The M180 runs along the south side of the Humber, connecting the wider world (via the M18) to Scunthorpe, the Humber Bridge and Grimsby. It is most famous for being very lightly used - most of the route is three lanes wide, and most of the time that feels rather extravagant. It would be nice if that third lane could be picked up and moved to somewhere more in need of it.
It was built to relieve the A18 and to service the expanding ports and industrial estates in this part of the world. Into the bargain it provides a good route to the large towns in the area from Doncaster to Grimsby. It now provides a fast, safe route for the large volumes of heavy goods traffic that has cause to use it, and allows faster cars and vans to make the same journey without being delayed by them. Even so, it doesn't go as far as it could, giving up at Elsham. The road on from there is the A180 - and is virtually indistinguishable from its motorway counterpart. The end of the motorway in this arbitrary location is inexplicable, though unfortunately no more unusual than, say, the M27 giving up at Portsmouth or the M62 at North Cave.
The M180 is let down slightly by its unimportant-sounding number - all the other motorways with three-digit numbers are short spurs of local significance (such as this road's little brother the M181). This is, arguably, an important strategic route and, despite its emptiness, is more important than a lot of motorways with more important sounding numbers (M45 and M49 spring to mind as much shorter and less nationally valuable routes). It could easily have been given a number like M19, but it appears that no such thing was ever contemplated.
That this route was built while other heavily-used corridors remain without a motorway upgrade to this day can be put at least partly down to the fact that this is a very flat area. Land here costs next to nothing and plonking down 26 miles of new motorway - especially at 1970s prices - was instant value for very little money.