From Roader's Digest: The SABRE Wiki
Central Reservations are used to separate opposing traffic flows on dual carriageways, including most motorways. They come in a variety of formats, with and without Crash Barriers depending on the road design and use.
In the past, Central Reservations were most commonly wide grassy strips between carriageways, often but not always with a Crash Barrier running along their length. The earliest motorways did without barriers, but they were quickly added to contain any accidents on a single carriageway as best as possible. These days, with motorways ever busier and extra lanes being added, many central reservations have been reduced in width, surfaced and had concrete barriers installed to both reduce their land-take and also provide a more resilient barrier.
In a few situations, most famously on the M6 in Cumbria, the M62 over the Pennines and the A74(M) in southern Scotland, the central reservation is much wider, with fields, road junctions and farms lying between the two carriageways. In other places, the vertical displacement between the carriageways see substantial Retaining Walls built, such as on the M5 in North Somerset.
Maintenance of central reservations has long been a problem, with vegetation taking hold and growing to unmanageable extents. The removal of the inevitable litter has also caused problems, although on the flipside they have become wildlife corridors, with rich ecosystems in some places.
Central Reserve Gaps
To allow for the use of contraflows, where traffic runs on the wrong carriageway during roadworks, (and originally also for places emergency vehicles could U-Turn) 'gaps' were provided in most central reservations. Whilst Gap is a bit of a misnomer in the modern era, these were tarred sections where the barriers were designed to be removable, to minimise the effort in providing the contraflow. In earlier times no physical separation was present at many gaps, and even up until the 1980s many gaps only used a series of semi-permanent plastic cones to prevent unauthorised use.
Away from the motorway network, gaps were also used to provide at-grade junctions, where right-turning traffic had to cross the opposing traffic flow. In the early days, the amount of room for queueing traffic was minimal, and even today on urban dual carriageways traffic often queues in the erstwhile 'fast lane', causing congestion, sucha s on the A82 at Dumbarton. However, it didn't take too long for much larger areas for turning traffic to be provided. The inherent dangers of such a manouvere are obvious, and as traffic flows increased, many have been closed up, either forcing traffic to proceed to the next junction and double back, or forcing authorities to build new Grade Separated Junctions or local access roads to maintain the accesses.