Transportation needs within the county of Kent in South East England has been served by both historical and current transport systems. Roads The earliest footpaths and roads in Kent ran in a north-east/south-west direction. They followed the natural contours of the land and took advantage of the gaps in the Downs and Weald created by streams and rivers. Some of these paths and tracks have over time have been widened and metalled but rarely straightened. With the Roman invasion a road network was constructed to contact London to the Channel Ports of Dover, Lympne and Richborough. The London-Dover road was part of Watling Street. These routes are now approximately followed by the A2, B2068, A257, and the A28. When the roads were turnpiked in the 18th century, they were used to carry foodstuffs from Kent to London, and the roads radiated from there. The A2 ran from London to Dartford (A207), Gravesend, Rochester, Canterbury and Dover. The A20 ran to Eltham, Wrotham, Maidstone, Charing, Ashford, Hythe, Folkestone and Dover. The A21 ran to Sevenoaks, Tonbridge, and Tunbridge Wells. With the increased traffic on these roads, the main towns were bypassed; and then in the 1960s two motorways were built: the M2 from Medway to Faversham, and the M20 from Swanley to Folkestone. Part of the M25 runs through Kent, from Westerham to the Kent and Essex tunnel at Dartford. Even these roads have failed to provide adequate capacity, and are upgraded and widened from time to time. The Dartford Tunnel was supplemented by the Queen Elizabeth II Bridge, allowing four lanes in each direction. This is a toll crossing. Water The Cinque Ports were important mediaeval ports. They were Sandwich, Dover, Hythe, Romney and Hastings, and later Rye and Winchelsea. All of these have silted up, with the exception of Dover which is a busy RORO ferry port. Ramsgate is a container port. The Medway Estuary has been an important port and naval base for 500 years. The River Medway is tidal up to Allington and navigable up to Tonbridge. There are two canals in Kent, the Royal Military Canal between Hythe and Rye, which is still extant, and the Thames and Medway Canal between Strood and Gravesend. Built in 1824, it was bought up by the railways in 1846 and backfilled. The Port of Dover is the major port. Railways The earliest locomotive driven, passenger carrying railway in Britain