The M25, the London Orbital Motorway, is the UK's busiest road, running almost in a loop around the Capital for 117 miles / 188km, although the Dartford Crossing over the River Thames is the non-motorway A282. It's frequently congested, despite being 4 lanes wide in most sections.
The M25's conception was in the Ringways plans in the post-war period - for a series of ring roads of increasing distance and standard. The M25, as now constructed, frequently runs close to the planned line of either ringway 3 or 4 for much of its length. The M25 number was chosen in the mid-1970s, although some sections were originally due to be numbered M16. The motorway was constructed between 1976 and 1986.
This video shows a clockwise journey along the M25 in Hertfordshire, from Junction 19 for the A41 at Watford, to Junction 21 for the M1 Motorway, and J21a for the A405 near St Albans.

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