2, Canterbury (M2), Dartford (A225) Ebbsfleet International, Bluewater, Gillingham  Gillingham A2  Dover, Chnl Tnl (M20)  M25(S) (M23)  Rochester A2
|September 1986 (northbound)April 1977 (southbound)
|- style="text-align:center;"
|8.7
|14.0
| London (SE & C) A20 Maidstone, Channel Tunnel, Folkestone M20 Swanley B2173
! scope=row| J3
| Maidstone, Channel Tunnel M20 London (SE & C), Lewisham A20
|April 1977 (northbound)February 1986 (southbound)
|- style="text-align:center;"
|12.2
|19.6
| Bromley A21 Orpington A224
! scope=row| J4
| Bromley, London (SE & C) A21 Orpington (A224) Hayes
| February 1986
|- style="text-align:center;"
|16.316.4
|26.226.4
| Sevenoaks, Royal Tunbridge Wells, Hastings A21
! scope=row| J5
| Maidstone, Channel Tunnel, Dover M26 (M20)  Sevenoaks, Hastings A21
| July 1980
|- style="text-align:center;"
|21.0
|33.8
|Clacket Lane services
! scope=row| Services
|Clacket Lane services
|July 1993
|- style="text-align:center;"
|25.8
|41.6
| East Grinstead, Eastbourne, Caterham, Godstone A22Redhill, Westerham (A25)
! scope=row| J6
| East Grinstead, Eastbourne, Caterham, Godstone A22Redhill, Westerham (A25)
| November 1979 (eastbound)February 1976 (westbound)
|- style="text-align:center;"
|28.6
|46.0
| Gatwick Airport, Crawley, Brighton, Croydon M23
! scope=row| J7
| Gatwick Airport, Crawley, Brighton, M23(S), Croydon M23(N)
| February 1976
|- style="text-align:center;"
|31.9
|51.4
| London (S & SW), Reigate, Sutton A217 Kingston (A240)
! scope=row| J8
| London (S & SW), Reigate, Sutton A217 Kingston (A240)
| February 1976 (eastbound)October 1985 (westbound)
|- style="text-align:center;"
|38.539.5
|62.063.5
| Leatherhead A243, Dorking, (A24)
! scope=row| J9
| Leatherhead A243, Dorking (A24)
| October 1985
|- style="text-align:center;"
|42.643.2
|68.669.5
| Cobham services
! scope=row| Services
|Cobham services
|September 2012
|- style="text-align:center;"
|45.0
|72.4
| London (SW & C), Guildford, Portsmouth A3
! scope=row| J10
| London (SW & C), Guildford, Kingston A3
| October 1985 (eastbound)December 1983 (westbound)
|- style="text-align:center;"
|49.8
|80.2
| Chertsey A317, Woking A320
! scope=row| J11
| Woking A320, Chertsey A317
| December 1983 (southbound)October 1980 (northbound)
|- style="text-align:center;"
|52.1
|83.8
| Basingstoke, Southampton, Richmond M3
! scope=row| J12
| Basingstoke, Southampton, Richmond M3
| October 1980 (southbound)December 1976 (northbound)
|- style="text-align:center;"
|55.2
|88.8
| London (W & C), Hounslow, Staines A30
! scope=row| J13
| London (W & C), Hounslow, Staines A30
| November 1981 (southbound)August 1982 (northbound)
|- style="text-align:center;"
|57.0
|91.8
| Heathrow Airport (Terminals 4, 5 and Cargo) A3113
! scope=row| J14
| Heathrow Airport (Terminals 4, 5 and Cargo) A3113
| August 1982 (southbound)September 1985 (northbound)
|- style="text-align:center;"
|59.0
|95.0
| The WEST, Slough, Reading, London (W & C), Heathrow Airport (Terminals 2 and 3) M4
! scope=row| J15
| The WEST, Slough, Reading M4(W)  London (W & C), Heathrow Airport (Terminals 2 & 3) M4(E)
| September 1985
|- style="text-align:center;"
|63.8
|102.6
|  The NORTH, Birmingham, Oxford, Uxbridge, London (W & C) M40
! scope=row| J16
|  Birmingham, Oxford M40(W)  Uxbridge, London (W & C) M40(E)
| September 1985 (southbound)January 1985 (northbound)
|- style="text-align:center;"
|68.7
|110.5
| Rickmansworth, Maple Cross (A412)
