Its best-known section is the Hog's Back, a ridge forming part of the North Downs between Guildford and Farnham in Surrey.

The west branch of the M27 motorway – M3 interchange has a flyover junction with the winding westbound A3090 on the western boundary of Cadnam, having passed through the conservation area village centre, from where the resumed A31 continues as a dualled trunk road through the rest of the New Forest, past Ringwood and enters Dorset just before it passes Ferndown where it resumes single carriageway width.

It passes Wimborne Minster and by-passes Charborough Park before terminating at a roundabout junction with the A35 road at Bere Regis.

The Highways Agency's route management strategy (RMS) for the A31 has identified that the westbound carriageway needs realigning at Cadnam where it undulates rapidly before negotiating a sharp right turn and passes an at-grade U-turn box, though there is no timetable for this at present.

[8] In December 2014 the Government announced a series of schemes as part of a £15bn investment in transport infrastructure that included widening the Westbound carriageway to 3 lanes.

[9] Work to widen the westbound section to 3 lanes began in January 2022 in conjunction with a wider scheme improving safety, and replacing two bridges over the River Avon and Bickerley Millstream.

[10][11] A Highways Agency review in 2010 records that the collision rate between the A338 Ringwood junction and the Poulner Hill Interchange was 4.5 Personal Injury Collisions (PICs) per 100million vehicle kilometers, whilst the rate for the stretch between Poulner Hill and Picket Post is 11.7 PICs per 100million vehicle kilometers.

Here it ended at a junction with the A31, which at this time, having merged with the by-pass east of Winchester, re-emerged from the A33 at Otterbourne and ran to Hursley and on along the current route of the A3090.