London Underground infrastructure Tube Programme, investigating replacing the trains for the Bakerloo and Piccadilly lines, is looking for trains with better energy conservation and regenerative braking. The Underground serves 270 stations. Fourteen Underground stations are outside Greater London, of which five (Amersham, Chalfont & Latimer, Chesham, and Chorleywood on the Metropolitan line and Epping on the Central line) are beyond the M25 London orbital motorway. The longest distance between two stations is between Chalfont & Latimer and Chesham on the Metropolitan line. The shortest distance between adjacent stations is the between Leicester Square and Covent Garden on the Piccadilly line. The station furthest
M25 motorway are in Buckinghamshire, 17–25 are in Hertfordshire, and 26–31 are in Essex. Policing of the road is carried out by an integrated policing group made up of the Metropolitan, Thames Valley, Essex, Kent, Hertfordshire and Surrey forces. The M25 is one of Europe's busiest motorways. In 2003, a maximum of 196,000 vehicles a day were recorded on the motorway just south of London Heathrow Airport between junctions 13 and 14. The idea of an orbital road around London was first proposed early in the 20th century. An outer orbital road around London had first been proposed in 1913, and was