The motorway was constructed between 1972 and 1975, at the same time as the southern section of the M25 from Godstone to Reigate. The current northern terminus at junction 7 uses the original sliproads to meet the A23 and a flyover above the junction built for the onward northern continuation remains unused. The cancellation of the unbuilt northern section from the M25 in towards Central London has resulted in the A23 carrying the majority of traffic through South London to the motorway. This is largely a single carriageway route, with many level junctions, traffic lights and awkward interchanges. It travels largely through residential areas and is inadequate for the level of traffic it carries. A new junction was opened in 1997, between J10 and J11, for access to the new Crawley neighbourhood of Maidenbower. It was financed as part of the development of Maidenbower by the construction consortium. It gives only had North facing slip roads with no Southern facing access.
Unbuilt sections
The M23 was planned to relieve congestion on the A23 through Streatham, Thornton Heath, Purley and Coulsdon in south London and was originally intended to terminate in Streatham Vale at a junction with the controversial London Ringways Plan's Ringway 2. In an earlier version of the Ringways Plan it would have continued into central London where it would have met the Balham Loop spur from Ringway 1 at Tooting. This was dropped in 1967 when the northern terminus was changed to Ringway 2. While a definite route had not been chosen at that time for the northern section, approval was met for the route south of the Greater London boundary at Hooley. By 1972 the southern section of Ringway 2 had been dropped from the plan, with an alternative proposal that the M23 continue further into London to end on Ringway 1. This was immediately countered in the same year by the GLC, who announced they would not be building that Ringway, which meant that had the M23 continued north into inner London it would not have had the motorway required at its northern end to distribute traffic to the east and west. The M23 plan was gradually scaled back further to omit the section across Mitcham Common and end the motorway at an unsuitable location on Croydon Road before the plan was postponed indefinitely. By the late 1970s, the area along the proposed line of the motorway had become affected by blight, and while the proposals were finally dropped in the mid-1980s, much of the land reserved for the route was not released by the Department for Transport until the mid-1990s. The missing section of motorway and the missing six junctions north of Hooley were not constructed due to the refusal of the GLC to finance the project, having encountered large scale opposition to the construction of Ringways elsewhere in London. However, the scale of the four-tier junction between the M23 and the M25, one of only three stack interchanges in the UK, is indicative of the importance attached to the M23 at that time.
Smart motorway
Between July 2018 and July 2020 works took place to upgrade the M23 between J8 & J10 to an all-lane-running motorway. The upgrades took place mostly for more reliable journeys to Gatwick Airport and increase the overall capacity of the route. The upgrades include an all-lane-running motorway, 12 new emergency areas, a new concrete central barrier replacing the current steel one, new roadside sound barriers, variable speed limits and two new emergency access slip roads. Changes were also implemented on the M23 J9 Spur, with the hard shoulder west bound converted to a running lane, and static 50 mph speed limits implemented in both directions. In January 2020 all Smart Motorway Projects were put under review due to concerns about their safety, during this period no new Smart Motorways could open, the review was published on the 12thMarch 2020 and required no immediate amendments to the design or construction, however new technology to detect static vehicles is required to be fitted within 3 years, and a review on spacing of Emergency Refuge Areas to be carried out in due course. The fourth lane was opened to traffic on 2nd April 2020, however with a temporary 50 mph speed limit, works are scheduled to take place until July 2020 to finish installation of technology, signs followed by testing and commissioning of the technology before the project is completed and can run at National Speed Limit.
Junctions
Data from driver location signs are used to provide distance and carriageway identifier information. Where a junction spans several hundred metres and start and end points are available, both are cited.
