M25 Motorway Guide | London’s Orbit

The M25 is really two motorways wearing one number. It was assembled from two separately planned schemes — Ringway 4 and the M16, or Ringway 3 — stitched together between junctions 3 and 5 and again between junctions 19 and 23. Some sections even opened under different numbers, the A405 and the A1178, before the whole thing was rebadged.

Getting there was not quick. Almost forty public inquiries took place before construction went ahead. Building ran in sections from 1975, finished ahead of schedule in October 1986, and involved planting more than 2.1 million trees and shrubs. Margaret Thatcher unveiled the completed road once the final Hertfordshire section was done.

It has never quite been a circle. The Dartford Crossing completes the orbital route but is not motorway at all — it is a trunk road designated the A282, which is why the toll applies there and nowhere else.

The M25 was also England's proving ground for traffic management: it was home to the country's first variable speed limits on motorways, between the A3 and the M4, in the early 1990s. That western stretch is still the busiest — widened to twelve lanes between junctions 14 and 15 near Heathrow in November 2005.