The London Loop, one of the Mayor of London's key routes, almost completely encircles Greater London, covering a total distance of around 240km (150miles). It has been described as 'the M25 for walkers', although it rarely comes anywhere near the motorway. The only gap in the circuit is between Sections 24 at Purfleet and 1 at Erith, where the River Thames provides a substantial barrier of almost a kilometre (half a mile) with no convenient crossing point.