This is largely a single carriageway route, with many level junctions, traffic lights and awkward interchanges.

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 (the intended replacement of the South Circular Road (A205)).

This was immediately countered in the same year by the GLC, who announced they would not be building that Ringway,[7] 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.

[7] 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.

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.

Works were scheduled to take place until July 2020 to finish installation of equipment, followed by testing and commissioning, before the project is completed and can run at the national speed limit.