Construction

The M20 was, in common with many United Kingdom motorways, opened in stages:^[1][2]

• Junctions 5 to 7 opened in 1960
• Junctions 7 to 8 opened in 1961

These sections of the M20 were known as the 'Maidstone Bypass'. This road was then numbered as the A20(M) as it bypassed the stretch of A20 through Maidstone which was renumbered A2020.^[3] This was the first stretch of motorway to open south of London.^[4] Plans for a bypass of Maidstone had existed since the 1930s, originally as an all-purpose project, before being upgraded to motorway standard in the 1950s.^[5] When the motorway was extended westwards towards London in the 1970s, it was renamed M20 and the A2020 reverted to A20.

• Junctions 3 to 5 in 1971
• Junctions 1 to 2 in 1977

This section ended at a temporary junction near West Kingsdown.^[6]

• Temporary terminus to junction 3 in 1980

This section of the route was difficult to construct due to its steep descent down the North Downs escarpment.

• Sellindge to junction 13 in 1981 – constructed by McAlpines
• Junction 9 to Sellindge in 1981 – constructed by Dowsett

The section around Ashford (junctions 9–10) was originally the A20 Ashford Bypass with actual construction having started before World War 2 – although the route itself was not opened until 19 July 1957.^[7] The bypass started at Willesborough near the current location of junction 10 and terminated south of the existing junction 9 at the current Drover's Roundabout. A section of the old bypass is still visible now named Simone Weil Avenue. The original bridge that brought Canterbury Road over the bypass is still visible as the bridge was not reconstructed when the motorway was constructed. This section of motorway has no hard shoulder indicating the smaller width of the old bypass.

This left the motorway in two sections, with the 14-mile (23 km) gap running via the A20 – this was referred to locally as 'The Missing Link'.^[8] The level of traffic was not considered necessary to complete the route. Most of the traffic for the Channel ports was using the A2/M2 route. When the Channel Tunnel was ready for construction, it was decided to complete the M20 between junctions 8 and 9 and this opened in 1991. Concurrent to this was the extension to Dover as part of the A20 which opened in 1993.^[6] A new junction 11A was also constructed to serve the Channel Tunnel.

Operation

M20 near Maidstone showing separated distributor roads

Following completion of the junction 8 to 9 section, the M20 was 3 lanes either side of the original A20(M) section. This was a bottleneck, so it was decided to widen this section of motorway. The road here was increased to a dual 3 or 4 lane road with 2 lane distributor roads either side. This section was opened in 1995.^[6]

To the north of Maidstone, there is an overlap between the slip roads for Junctions 5 (A20) and 6 (A229).

Between 2006 and 2007 junction 10 near Ashford was remodelled to increase capacity when the bridges across the motorway were modified to provide three lanes of traffic at the roundabout, and local approach roads were widened, with new traffic lights to control traffic flows at the junction between the A292 Hythe Road and the London-bound M20 entry slip road. A new footbridge was also constructed across the motorway.^[9] The cost was £4.9 million.^[10]

A Controlled motorway scheme was introduced in West Kent between junctions 4 and 7, with variable speed limits.^[11]

In August 2016 part of a pedestrian footbridge connecting areas of Ryarsh divided by the motorway was brought down - initially suspected to be the result of an impact by a digger from nearby works to widen the southbound bridge at junction 4 being carried on a low-loader that was moving along the hard shoulder.^[12] In the incident, the southern section of the bridge - which rested on a plinth south of the motorway and the cantilevered northern section^[13] - was dislodged and fell onto the carriageway below, landing on the trailer of a passing HGV and being narrowly avoided by a motorcyclist who suffered broken ribs taking avoiding action.^[14][15] Both carriageways of the motorway were closed to enable the removal of the broken section. The motorway reopened with the Highways Agency having declared that the northern part of the bridge was structurally intact. However this section of the motorway was again closed on the weekend of 3 and 4 September 2016 for the demolition and clearance of the northern bridge element.^[16] A replacement pedestrian and cycle bridge was opened in March 2021 at a cost of around £1.5 million.^[17]

The Highways Agency proposed a new M20 junction 10a and link road to the A2070 at Ashford in Kent, east of junction 10 ^[18] to support the development of South Ashford which has been identified as a growth area in the South East. In May 2012, it was announced that the scheme would be postponed for the short-term future. Planning recommenced in 2016.^[19] Work started on building the scheme in January 2018, with works planned to complete in May 2020.^[20] The coast-facing sliproads at the existing Junction 10 were closed to allow the final works on the London-facing sliproads at Junction 10a and the new junction opened on 31 October 2019.^[21]