It is a dual carriageway where it is furthest from that river, in Spelthorne, Surrey and forms one of the motorway spurs to the large town of Maidenhead.

Other key settlements served are Fulham, Kingston (London), Staines upon Thames, Windsor and a minor approach to Marlow The South Kensington to Fulham section starts at the A4 road opposite Brompton Oratory and follows Fulham Road south-west past Chelsea and Westminster Hospital, where it jumps south a block to takeover the Kings Road.

It occupies a space carved out of the Royal Estate for it between Bushy Park and Hampton Court Park, and passes Hampton Court Palace including the roundabout opposite the Palace Gates, before continuing west through broad Hampton, passing its prominent parish church and waterworks to exit Greater London.

[3] As Staines Road East, the A308 winds past Kempton Park Racecourse, Sunbury-on-Thames and adopts instead of south-west followed after Kingston by west, the direction WNW, allowing it to meet Junction 1 of the M3/the A316 at the Sunbury Cross area also within Sunbury on Thames.

On this line, it passes the Sunbury Common part of that town and the south side of the slightly larger town of Ashford, as Staines Road West and a dual carriageway, passing the Queen Mary Reservoir to form the by-pass to the large town of Staines-upon-Thames, after which it goes back to single carriageway to turn briefly south-west to take the high street roads network, rather than a straight continuation, which becomes the A30, then ends this street as Clarence Street, crosses the Thames (for a second time) at Staines Bridge, and resumes its main third-part orientation.