|To:||Burchett's Green (SU856809)|
|Length:||2.3 miles (3.7 km)|
|Meets:||M4, A308(M), A4, A404|
|Former Number(s):||M4, A4(M), A423(M)|
|Route outline (key)|
The road begins at Holyport Interchange: a large square-shaped signalised roundabout with slip roads to the M4, whose mainline passes underneath the roundabout, and the A308(M), also coming off the roundabout.
A short distance later, the A404(M) curves onto the historic A4(M)/M4 Maidenhead bypass line (whose route is still visible from the air) and, almost immediately, there is a tight junction with unclassified roads to Cox Green and White Waltham. The slip roads are so tight that they have 30mph advisory speed limits signed on the mainline and the fork sign is a 'Mr Floppy'.
Continuing around Maidenhead, the motorway ends at The Thicket Roundabout, passing underneath and becoming the all-purpose (and still trunk) A404. The underpass at The Thicket is the newest bit of the motorway, built as part of the A404 dual carriageway route in the early nineties. Previously the motorway ended at a smaller roundabout on the A4, with the A423 heading straight on.
After the M4 south of Reading opened, the leftover bit of motorway was diverted to a new junction at Holyport. As the A423 ended at the other end of the motorway, this section of motorway was numbered A423(M).
In the early 1990s, a new north-south link, linking the Marlow and Maidenhead bypasses, was created by dualling parts of the A404 and A423 and grade-separating Burchetts Green Roundabout and The Thicket Roundabout. This, coupled with the loss of the A423's importance due to the M40 being finished prompted a large scale renumbering of the A423 south of Banbury, with the A423(M) becoming A404(M) and creating a one-number high-quality route between the M4 and the M40 at High Wycombe.