This motorway, which surrounds Greater London like a beltway, has been called the world's busiest motorway. It's identified by a letter and a number. What is it?

This major ring road is the M25, a critical piece of the UK's transport network (Review). The "M" designates it as a motorway, and it serves as the primary orbital route for Greater London. At 117 miles (188 km) long, it is one of Europe's longest city bypasses, passing through several of England's "home counties" as it encircles the capital.

Its reputation as the "world's busiest motorway" stems from the immense volume of traffic it handles daily. The western section, particularly the stretch near Heathrow Airport, is one of the most heavily used in Europe, often carrying close to 200,000 vehicles per day. This constant flow of cars, vans, and lorries leads to its infamous and frequent traffic jams.

The concept of a London orbital road was proposed as early as 1905, but the M25 as we know it was constructed in sections between 1975 and 1986. For millions of drivers, its legendary congestion has earned it less flattering nicknames, including "The Road to Hell," famously used as the title of a song by musician Chris Rea.