The A329(M) traverses a patch of urban Berkshire, connecting Bracknell to Reading and linking both to the M4 - or rather, strictly speaking, connecting the edges of Bracknell and Reading, since it doesn't get very close to the centre of either. It's effectively a two-way spur from the M4.
It used to be longer than it is now, continuing northwards to the A4. That section of road still exists, but it's not the A329(M) any more. The motorway was shortened in the early 2000s as part of a park and ride scheme when the northbound hard shoulder approaching the A4 was converted into a bus lane. An employee of Berkshire County Council confirmed that the intention was simply to put a bus lane on the A329(M), but the Highways Agency stepped in and refused to allow them to do it without downgrading the motorway to an A-road, for no better reason than that they were developing the M4 Bus Lane and wanted to be the first to put a bus lane on a motorway. The northern section of the motorway was duly downgraded and is now the A3290. The M4 Bus Lane, meanwhile, has since been removed, but there's no sign of the A329(M) being restored to its former length.
The reason the A329(M) has such a huge free-flowing interchange with the M4 is because it was supposed to be much longer and more prestigious than it is now. At its northern end, it was supposed to continue parallel to the Thames to reach the Reading Inner Distributor Road - there's even space on Vastern Road in Reading town centre for a flyover to be added as part of its terminal junction. At its southern end, it was meant to be extended to form the M31, a motorway proposed in the 1960s that would have cut the corner between the M4 and what is now the southern section of the M25. This much grander motorway scheme would have seen a motorway from Reading to the M3 via Bracknell Forest, possibly with a further connection to directly interchange with the M25 as well. Pathetic Motorways has details on the proposals, including a map of what the A329(M) might have been.
Some evidence of those grand plans can be seen on the northern part of the route, and on the A3290, which have an unusually wide central reservation that was designed to enable a third lane to be added to the motorway when it was required.
Even without that extension, the A329(M) still performs that job today, in a more low-key fashion. Considerable volumes of traffic use it - and roads through Bracknell town centre to the A322 - between the M4 and M3, mostly in a bid to avoid part of the M25. The M31 is, unusually, a road that is being used even though it was never built.
