The crossing's development started in the late 1930s, but was interrupted due to the Second World War and resumed in the 1950s. The original tunnel catered for a single lane of traffic in each direction, but rising traffic levels required the second tunnel to be built. The M25 connected to the tunnels at both ends when completed in 1986, and this increased traffic put pressure on the tunnels' capacity. A Private Finance Initiative scheme was started in 1988 to build the bridge. The combined crossing now handles four lanes of traffic in each direction.