The A421 is an important highway connecting the M1 at Junction 13 to the A1 at the Black Cat roundabout and is a major link between Oxford, Milton Keynes, Bedford and Cambridge. It was opened to traffic in December 2010.

The road was originally operated largely as a single carriageway with a short length of dual carriageway at Marston Moretaine. It suffered from considerable congestion at Junction 13 during peak travel times.

The road also had considerable inbuilt safety problems because of the large number of side-roads, private properties and businesses with direct access to it.

The intention for the overall improvement of the road was to provide a new fast dual carriageway route (two in each direction) to link the 13km from the Black Cat roundabout (junction with the A1) via the A6 at Kempston to Junction 13 of the M1. The project involved the construction of two new grade-separated interchanges, major works on the M1 Junction 13, a number of side road crossings and widening of the bridge over the Bedford to Bletchley railway.

“The project scheme was developed by Hyder Consulting between 2001 and 2005.”


The £231m project was undertaken by the UK Highways Agency, which had awarded the project to Balfour Beatty under an early contract involvement scheme in November 2005.

In the same month Scott Wilson was appointed designer for the scheme. It provided advice on engineering design, statutory procedures and environmental matters. The draft orders and environmental statement for the project were published in March 2007.

The project scheme was developed by Hyder Consulting between 2001 and 2005. It was also responsible for the initial scheme design and construction of traffic models, and development of the Environmental Statement.

A public inquiry was held between December 2007 and February 2008. Orders giving the go-ahead were published in July 2008 and construction began in October 2008 (major works began in early 2009). The project was completed in December 2010.


The scheme was composed of 13km (8 miles) of new dual two- and three-lane carriageways between Junction 13 on the M1 and the south side of Bedford (the Bedford Southern Bypass).

“The scheme was composed of 13km (8 miles) of new dual two- and three-lane carriageways between Junction 13 on the M1 and the south side of Bedford.”

The majority of the new road consists of a dual two-lane, constructed to the side of the existing A421, except for the 1.2km section from Marsh Leys to the A6, which is a dual three-lane and on the original alignment. The two new level junctions have been built at Marston Moretaine and Marsh Leys.

The circulatory carriageway at Marsh Leys was constructed as part of the Bedford Western Bypass scheme. Junction 13 of the M1 was redesigned to have a new bridge carrying the new A421 across it, while the existing bridge to the south-east is being used for local traffic movements and connection to the new Ridgmont Bypass. On the A6 there was an upgrade of the road between the A421/A6 junction and Elstow Road, ostensibly for traffic using the Interchange Retail Park (agreed with Bedfordshire County Council), which included remodelling and signal installation at three roundabout junctions at Progress Park (park and ride), Cowbridge and Elstow Road.

This project also involved tree and shrub planting (about 250,000 new trees sourced from Marston Forest) and extensive landscaping. Drainage from the new road surfaces occurs via new balancing ponds before the water is discharged into existing watercourses.


In recent years, because of serious restrictions on landfill, new materials have been used for fill materials during engineering and construction projects. Tyres are a major disposal problem, they can no longer be sent to landfill (they were banned in 2003).

Tyre bails are being used in construction projects for slope stabilisation, as waterway control barriers and even as artificial reefs. The A421 used tyre bails in some areas to build a lightweight embankment on soft ground next to Brogborough Lake to allow the A421 re-alignment. Each tyre bale measures 1.5m × 0.8m × 1.3m and consists of 100 to 115 tyres.

Approximately 4,500 tyre bales were needed for the embankment, equating to about 500,000 tyres. A 14m-deep cutting was constructed at Brogborough, although the spoil could not be transported to landfill. Instead the material was used to form alignments of the southbound off-slip from the M1 and the new Salford Road.

The M1 west-side embankment was completed between October 2008 and October 2009. A new rail bridge was built over the Bedford to Bletchley line to carry the new westbound carriageway.

Ongoing works include replacing the existing concrete parapets with new metal parapets on three bridges (Brogborough Interchange, Aspley Guise Footpath Bridge and Portsfields Farm Access Bridge) over the M1 Motorway. The project, which started in January 2011, is expected to be completed within six months.