|Location Map ( geo)|
|The Junction viewed from a local road.|
| South Mimms|
|M25, A1, A1(M), A1081|
|Junctions related to the A1081|
Bignell's Corner (also known as South Mimms Interchange) is a large three-level roundabout interchange providing full access between the M25, A1(M), A1081, South Mimms services and local roads. It is junction 1 of the A1(M), and junction 23 of the M25. The A1(M) passes underneath the roundabout in a north-south direction at the lowest level, with the M25 passing over the roundabout at the highest level in an east-west direction. The junction marks the point where the A1 heading north out of London becomes the A1(M) motorway, and also the start of the M25 all lane running eastbound between the A1(M) and the M11; this was the first All Lane Running scheme to be launched on the UK motorway network.
The roundabout is signal controlled, and is one of the largest roundabouts in the UK.
The earliest modern major route through this area was the A6 running from Barnet in the south-east towards St Albans (and ultimately on towards Carlisle) north-west of the junction. When the Barnet bypass was built (as A555), the Barnet bypass crossed the A6 just to the north of the present interchange. The A555 was renumbered as A1, and these cross roads became a roundabout, indeed there is still a roundabout at that location, just to the east of the local road that crosses the A1(M) immediately north of the main interchange.
The three-level stack roundabout was built to the south of the original interchange location in stages to accommodate the construction of the linking motorways. The A1 was grade separated when the M25 eastwards opened in September 1975 (initially as an all-purpose road, the A1178). The A1 (now A1(M)) was built through the junction as dual 3 lane motorway (D3M) to tie in with the 3 lane Barnet bypass to the south.
The A6 link to the north-west was upgraded as part of the final section of the M25 which opened in 1986. It was as part of this scheme that the bridge carrying the M25 over the roundabout was built and the junction took on the form we see today.
The A1(M) has been re-marked as dual two-lane motorway through the interchange in order to improve traffic flow. Prior to this change, the A1(M) used to widen from D2M to D3M about half a mile north of the south-bound off-slip. Traffic waiting to leave the A1(M) blocked back into the new lane 1 of the A1(M), conflicting with traffic intending to stay on the A1. By remarking the A1(M) as a two lane carriageway, the new lane 1 became a dedicated slip road for the M25 and service area. There was an added benefit south of the interchange as traffic joining from Bignell's Corner could now do so via a dedicated lane.
On the northbound carriageway, similar changes have been made - previously 3 lanes of A1(M) and 2 slip road lanes merged in to three lanes and then into two lanes over a very short length. By giving the interchange a dedicated lane at the northbound off-slip, the northbound merge has been made much cleaner and safer.
See Main Article: South Mimms services
The roundabout also provides access to a large Welcome Break motorway service area which caters mainly for cars and coaches. The car service area was built in 1986 (after the M25 opened), and the facilities building re-built after a fire in 1998. There is also a hotel at the site, as well as a large truck stop just to the north. The truck stop pre-dates the M25.
This section of the interchange is known as Bignell's Corner, after the garden centre that used to stand here.
|The NORTH, Hatfield||was A555 (originally (A5092).|
|Heathrow Airport (M4), Watford (M1), St Albans||was A6.|
|(M20), Dartford Tnl, Stansted Airport (M11), Potters Bar (A111), Harlow (M11)||was A1178 and planned to be M16.|
|London (C) & (NW)||was A555 (originally (A5092).|
|South Mimms services|
|Brookmans Park, South Mimms, Royal Vet College, Colney Heath|
|Watford & M1, Heathrow Airport|
This page was Article of the Month for June 2009