Bignells Corner

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.

History

In 1922, there was no major junction in the area

The earlier crossroads was to the north of the present roundabout, shown here in the 1960 OS One Inch map

The first stage of construction of the roundabout is shown on the 1962 Half Inch map of Greater London

The original roundabout built as part of the A6 South Mimms Bypass, shown here in 1963

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 A5092), the Barnet bypass crossed the A6 just to the north of the present interchange. The A5092 was renumbered as A555 in 1935, finally becoming part of the A1 in 1951-52. At the same time the A6 was truncated back to this junction in order for it to continue to start on the A1; the road to the southeast of the junction gaining the number A1081. The location of the original junction is at a roundabout just to the east of the local road that crosses the A1(M) immediately north of the main interchange, although the roundabout here is a later addition.

In the 1962, the South Mimms Bypass opened, moving the junction to its present location slightly to the south-west of the original, with a flat roundabout being created at the new location, with mapping evidence (left) showing the A1081 connection from the southeast being opened first.

The three-level stack roundabout was built around the location of this later roundabout 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 A1178 became motorway when the adjoining section of M25 from Junction 24 to 25 was opened. A1(M) junctions 1 to 2 opened in 1979, as dual 2 lane motorway (D2M).

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.

In 1989, the South Eastern Road Construction Unit reported that this interchange had "severe operational problems", and that it could not be improved any further, without providing "corner cuts" between the A1(M) and both sides of the M25. Despite this, the only changes to the junction have retained the existing roundabout.

Service Area

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.

Routes

Route	To	Notes
A1(M)	The NORTH, Hatfield	was A555 (originally (A5092).
M25	Heathrow Airport (M4), Watford (M1), St Albans	was A6.
M25	(M20), Dartford Tnl, Stansted Airport (M11), Potters Bar (A111), Harlow (M11)	was A1178 and planned to be M16.
A1	London (C) & (NW)	was A555 (originally (A5092).
A1081	Barnet	was A6.
	South Mimms services
C173	Brookmans Park, South Mimms, Royal Vet College, Colney Heath
M16	Watford & M1, Heathrow Airport

Links

• Roads.org.uk: Bad Junctions

	This page was Article of the Month for June 2009