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Chiswell Interchange, exactly 30 years later

This page outlines other technical information of note, such as chainage and street lighting.

Distance and markers

The M25 is widely accepted to be 117 miles (188.3 kilometres) long.[1][2]

The lowest and highest numbered distance markers are on the A282, at the clockwise entrance to the Dartford Crossing Control on the north side of the Thames. They are km 2.1 and km 187.4 respectively,[3][4] although some equipment are marked up to km 187.7.[5] This gives a total signed chainage of 185.3 kilometres, which would technically place km 0.0 (signed as km 185.3)[6] just under the northern bridge of the Mar Dyke Interchange.

Emergency access connections

Location km
London Road, near Fort Halstead [disused] 24.2
Hogtrough Hill and Station Road, near Brasted 29.2 29.4
Palmer’s Moor Lane, near Iver 98.5
Fulmer Lane and Pinstone Way, north of Denham Interchange 103.5 103.6
Coldharbour Farm 106.0
West of Theydon Bois Interchange 158.2
Mount Road, East of Theydon Bois Interchange 162.3

Emergency access connectors also exist on the slip roads of the Theydon Bois Interchange, which can also be improvised as a diversion if the main carriageway is blocked.


P2901 - Chopsticks Sign (M25 Variant).JPG

A number of entrances to the M25 use a variant of the Chopsticks sign (drawing P2901) with the “London Orbital” header, most of which date from the completion of the M25 in 1986. No version of the Traffic Signs Regulations and General Directions have supported this variant, and it is unclear if a special authorisation existed at the time: newer signs simply use the route number.

As of 2019, you can still find such signs at the following entrances:

  • Junctions 18, 25 and 29 (only clockwise)
  • Junctions 17 and 24 (only anticlockwise)
  • Junction 26

Street lighting

The M25 has many sections with street lighting, although the location of such sections have changed over time. The Philips MA series of low pressure sodium lanterns used to be the dominant model in use, supplemented by catenary lighting around junction 12 (km 82.9–84.6) and junctions 13–15 (km 87.7–km 95.0). They were gradually replaced by various models of high pressure sodium lanterns, such as the Philips Iridium, Philips Trafficvision, Schréder Sapphire and Urbis ZX3. A short section of LED lighting (using CU Phosco P860) was recently installed as part of a temporary junction for the construction of High Speed 2 (km 108.0 to km 108.4).

In 2001, street lighting (on the main carriageway) covered the following sections (in clockwise direction unless indicated):

Start End Notes
km Description km Description
133.3 Blanche Lane Overbridge, west of junction 23 9.5 Hawley Road underbridge, south of junction 2 Via the Dartford Crossing.
24.7 Gantry 4248J, between the Morants Court Road overbridge and junction 5 27.8 Marker between the Chevening Road and Coombe Bank Drive overbridges, west of junction 5 Carriageways A/B.
26.9 Footbridge 26 2/3, south of junction 5 Carriageways J/L towards the A21.
42.0 Footbridge over the western slip roads of junction 6 104.2 Gantry 5041B (anti-clockwise),[7] north of junction 16 (now removed) Longest section, at the time around 63.2 kilometres.
112.1 Northern end of a viaduct over the Metropolitan line, south of junction 18 124.6 Marker between the Lye Lane and Park Street Lane overbridges, east of junction 21A Excludes the spur to Hunton Bridge.

In September 2020, street lighting covered the following sections:

Start End Notes
km Description km Description
156.5 Traffic camera 156/5B (anti-clockwise), west of Bell Common Tunnel 5.7 ⅔-mile fork sign for the junction 1b, between the slip roads of junction 1a Via the Dartford Crossing.
7.7 Gantry 7 7/S, north of junction 2 13.2 A marker, north of junction 3
24.7 Gantry 4248J, between the Morants Court Road overbridge and junction 5 27.6 A marker, between the Chevening Road and Coombe Bank Drive overbridges, west of junction 5 Carriageways A/B.
26.9 Footbridge 26 2/3, south of junction 5 Carriageways J/L towards the A21.
42.0 Footbridge over the western slip roads of junction 6 135.5 A marker east of junction 23 Longest section, now around 98.4 kilometres; excludes the spur to Hunton Bridge.
137.1 A marker west of junction 24 139.5 Traffic camera 139/5A (clockwise),[8] east of junction 24 The traffic camera is accessible via Traffic England.
146.0 A marker west of junction 25 148.5 Western end of a bridge over the West Anglia line, west of the Holmesdale Tunnel
151.6 A marker west of junction 26 154.0 A marker (clockwise) or tourist sign (anti-clockwise), east of junction 26

Traffic management

A320 junction - Coppermine - 3645.jpg

The M25 was one of the first motorways to adopt Variable Speed Limits (VSL), a precursor to the current Smart Motorway system. They are recognisable by the use of red rings for speed limits, in place of flashing amber lamps.

The Highways Agency introduced VSLs between junction 10 and junction 15, in August 1995. The system, known at the time as Controlled Motorways, was originally based on historical traffic data, but driver complaints brought forward plans to use actual traffic flows instead of historical data. VSLs were supplemented by Enhanced Message Signs, introduced in stages from October 1998 to July 1999.[9]

Today, the remaining sections of the London Orbital without VSLs are:

  • Between junctions 3 and 5
  • The Hunton Bridge spur at junction 19
  • The A282 between junctions 30 and 2 (via the Dartford Crossing)


Addlestone Interchange • Bignells Corner • Brentwood Interchange • Bricket Wood Junction • Brook Street Interchange • Chandlers Cross • Chevening Interchange • Chipstead Interchange • Chiswell Interchange • Chorleywood Interchange • Colnbrook Interchange • Cranham Interchange • Darenth Interchange • Denham Interchange • Denham Roundabout • Godstone Interchange • Hewitts Roundabout • Hunton Bridge • Iver Heath Interchange • Kings Langley Interchange • Leatherhead Interchange • M25 HS2 Works Access • Maple Cross Interchange • Mar Dyke Interchange • Merstham Interchange • Ockendon Interchange • Potters Bar Interchange • Poyle Interchange • Reigate Hill Interchange • Runnymede Interchange • Sarratt Interchange • Swanley Interchange • Terminal 5 Roundabout • The Bell Roundabout • Theydon Bois Interchange • Thorpe Interchange • Waltham Abbey Interchange • Waltham Cross Interchange • Westerham Interchange • Wisley Interchange
E15 • E30 • M25 • T5 Spur
Related Pictures
View gallery (350)
- Coppermine - 3925.jpg- Coppermine - 3912.jpgM25 after Reigate junction - Coppermine - 3647.jpgDenham Roundabout Interchange April 1970 Scheme B.pngM23-M25 Interchange in 1979 - Geograph - 2466993.jpg

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