The A823(M) is a tiny spur from the M90 just north of the Forth Road Bridge. It was built in the early 1960s, at the same time as the adjoining section of the M90, to connect South Dunfermline to the new bridge. But it was never designed to just be a short spur - it was meant to go much further.
Plans existed at the time for a motorway running east-west across Fife, sometimes referred to as the M92. That route would have run towards the Kincardine Bridge in the west and along the coast to Kirkcaldy and then Glenrothes in the east. The A823(M) was designed to be the first part of that motorway project.
At the western end of the route, there's a huge roundabout with lots of space for an underpass for the motorway to continue west. At its eastern end, it connects to the M90 at a decidedly odd interchange where the carriageways split apart and take strange courses involving sharp corners and gradients that appear quite unnecessary. In fact the layout, the bridges and the considerable expanses of empty land between them were all provided as the first phase of an eventual free-flowing four-way interchange that would allow an eastward extension. In fact, given the amount of open space that exists between its two carriageways, most of the present A823(M) was only ever intended to be the sliproads for each of these junctions and very little of it was ultimately meant to be the mainline of the motorway at all.
Since the early 1960s, the plan changed, and a high-speed link to Glenrothes and Kirkcaldy now exists north of the proposed M92 line. It's an upgrade of the A92, missing out the coastal towns on the way that the M92 would have connected, and it meets the M90 at junctions 2A and 3, making any further eastward extension very unlikely. A short westward extension to provide a bypass for Rosyth was contemplated in the early 1990s, but has yet to materialise. The A823(M) is now almost certain to remain a little spur with ambitions of extension in both directions that will never be fulfilled.