Unlike many of the "First 99" roads that start in London, the A23's London section hasn't really changed since 1935, although that's more due to cost and political willpower as the road is still one of the most congested in London. The southern end, on the other hand, particularly south of the M23, has seen much many more improvements, resulting in today's road being mostly D3 south of the M23 as far as Patcham with the A27.
Westminster - Gatwick Airport
The original start of the A23 was at Purley Cross, where it met the A22 and A235. The remainder of the route north, including the then-unbuilt Purley Way (which opened in April 1925), was part of the A22. The numbers switched in the 1935 Road numbering revision.
Large parts of the A23 in Greater London were intended to be bypassed by the M23. After the plans stalled in the late 1970s, then finally cancelled outright in 1995, a watered down improvement was announced in 1998 in the form of the Coulsdon Relief Road. Construction of this was delayed further when control of the road transferred to Transport for London in July 2000, at which point the design of the road changed from D2 to S2 + bus lane. The road was finally opened in December 2006. A few months later, it was reported that no buses actual ran on the bus lanes.
Gatwick Airport - Pease Pottage
The A23 was re-routed as a dual carriageway around Gatwick Airport when the airport was expanded in 1956 to 1958. The original route ran to Povey Cross, where it met the A217, then south to Lowfield Heath, where it rejoined the modern route. Parts of this original road can seen in aerial photographs of the airport.
The A23 originally ran through the centre of Crawley which was bypassed in the late 1930s. The old road is now the A2219, though that itself has been rerouted around the town centre to avoid the High Street.
Pease Pottage - Patcham Interchange
The A23 originally ran through both the centre of Pease Pottage. This was bypassed in the 1950s, and the bypass forms part of the current A23 mainline (the B2214 via Pease Pottage Services is a newer addition]]). Similarly, Handcross was bypassed in the 1950s, and ran to the west of the recreation ground before meeting the current route of the road. The modern A23 alignment dates from an upgrade to D3 in the 1990s as far as Handcross Junction.
By the time the road was largely upgraded in the 1990s, the Handcross to Warninglid section became a D2 section between two stretches of D3 either side, and featured many tight bends and black spots as well as the overall steep gradient. As such, it quickly became a bottleneck. However, work undertaken in the 2010s widened the road to D3 while also reducing the severity of the bends. South of here the entire road was upgraded to grade-separated dual carriageway in the 1990s, including a bypass for Sayers Common and Albourne. The junction with the A273 was formerly a set of traffic lights. The pillars that can be seen approaching Brighton used to sit on either side of the road. The junction with the A27 was built at the time of the construction of the A27 Brighton Bypass.
Patcham Interchange - Brighton A259
The A23 runs largely on its original alignment south of the A27. A small bypass of Patcham opened around 1926, and some of the original route is now one-way, with traffic in the opposite direction running on a nearby parallel road. The very southern end of the A23 around Valley Gardens got major changes in 2019 and 2020 as before of two carriageways either side of the gardens carrying the A23, which some contraflow traffic. This changed as all non-bus, taxi and cycle traffic was moved to the western side of the gardens and all other traffic is only allowed on the western side on a vary width of S2 to S4.
The Future north of the M25 looks quite mild, with a few adjustments to lanes. However South of the M23 the A23 may alter a lot due to its near motorway usage and layout. It is one of the routes that may become an Expressway if Highways England takes that route.
South Coast Central Report 2017
In 2017 the A23/M23 between Burgess Hill (A2300) and Crawley (M23 J10a) was highlighted as a potential for investment for the RIS2. The A23 for this section of the route is a D3, becoming a D2 south of the A2300 and runs on a fairly new alignment (all built since 1990) and the newest section realigned in 2014 between handcross and warninglid, the M23 on this section is a D3M, becoming a D4SM at junction 10, just north of junction 10a. Highway England's says:
- Congestion issues may be exacerbated due to major growth planned in and around Crawley, restricting growth.
- There are persistent safety and congestion issues, which are exacerbated by the lack of incident monitoring facilities.
So Highways may only make slight adjustments to improve visibility and add CCTV coverage to the road and make little physical adjustments, however there is a potential for it to become a Smart Motorway.