River Thames Thames Estuary), the river is subject to tidal activity from the North Sea. Before the lock was installed, the river was tidal as far as Staines, about upstream. London, capital of Roman Britain, was established on two hills, now known as Cornhill and Ludgate Hill. These provided a firm base for a trading centre at the lowest possible point on the Thames. A river crossing was built at the site of London Bridge. London Bridge is now used as the basis for published tide tables giving the times of high tide. High tide reaches Putney about 30 minutes later than
M25 motorway and Dartford. This crossing, which consists of twin two-lane tunnels and the four-lane QE2 (Queen Elizabeth II) bridge, is named "Canterbury Way". Passage across the bridge or through the tunnels is subject to a toll, its level depending on the kind of vehicle. This stretch being non-motorway, it allows traffic, including that not permitted to use motorways, to cross the River Thames east of the Woolwich Ferry; the only crossing further to the east is a passenger ferry between Gravesend, in Kent, and Tilbury, in Essex. However, in 2017 Highways England published plans to build another motorway-grade Thames tunnel to