A short spur from the M180 to Scunthorpe, with nothing much to see on the way, the M181 is so short that you have to ask whether it needs its own number at all.
Travelling north from junction 3 of the M180 to a point west of Scunthorpe, the M181 has few features to interest the traveller. It has a free-flowing interchange with its parent motorway; a gentle bend at one point; and views on both sides of flat Lincolnshire fields all the way to the flat horizon. It is now, however, in danger of being extinguished altogether.
If you're building a motorway - and we've all done it - there will be occasions on which it is necessary to provide a short spur road to connect your new highway with some part of the existing road network that is slightly out of reach. Because the little spur inevitably leads to a motorway, it should be a motorway too; otherwise cyclists and horse riders might get to the end and find themselves stuck.
There is, however, no need to number the little spur. It's just part of its parent route, like the little M23 spur near Gatwick or the branch from the M6 to Wigan. It's like a set of extended sliproads.
Every rule has exceptions, of course, especially where motorways are concerned. The M181 is one of them. This little connection from the M180 to the A18 west of Scunthorpe has ideas above its station and, for some reason now concealed by the ragged and threadbare dungarees of history, was granted its own number when other spurs of greater length do perfectly well without.
The M181 stood as a nondescript little spur, whose only indiosyncracy was the fact it had been given its own route number, until 2020 when North Lincolnshire Council took over control of the road from Highways England and began changing it to open up the land to either side as part of their Lincolnshire Lakes development. A new roundabout was built at Brumby Common, cutting the motorway short by almost a mile. Plans are in hand to build another roundabout at the B1450 Burringham Road, which will curtail it further, leaving just a very short motorway connection to the M180. Current plans suggest it will lose its route number at this point.
At the time of writing, however, the M181 has been setting the world of road enthusiasts ablaze, since its orphaned northern section - cut off from the motorway by the new Brumby Common Roundabout - has weirdly kept its motorway status and is now called A1077(M). This new motorway might or might not be permanent, but for now, the M181 appears to have been sliced in half, with the result that one half is still M181 while the other half has taken on a whole new identity of its own.