It is for the most part a three-laned motorway although there are a few stretches which are two-laned and a few (seemingly shorter!) stretches which are four-laned. It is thought to be Europe's busiest motorway: an estimated 200,000 vehicles a day make use of it, up from 100,000 a day in 1987.
The idea of an orbital road around London was first proposed early in the 20th century, through the Lutyens and Bressey plans of 1937 to the Abercrombie Plan of 1945 which proposed a series of five individual roads around the capital. Over time successive governments reduced this grandiose scheme to the Greater London Development Plan - a combination of two rings into one, the M25, and a smaller inner ring, initially hoped to become the M15, but currently still the A406.
The entire orbital was constructed in a number of stages from around 1975 up until 1985. The sections were not constructed contiguously but in small sections, such as Dartford to Swanley (Junction 1 to Junction 3) or Potters Bar to Waltham Cross (J24 to J25), and later joined. Each section was presented to planning authorities in its own right and was individually justified; there were almost forty public inquiries relating to sections of the route.
The M25 was officially opened in October, 1986 with a ceremony by Margaret Thatcher opening the section between Junctions 22 and 23 (London Colney and South Mimms).
More recently, the perenially congested south-western stretch of the M25 (near Woking) was fitted with an experimental automated traffic control system called MIDAS (Motorway Incident Detection and Automatic Signalling). This consists of a distributed network of traffic and weather sensors, speed cameras and variable speed signs which control traffic speeds with little human supervision. The system successfully reduced congestion and it is hoped that MIDAS will be fitted to the rest of the M25 in due course.
The M25 is known for its frequent jams. This has inspired jokes ("the world's biggest car park"), songs (Chris Rea's "The Road to Hell") and the following tongue-in-cheek theory: