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
Greater London boundary with Kent, at 245 metres (804 ft). Central government has implemented small boundary changes. The greatest were the 1969 transfers of Farleigh to Surrey and Knockholt to Kent. Others have included exchange of two Thames islands with Surrey and adjustments during the 1990s to parts of the boundaries of three boroughs near the M25. The only part of Greater London outside the motorway is North Ockendon, the furthest land unit from its centre. The majority of Greater London forms the London low emission zone. The London postal district does not cover all of Greater London. The Mayor of London