This tiny little spur is the highest numbered motorway in the UK. It connects the Erskine Bridge to the motorway network at the M8, and that's it.
It's a motorway designed and built in the earlier part of Scotland's motorway programme, and so it was designed to a set of standards that allowed less heavily-used routes to be built without hard shoulders and some other concessions. That sort of route would have emergency lay-bys at regular intervals instead, but at one mile in length, it's too short to have those either, so really it's just a dual carriageway with delusions of grandeur.
Its most impressive point, the three-level free flowing interchange with the M8, is less impressive when you realise its sliproads are all only one lane wide! When the motorway first opened each sliproad had two narrow lanes and no hard shoulder, which made it unwise to actually attempt overtaking, so it's for the best that the paintwork has been revised and the number of lanes reduced.
The motorway is, in fact, so short that by the time the sliproads have come together to form the mainline, you're already passing the direction signs for the terminus.
As of mid-2006, the Erskine Bridge tolls have been removed and the crossing is free, which means that the motorway's northern terminus, Toll Plaza Interchange, is no longer very appropriately named.