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The M25 or London Orbital Motorway is a major road encircling almost all of Greater London. The 117-mile (188-kilometre) motorway is one of the most important roads in England and one of the busiest. Margaret Thatcher opened the final section in 1986, making the M25 the longest ring road in Europe. The Dartford Crossing (A282 road) is part of the orbital route but is not part of the motorway. In some cases, such as the Communications Act 2003, the M25 is used as a de facto reference to Greater London.

In the 1944 Greater London Plan, Patrick Abercrombie proposed an orbital motorway around London. In the early 1960s, this had evolved into the London Ringways project. By 1966, planning had started on two projects, Ringway 3 to the north and Ringway 4 to the south. By the time the first sections opened in 1975, it was decided the ringways would be combined into a single orbital motorway. The M25 was one of the first motorway projects to consider environmental concerns and almost 40 public inquiries took place. The road was built as planned despite some protests that included the section over the North Downs and around Epping Forest which required an extension of the Bell Common Tunnel.

Although the M25 was popular during construction, it quickly became apparent that there was insufficient traffic capacity. Because of the public inquiries, several junctions merely served local roads where office and retail developments were built, attracting even more traffic onto the M25 than it was designed for. The congestion has led to traffic management schemes that include variable speed limit and smart motorway. Since opening, the M25 has been progressively widened, particularly near Heathrow Airport.
This extract uses material from the Wikipedia article "M25_motorway", which is released under the Creative Commons Attribution-Share-Alike License 3.0.