This is one of the few journeys we cover with two genuinely competing road routes. Which one the driver chooses depends on the hour, the state of the orbital, and whether the coastal towns are moving. Your fare does not change either way — that is the whole point of agreeing it before travel.
Two routes, one destination — the driver picks on the day
wavesThe coastal route
Shorter in miles, slower per mile
Out of Southampton on the M27 east, then the A27 along the coast through or beside Fareham, Cosham, Havant, Chichester, Arundel, Worthing and Shoreham-by-Sea. Then the A24 north towards Horsham, then the A264 east to the M23, joining the motorway at junction 11 near Pease Pottage.
The single-carriageway stretches of the A27 through the Sussex towns carry most of the variation in the journey time. Off-peak the coast can be a pleasant drive.
directionsThe motorway route
Longer in miles, often quicker on a bad afternoon
The M3 north east out of Southampton, then the M25 clockwise round the southern side of London, then the M23 south.
The M25 flows fast off-peak and stops entirely at peak, so the driver on the day makes the call rather than a page in advance. At peak, when the single carriageways queue, the motorway wins.
mergeBoth converge at junction 9 of the M23 — the final few miles are identical either way.
The honest time range is 89 to 135 minutes, and almost all of the variation sits in the middle third of the drive rather than at either end.