The M27 forms a bypass and distributor road for what amounts to a sizeable conurbation along the south coast in Hampshire, including Portsmouth, Gosport and Southampton.
On the road, it often looks and feels every bit the long-distance, inter-urban motorway: fast, with a smooth alignment cutting across countryside and through woods, with widely-spaced junctions and mostly three lanes wide. But its appearance belies the fact that it's really of regional importance, linking up cities and towns around the Solent and filling up daily with commuters.
There's a good case for it to travel further east and west to help relieve the busy south coast trunk route, reaching Bournemouth and Brighton, but the motorway is constricted by the New Forest National Park in the west, whose guardians the Verderers are unwilling to see road building in the forest, and in the east by - er, nothing. The A27 continues from the M27's eastern end as a four-lane motorway-standard road. For no real reason, this remains part of the A27.
One of the reasons that Southampton and Portsmouth are linked by such a good road is that, in the 1970s as this motorway was being built, there were ambitious plans for a major New Town on the south coast between the two cities: Solent City would have been a vast new urban area, requiring good motorway access. The new city was, in the end, never built.
The M27 forms perhaps the most effective terminus of any London radial motorway. The M3 approaches Southampton via Basingstoke and Winchester, but it's stopped from crashing headlong into the city by the M27 which distributes its traffic east and west.