South of Johns Cross, the A21 originally followed the present day A2100 road passing Mountfield and heading through Battle and approaching Silverhill via Hollington.

Included in the scheme is Britain's first[10] land bridge at Scotney Castle which facilitates safe migration of wild animals over the road.

There were proposals to upgrade this section of A21 to a dual carriageway standard following the Pembury bypasses completion, however they were delayed multiple times.

Trees and shrubs were relocated to adjacent land and nesting boxes were installed to protect endangered species such as the dormouse.

As a result of the widening of the carriageway a number of buildings were demolished, including a Grade 2 listed 18th century barn.

Passing through Catford, Bromley and Farnborough, twenty miles (32 km) from the start of the journey, it reaches the Kent border and the open countryside.

The A21 originally entered Kent here and climbed to the scarp of the North Downs at Polhill, and then descended through Dunton Green and up the valley of the River Darent to Sevenoaks; through the town centre and then down into the Medway valley via Hildenborough to Tonbridge.

The London Road at the north of the town is now the B245; it continued through the long High Street, over the many bridges of the river (during which time it was also part of the A26 from Maidstone ).

As the road began to climb out of the valley it took a left fork; shortly after this the route of the modern A21 is rejoined.

The M25 then has to use a slip road in the left lane and the A21 takes priority although is still technically a motorway until the junction with the A25 to Sevenoaks and the M26.

The road passes to the west of the town, running through a nearby valley until it meets the A225 and B245 at Morley's Interchange near Sevenoaks Weald.

Around this point, the road enters the High Weald Area of Outstanding Natural Beauty.

As the dual Carriageway ends, the road enters East Sussex and meets the A268, taking traffic to/from Rye.

After a hill descent, the road reaches a roundabout where the Robertsbridge bypass begins, taking traffic away from the main street in the village.

After Johns Cross roundabout, the A21 takes a relatively straight, though undulating, journey, through Whatlington and passes Sedlescombe before climbing a four-mile (6.4 km) long hill to enter Hastings where the first junction reached is the Baldslow Interchange where currently the A28, A2100 and B2093 roads all terminate.

The next section of the A21 heads around partly on a one-way system near the railway station and the Priory Quarter business development.

There have long been plans are to upgrade some of the remaining stretches of single carriageway to alleviate congestion, safety and accessibility problems in the villages along the route.

[26][27][28][29][30][31] When the Pembury bypass ends at Kippings Cross, the next section of A21 is a low quality single carriageway road with several steep gradients across the Weald.

The Kippings Cross to Lamberhurst section has a high accident rate and congestion occurs particularly at peak times.

[35] The improvement will commence at the B2079 junction (Lady Oak Lane) on the short section of existing dual carriageway north of Flimwell and terminate at the roundabout at the northern end of the Robertsbridge Bypass.