! scope=row| J17
| Rickmansworth, Maple Cross A412
| January 1985 (southbound)February 1976 (northbound)
|- style="text-align:center;"
|69.9
|112.5
| Chorleywood, Amersham A404
! scope=row| J18
| Chorleywood, Amersham A404
| February 1976
|- style="text-align:center;"
|71.5
|116.4
| Watford A41
! scope=row| J19
| Exit via J20 – Watford A41
| September 1976
|- style="text-align:center;"
|73.5
|118.2
| Hemel Hempstead, Aylesbury A41
! scope=row| J20
| Hemel Hempstead, Aylesbury, Watford A41
| August 1986
|- style="text-align:center;"
|76.3
|122.8
| The North, Luton & Luton Airport M1
! scope=row| J21
| The North, Luton & Luton Airport M1
| August 1986
|- style="text-align:center;"
|76.9
|123.7
| Watford A405 Harrow (M1 South)
! scope=row| J21A
| St Albans A405 London (NW & C) (M1 (South))
| August 1986
|- style="text-align:center;"
|80.6
|129.7
| London Colney A1081
! scope=row| J22
| St Albans A1081
| August 1986
|- style="text-align:center;"
|83.3
|134.0
| Hatfield A1(M), London (NW & C) A1, Barnet A1081South Mimms services
! scope=row| J23
| Hatfield A1(M), London (NW & C) A1, Barnet A1081South Mimms services
| August 1986 (westbound)September 1975 (eastbound)
|- style="text-align:center;"
|85.9
|138.2
| Potters Bar A111
! scope=row| J24
| Potters Bar A111
| September 1975 (westbound)June 1981 (eastbound)
|- style="text-align:center;"
|91.4
|147.1
| Enfield Town, Hertford A10
! scope=row| J25
| Enfield, Hertford, London (N & C) A10
| June 1981 (westbound)January 1984 (eastbound)
|- style="text-align:center;"
|94.9
|152.7
| Waltham Abbey, Loughton A121
! scope=row| J26
| Waltham Abbey, Loughton A121
| January 1984
|- style="text-align:center;"
|99.2
|159.7
| London (NE & C), Stansted Airport, Harlow, Cambridge M11
! scope=row| J27
| London (NE & C) M11(N), Stansted Airport, Harlow, Cambridge M11(S)
| January 1984 (westbound)April 1983 (eastbound)
|- style="text-align:center;"
|107.1
|172.4
| Chelmsford, Witham, Colchester A12 Brentwood A1023
! scope=row| J28
| Chelmsford, Romford A12 Brentwood A1023
| April 1983
|- style="text-align:center;"
|109.9
|176.8
| Romford, Basildon, Southend A127
! scope=row| J29
| Basildon, Southend, Romford A127
| April 1983 (northbound)December 1982 (southbound)
|- style="text-align:center;"
|115.2
|185.4
| Tilbury, Thurrock, Lakeside A13(E), London (E & C) A13(W) Thurrock services
! scope=row| J30
| London (E & C), Barking, Tilbury, Basildon, Dagenham, Rainham A13
| December 1982
|-  style="background:#00703c; text-align:center; color:#ffd200; font-size:120%;"
| colspan="6" | A282 (Dartford Crossing)
|- style="text-align:center;"
|115.9
|186.6
| Exit via J30 – Purfleet (A1090), South Ockendon, Thurrock services A1306
! scope=row| J31
| Thurrock (Lakeside), Thurrock services A1306, Purfleet (A1090), West Thurrock (A126)
| December 1982
|- style="text-align:center;"
|
|
|style="background:#dcdcfe;"| Dartford Crossing South(Queen Elizabeth II Bridge)
! scope=row| RiverThames
|style="background:#dcdcfe;"| Dartford Crossing North(Dartford Tunnels)
| November 1963 (west tunnel)May 1980 (east tunnel)October 1991 (bridge)
|-
|colspan=6|Notes
 Distances in kilometres and carriageway identifiers are taken from driver location signs/location marker posts. Where a junction goes across several hundred metres and the data is available, both the start and finish values for the junction are shown.
|-

Notes

References 

Motorways in England