M25 motorway Europe's second longest orbital road after the Berliner Ring, which is . Originally built almost wholly as a dual three-lane motorway, much of the motorway has been widened: to dual four lanes for almost half, to a dual five-lanes section between junctions 12 and 14 and a dual six-lane section between junctions 14 and 15. Further widening is in progress of minor sections with plans for managed motorways in many others. To the east of London the two ends of the M25 are joined to complete a loop by the non-motorway A282 Dartford Crossing of the River Thames between Thurrock
M25 motorway began in 1973. The first section, between South Mimms and Potters Bar in Hertfordshire (junction 23 to junction 24) opened in September 1975 and was given the temporary general purpose road designation A1178 (a section of motorway-standard-road, originally the M16, which eventually was incorporated into the M25) was completed and operational before this. A Watford-avoiding route between the M1 and the A40 between north Watford and Denham was locally known as the Croxley Green/Rickmansworth bypass, and was operational about 1973/4; a section south of London (junction 6 to junction 8) opened in 1976. A section of Ringway 3 south of